Asimov on Astronomy

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ASIMOV ON ASTRONOMY In this collection of essays on astron-

omy

Asimov again displays one

Isaac

facet of the

him

many

a Renaissance

talents that

Man

make

in the twenti-

eth century.

The seventeen chapters

in this

book

have been selected from Asimov’s earlier works. Here updated and enhanced with new photographs, the essays reflect Asimov at his best— explaining the subtle puzzles that a entific (it

is

sci-

view of the universe presents the tides that aflFect time and

make our days twenty-four hours engaging in the heady delight of ungoverned speculation ( what might a possible tenth planet be like?), long),

or offering practical applications of

phenomena

mathematical verities are used to solve the problem of disposing of radioactive waste )

scientific

(

As Dr. Asimov moves from the earth outward to the solar system and the stars,

we

working is

see a piercing intelligence

to describe the universe that

our hv 'ne. All this from a

by

hi^

one

.

own

man who,

confession, has never taken

urse in astronomy!

523 As

Asimov

Asimov on astronomy

S.Cat*

CADILLAC-WEXFORD PUBLIC LIBRART' CADILLAC, MICHIGAN. 49602

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ASIMOV ON ASTRONOMY

ESSAYS

ON SCIENCE

by Isaac Asimov

From The Magazine

of Fantasy

and Science Fiction

fact and fancy VIEW FROM A HEIGHT adding a dimension

OF TIME AND SPACE AND OTHER THINGS FROM EARTH TO HEAVEN SCIENCE, NUMBERS, AND I THE SOLAR SYSTEM AND BACK

THE STARS IN THEIR COURSES THE LEFT HAND OF THE ELECTRON ASIMOV ON ASTRONOMY

From Other

Sources

ONLY A trillion IS ANYONE THERE? TODAY AND TOMORROW AND

ASIMOV

ON ASTRONOMY ISAAC ASIMOV

Doubleday

6*

Company,

Inc.,

Garden

City,

New

York

1974

.kDILUC-r.'':),'

CAb'iLLAC,

i

-I)

i

tilC LIBRART

49601

1

ISBN:

1

Library of Congress Catalog Card

©

Number y^- 8 og ^6

974 by Isaac Asimov 1966 by Mercury 1959, i960, 1961, 1962, 1963, 1964, 1965, Copyright

Copyright

©

-X

1

All Rights Reserved

Printed in the United States of America

A I

'

Press, Inc.

To

Paul R. Esserman^

easily the best internist in the

world

Digitized by the Internet Archive in

2017 with funding from

Kahle/Austin Foundation

https://archive.org/details/asimovonastronomOOisaa

volume

from The Magazine of Fantasy and Science Fiction. Individual essays appeared in

All essays in this

are reprinted

the following issues:

Planet of the Double Sun

June 1959

Home

The

Sight of

The

Flickering Yardstick

February i960

March i960

Beyond Pluto

July i960

Stepping Stones to the Stars

October i960

Heaven on Earth

May

The Trojan Hearse

December 1961

Superficially Speaking

February 1962

By Jove

May May

Just

Mooning Around

1961

1962

1963

Twinkle, Twinkle, Little Star

October 1963

Round and Round and

January 1964

The Black

A

of

Galaxy at

Harmony

in

The Rocks

a

Night

Time

Heaven

of

Damocles

Time and Tide

.

.

.

November 1964

May

1963

February 1965

March 1966

May

1966

It

%

%

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CONTENTS

INTRODUCTION

XI

1

Time and Tide

2

The Rocks

3

Harmony

4

The Trojan Hearse

44

5

By

62

Damocles

of in

1

Heaven

29

Jove!

6 Superficially Speaking 7

Round and Round and

8

Beyond Pluto

9 Just

7S .

.

.

89 101

Mooning Around

10 Steppingstones to the Stars 1 1

TTie Planet of the Double Sun

12 Twinkle, Twinkle, Little Star 1

3

Heaven on Earth

14 I’he Flickering Yardstick 1

5

The

16 s ij6 192

of

Night

20 ^

at a

Time

218

16

The Black

17

A

INDEX

^53

Home

Sight of

Galaxy

141

^33

I**

I

.•I

4

n.

INTRODUCTION

Back

in 1959,

I

began writing

a

monthly science column

azine of Fantasy and Science Fiction.

I

was given carte blanehe

subject matter, approach, style, and everything else, and of that.

I

The Mag-

for

I

made

full

have used the column to range through every science

informal and very personal way so that of a great deal)

nothing gives

me

so

much

all

in

use

an

do (and I do these monthly es-

the writing

pleasure as

as to

I

savs.

And time

I

them

as

though that were not pleasure enough

complete seventeen

into a

A

Few

tenth

Doubleday & Company

book and publishes them. As

published nine books of says.

essays.

is,

my F & SF

why, every

in itself,

of this

moment,

Inc., puts I

have had

essays, eontaining a total of 153 es-

of course, in the works.

books, however, can be expected to

sell indefinitely;

at least not

enough to be worth the investment of keeping them forever in print. The estimable gentlemen at Doubleday have, therefore (with some reluctance, for they are fond of me and know how my lower lip well

tends to tremble on these oceasions), allowed the of essays to

Out

go out of

I

hasten to say. All

ishing in paperback so that they are

It is

is

of

my

books

print.

of hardback print,

theless, there

first five

a cachet

still

five of

the books are flour-

available to the public. Never-

about the hardback that

I

am

reluctant to lose.

the hardbacks that supply the libraries; and for those

who

really

INTRODUCTION

Xll

want

books’^ there

My

their large personal collections of

permanent addition to

a

nothing

is

like a

Asimov

hardback.

impulse, then, was to ask the kind people at Doubleday to

first

wind. put the books back into print and gamble on a kind of second with This is done periodically in the case of my science fiction books (even

success

editions are simultaneously available).

when paperback

ever could see that the case was different. My science fiction is for the advance of fresh, but science essays do tend to get out of date,

But

I

science

And

inexorable.

is

then

I

got to thinking

.

.

.

satisfy deliberately range widely over the various sciences both to

I

my own

restless interests

audience a chance to result

is

and

to give each

own

satisfy his

of

my

heterogeneous

now and

particular taste

then.

The

some on astronomy, some on some on biology, and so on.

that each collection of essays has

some on physics, But what about the reader who

chemistry,

ularly interested in

nomical

member

astronomy?

interested in science, but

is

He

partic-

has to read through the non-astro-

each book and can find only four or

articles in

is

five,

perhaps, on

his favorite subject.

Why

out the not, then, go through the five out-of-print books, cull

and put seventeen of them together in a volume we put can call Asimov on Astronomy? Each individual article is old, but together like that, the combination is new. So here is the volume. It has six articles from Fact and Fancy, three astronomy

articles,

from View from a Height, one from Adding a Dimension, four from Earth to Of Time and Space and Other Things, and three from From Heaven. The articles are arranged, not chronologically, but conceptually.

As you turn the pages, you

broader scope.

The

first

them taking on broader and

will find

couple deal with Earth and

its

vicinity, the last

couple with the Universe as a whole.

Aside from grouping the

what more have

I

articles

done? Well, the

into a articles

more homogeneous mass, are anywhere from six to

shows here and there. I feel rather of pleased that the advance of science has not knocked out a single one the articles here included, or even seriously dented any, but minor thirteen years old

and

their age

changes must be made, and In doing this

I

I

have not revised the

would deprive you of the fun ^

Don’t

tell

me you

have made them.

of seeing

don’t have one!

articles

me

eat

themselves since that

my

words now and then,

INTRODUCTION or,

anyway, chew them

a little.

So

I

have made the changes by adding

footnotes here and there where something or where

I

was forced to make

a

Xlll

said

I

needed modification

change to avoid presenting misinforma-

tion in the tables.

In addition to that,

my

good friends

at

Doubleday decided

the book in more elaborate format than they have used for

and have added

to prepare

my

ordinary

which

I

have written

captions that give information above and beyond what

is

in the essays

essay collections,

illustrations to

themselves. Finally, since the subject matter in

my

which

is

so

much more homogeneous than

ordinary grab-bag essay collections, will,

I

hope, increase the usefulness of the book as reference.

So, although the individual essays are old,

new and

have prepared an index

I

useful just the same.

esty, exactly \\’hat

I

And

at least

I

have done and why. The

I

hope you

find the

have explained, rest

is

Isaac

New

up

in all

book hon-

to you.

Asimov York, September 1972

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ASIMOV ON ASTRONOMY

«

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41

TIME

What

AND

TIDE

with one thing and another, IVe gotten used to explaining var-

ious subtle puzzles that arise in connection with the scientific view of the universe. For instance, I have disposed of the manner in which elec-

and photons can be waves part of the time and particles the rest of the time in a dozen different ways and by use of a dozen different trons

analogies.

ve gotten so good at

I

it,

in fact, that at

dinner parties the word nerv-

ously goes about, “For heaven's sake, don’t ask

Asimov anything about

wave-particle duality.”

And no one

ever does.

I

primed and aching to explain, the party for me.

sit

there

all

and no one ever asks. It kills But it’s the simple thing that throws me. I’ve just been trying to write a very small book on the Moon* for third-graders and as part of the task I was asked to explain why there are two high tides each day. Simple, flexed *

my

I

thought, and a condescending smirk passed over

fingers

and bent over the

This has since been published: The

typewriter.

Moon

(Follett,

1967).

my

face.

I

ASIMOV ON ASTRONOMY

2

As the time passed, the smirk vanished and the hair

at

my

temples

managed at last, after a fashion, but if you don't mind. Gentle Reader, Fd like to try again. I need the practice. grew perceptibly grayer.

I

Tlie tides have bothered people for a long time, but not the good

whom

old Greeks, with reference to

you

see, lived

(and

still live,

start so

I

many

articles.

Tlie Greeks,

on the shores of the Medi-

for that matter)

terranean Sea. Tliat sea happens to be relatively tideless because

it is

so

nearly land-locked that high tide can’t get through the Strait of Gibraltar

before the time for

it

has passed and

it is

low tide again.

About 325 B.C., however, a Greek explorer, Pytheas of Massalia (the modern Marseilles), ventured out of the Mediterranean and into the Atlantic. There he came across good pronounced tides, with two periods and two periods of low water in between. Pytheas made good observations of these, undoubtedly helped out by the inhabitants of the shores facing the open ocean who were used to the tides of high water each day

and took them

The key

for granted.

observation was that the range between high water and low

water was not always the same.

increased and decreased with time.

It

Each month there were two periods

of particularly large range

between

high and low tides (''spring tides”) and, in between, two periods of particularly small range ("neap tides”).

What’s more, the monthly variations matched the phases of the Moon. The spring tides came at full Moon and new Moon, while the neap

tides

came

and

at first quarter

third quarter. Pytheas suggested,

by the Moon. Some of the

therefore, that the tides were caused

Greek astronomers accepted gestion lay fallow for

this,

but for the most part, Pytheas’s sug-

two thousand

Tliere were plenty of

men who

years.

believed that the

manner in which crops grew, the the way in which a man might turn

the

encountering spooks and goblins

seemed I

to

be going

later

Moon

influenced

rationality or irrationality of

men,

into a werewolf, the likelihood of

— but

that

it

might influence the

tides

a bit far!

suspect that one factor that spoiled the

Moon /tide

connection for

thoughtful scholars was precisely the fact that there were two tides a day.

For instance, suppose there

is

a high tide

when

the

Moon

is

high in

That would make sense. The Moon might well be drawing the water to itself by some mysterious force. No one in ancient and mediethe sky.

TIME AND TIDE val times

had any notion of

could at least give ter

just

how

3

such a force might behave, but one

name such as '‘sympathetic attraction/’ If the waheaped up under a high Moon, a point on the rotating Earth, passit

a

ing through the heap,

would experience

a high tide followed

by a low

tide.

But

a little over twelve hours later, there

Moon would

and then the on the other

Moon

be nowhere

would be another high

in the sky. It

would

tide

be, in fact,

side of the globe, in the direction of a man’s feet. If the

were exerting

sympathetic attraction, the water on the man’s side of the globe ought to be pulled downward in the direction of his

feet.

There ought

Or

could

a

to be a hollow in the ocean, not a heap.

be that the

it

Moon

the side of the Earth nearest side opposite.

Then

exerted a sympathetic attraction on

and

itself

a sympathetic repulsion

there would be a heap on both sides,

on the

two heaps

all

one rotation of the Earth, a point on the shore would pass through both heaps and there would be t\\'0 high tides each day, with

told. In

two low

The

tides in bet\^’een.

notion that the

Moon would

pull in

some

places

and push

in

other places must have been very hard to accept, and most scholars didn’t logical

So the Moon’s influence on the tides was put down to superstition by the astronomers of early modern times.

try.

astro-

In the early i6oos, for instance, Johannes Kepler stated his belief that the Moon influenced the tides, and the sober Galileo laughed at him. Kepler, after

Moon and

who

was an astrologer

all,

the planets on

would have none of

all sorts

that. Galileo

sloshing of the oceans back

believed in the influence of the

of earthly

thought the

and forth

as the

phenomena and tides

Galileo

were caused by the

Earth rotated— and he was

quite wrong.

Came

Newton at last! By using that law

Isaac

gravitation.

tational field

had

to exert

In 1685, he advanced the law of universal

became obvious that the Moon’s gravian influence on the Earth and the tides could it

well be a response to that field.

But why two

What difference does the Moon on the Earth

tides?

the force exerted by

or "gravitational attraction”?

How

could the

it

make whether we

call

"sympathetic attraction”

Moon, when

it

was on

the other side of the Earth, cause the water on this side to heap upward,

away from the Moon, llie Moon would still have to be pulling in one place and pushing in another, wouldn’t it? And that still wouldn’t

make

sense,

would

it?

ASIMOV ON ASTRONOMY

4

Ah, but Newton did more than change words and substitute for

“sympathy.” Newton showed exactly

in

The

the gravitational force

which was more than anyone before him had

varied with distance,

shown

how

‘'gravity’'

connection with any vaguely postulated sympathetic force.

gravitational force varied inversely as the square of the distance.

That means the

force grows smaller as the distance grows larger;

the distance increases by a ratio of

and

the force decreases by a ratio of

x,

Moon and

Let’s take the specific case of the

the Earth.

The

if

y?.

average

Moon’s center from the surface of the Earth nearest itself is 234,000 miles. In order to get the distance of the Moon’s center from the surface of the Earth farthest from itself, you must add the thickness of the Earth (8,000 miles) to the first figure, and that gives

distance of the

you 242,000 miles.

THE MOON This

is

the full

Moon, but not

exactly as

we

see

from Earth.

it

It

was photographed from Apollo 11, the first vehicle to deliver human beings onto the surface of our satellite, and the photograph was taken

on the homeward journey. The angle see the

Moon

from Earth's

Notice that the dark

Moon.

All of

There are like

them

virtually

tend to cluster toward one side of the

are located

on the Lunar hemisphere facing Earth.

no seas on the

hemispheres, hiars,

not quite that from which we

surface.

‘"seas”

a '‘mystery” but the

is

Moon

far side of the

is

Moon. This sounds

not unique in having two unlike

we now know, has one hemisphere

rich in craters,

the other rich in volcanoes. For that matter. Earth can be divided into

two hemispheres, one of which other

is

almost entirely water-covered and the

is

half land.

Actually, the

most amazing thing about the

Moon

are five satellites in the Solar system that are

somewhat

Moon, but all of those Earth. The Moon has 1/81

much

the

lite is it

circle giant planets

the mass of the Earth

There

its size.

is

than

larger

larger

than the

and no other

satel-

anywhere close to that mass in terms of the mass of the planet

circles.

The

result

the right

is

that the Earth experiences greater tides than

{perhaps)

to

expect,

and

it

swish of the tides that helped give sea

vade and colonize the land.

agency of the Moon.

We

all

may be life

the

the push

may be

it

has

inward-outward it

needed to

in-

here only through the

TIME AND TIDE

photo: national aeronautics and space administration

5

ASIMOV ON ASTRONOMY

6

we

If

at

1,

Moon

set the distance of the

to the near surface of the Earth

then the distance to the far surface

242,000/234,000 or 1.034. As

is

the distance increases from 1.000 to 1.034, the gravitational force de-

from 1.000 to 1/1.034^, or 0.93. There is thus a 7.0 percent difference

creases

by the

Moon

the Earth were

made

force exerted If

ing

somewhat

ent

amount depending on

to the

on the two

pull,

of gravitational

sides of the Earth.

of soft rubber,

Moon’s

amount

in the

you might picture

as yield-

it

but each part would yield by a

differ-

the strength of the pull on that particular

part.

The it

surface of the Earth on the

would be most strongly

would be attracted with

Moon’s

from the

would

yield

attracted. Tlie parts beneath

a progressively

The opposite Moon would move toward it

toward the Moon.

less

side

most since the surface

weaker force and move

less

and

side of the Earth, being farthest least of all.

There would therefore be two bulges; one on the part of the Earth’s

Moon,

surface nearest the

since that part of the surface

would move

the most; and another on the part of the Earth’s surface farthest from

Moon,

the

behind

since that part of the surface

would move the

least

and

lag

the rest of the Earth.

all

not

If that’s

clear, let’s try analogy.

Imagine

a

compact group

of run-

them run toward the finish line so that we might suppose some “force” is attracting them toward that finish line. As they run, the speedier ones pull out ahead and the slower ones ners running a long race. All of

fall

behind. Despite the fact that only one “force”

directed toward the finish line, there are

is

involved, a “force”

two “bulges” produced;

a bulge

of runners extending forward toward the finish line in the direction of

the force, and another bulge of runners extending backward in the direction opposite to that of the force.

Actually the solid body of the Earth, held together by strong inter-

molecular forces, yields only very slightly to the gravitational

by the

ential exerted

Moon

on the Earth. The liquid oceans, held

gether by far weaker intermolecular forces, }ield considerably

make two

“tidal bulges,”

As the Earth past the

first

tidal

There are thus tion of the If

the

rotates,

one toward the

Moon and

more and

one away from

an individual point on some seacoast

to-

is

it.

carried

bulge and then half a day later through the second.

t^^’0

high tides and two low tides in one complete rota-

Earth— or,

Moon

differ-

to put

it

more simply,

were motionless, the

in

one day.

tidal bulges

would always remain

TIME AND TIDE in exactly the

apart.

same

same

and high

place,

The Moon moves

tides

7

would be exactly twelve hours

about the Earth, however,

in its orbit

direction that the Earth rotates,

and the

in the

move with

tidal bulges

it.

By the time some point on Earth has passed through one bulge and is approaching a second, that second bulge has moved onward so that the Earth must rotate an additional half hour

in order to pass the

point un-

der question through high tide again.

The time between

high tides

is

twelve hours and twenty-five minutes,

and the time from one high tide to the next but one is twenty-four hours and fifty minutes. 1 hus, the high tides each day come nearly one hour later than on the day before. But why spring tides and neap tides and wdiat tween tides and the phases of the Moon? For that

W/'e

have to bring

The

influence on the Earth.

in the Sun.

It,

the connection be-

is

too, exerts a gravitational

gravitational pull of

two separate heavenly

bodies on the Earth varies directly wdth the mass of the bodies in question

and

inversely with the square of their distance

To make unit,

things simple,

let’s

use the mass of the

and the average distance of the

center) as the distance-unit. Tlie at

1

upon

Moon-distance

in

Sun

of the

distance from the Earth then, that the distances.

The

Sun

is

Moon from Moon possesses

Moon

as the mass-

the Earth (center to

Moon-mass and is other words, and the Moon’s gravitational pull

us can therefore be set at

The mass

from the Earth.

is is

or

i

i.

Moon and

27,000,000 times that of the

392 times that of the

Moon.

27,000,000 Moon-masses and

gravitational pull of the

is

We

at

Sun upon the Earth

can

392 is

its

say,

Moon-

therefore

27,000,000/ 392^ or 176. This means that the Sun’s gravitational pull upon the Earth is 176 times that of the Moon. You would therefore

expect the Sun to create tidal bulges on the Earth, and so it does. One bulge on the side tow'ard itself, natually, and one on the side opposite itself.

At the new Moon, the Moon Sun, and both Moon and Sun

is

on the same side of the Earth

are pulling in the

same

as the

direction. Tlie

bulges they produce separately add to each other, producing an unusually large difference

At the

between high and low

Moon,

Moon

tide.

on the side of the Earth opposite that of the Sun. Both, how'ever, are producing bulges on the side nearest full

the

is

them and on the side opposite them. The Sun’s near-bulge coincides with the Moon’s far-bulge and vice versa. Once again, the bulges pro-

ASIMOV ON ASTRONOMY

8

duced separately add to eaeh other and another unusually large enee between high and low tide TTierefore the spring tides

At

first

and

third quarter,

anee,

Moon,

Sun

as pulling

and

left of

produeed.

is

eome at new Moon and full Moon. when the Moon has the half-Moon

Earth, and Sun form a right triangle.

from the right and producing a

Moon

the Earth, then the

If

tidal

and

still

appear-

you picture the

bulge to the right

at first quarter

pulling from

is

above and producing a bulge up and down. (At third quarter, ing from below

differ-

it is

pull-

producing a bulge up and down.)

In either case, the two sets of bulges tend to neutralize each other.

What would

Moon’s low

ordinarily be the

tide

partially filled

is

by the

existence of the Sun’s high tide, so that the range in water level be-

tween high and low tide first

and

is

Thus we have the neap

cut down.

tides at

third quarter.

But hold on.

I

said that the

Moon’s low

tide

by

'‘partially filled”

is

Only “partially.” Does that mean smaller than the Moon’s tidal bulges?

the existence of the Sun’s high tide.

the Sun’s tidal bulges are It

sure does.

Moon’s

effect

Surely,

The

tides follow

but never abolishes

Moon. The Sun

the

it.

one ought to ask why that should be

Sun’s gravitational pull on the Earth

Why then should Tlie answer

is

it

be the

that

it is

Moon

The

so. I

have said that the

176 times that of the

is

Moon.

that produces the major tidal effect?

not the gravitational pull

the tides, but the difference in that pull

Earth.

modifies the

upon

itself

that produces

different parts of the

difference in gravitational pull over the Earth’s width de-

creases rapidly as the

body under consideration

is

moved

farther off,

since, as the total distance increases, the distance represented

width of the Earth makes up a smaller and smaller part of the

by the

total.

Thus, the distance of the Sun’s center from the Earth’s center

about 92,900,000 miles. The Earth’s width makes this case

than

far less difference in

tance from the Sun’s center to the side of the Earth near 000, while the distance to the far side

is

is

tional pull drops off to only

it

is

dis-

92,896,-

92,904,000. If the distance from

the Sun’s center to the near side of the Earth distance to the far side

The

the Moon’s distance.

in the case, earlier cited, of

is

is

set equal to 1,

then the

1.00009. In that distance, the Sun’s gravita1 /

1.00009-

0.99982.

In other words, where the difference in the Moon’s gravitational pull

from one side of the Earth to the other

is

7.0 percent; the difference of

TIME AND TIDE the Sun’s gravitational pull gravitational difference by

only 0.018 percent. Multiply the Sun’s

is

greater gravitational pull overall

its

176) and you get 3.2 percent. to that of the

is

We see

Sun

as 7.0

is

The

1 is

Moon

to 0.46.

then that the Moon’s effect on tides

much

(0.018X

tide-producing effect of the

to 3.2 or as

of the Sun, despite the Sun’s

A

9

more than twice that

is

greater gravitational pull.

second way of attaining the comparative gravitational pulls of two

bodies upon the Earth

to divide their respective masses

is

by the cubes

of their respective distances.

Thus, since the its

Moon

tide-producing effect

has

1

is

Moon-mass and is at 1 Moon-distance, or 1. The Sun with 27,000,000 Moon-

masses at 392 Moon-distances, has a tide-producing effect of 27,000,000/ 392^ or 0.46.

We

can easily see that no body other than the Sun and the

can have any significant

tidal effect

body other than those two

is

Moon

on the Earth. The nearest sizable

the planet Venus.

It

can approach as

closely as 26,000,000 miles, or 108 Moon-distances, at year-and-a-half intervals.

Even then

tidal effect

is

tides, in a

way, affect time. At

day twenty-four hours long. As the it

only 66/109^ or 0.0000051 times

Moon.

that of the

The

its

least, it

tidal

the tides that

tion

so

is

total over

this dissipation represents

to

much on

isn’t

the

human

must have been

rotating on

were more important

scale,

and

this rate of

but

the Earth has been

if

day-lengthening has

its

When

total of fifty thou-

the Earth was created,

axis in only ten

hours (or

in early geologic times

less,

if

it

the

than they are now,

might have been).

as they well

As the Earth’s as well,

must be

Still, it is

years.

time

sand seconds or nearly fourteen hours.

It

rota-

be slowing the Earth’s rotation and lengthening the day by

been constant throughout, the day has lengthened a

tum

Sea

only a very small portion of the

any particular year or even any particular century.

in existence for five billion years

tides

Irish

and the energy of Earth’s

one second every one hundred thousand This

and the

dissipated as frictional heat. Tlie energy of the Earth’s rotation

huge that

enough

make our

bulge travels about the Earth,

scrapes against shallow sea bottoms (the Bering Sea

are supposed to be the prime culprits)

is

is

but

rate of rotation slows this

angular

down,

momentum

retained, as angular

it

loses angular

momen-

cannot be dissipated as heat.

momentum,

elsewhere in the

Moon-

ASIMOV ON ASTRONOMY

lO

Earth system. this

What

by receding from the Earth.

momentum

angular

Moon must

the Earth loses the

as

Its

greater distance

turns, since angular

it

gain and

means

momentum

can do

it

a greater

depends not

only upon rate of turn, but also upon distance from the center about

which an object

The

is

turning.

effect of the tides, then,

to slow the Earth's rotation

is

Moon. how much

and

to in-

crease the distance of the

There

a limit to

is

the Earth's rotation will be slowed.

Eventually, the Earth will rotate about will

always face the

happens, the der the

will

Moon

turns in

No

more friction, no more then be more than fifty times

Moon

and the more distant

its

one side

When

orbit.

that

be “frozen" into place immediately un-

(and on Earth's opposite side) and

about the Earth.

now turns. Of course,

as the

tidal bulges will

Moon

Earth day

Moon

axis so slowly that

its

will

slowing.

no longer

The

travel

length of the

as long as the present day;

will turn in its orbit in twice the period

the tidal bulges of the Sun will

Earth some seven times a year and

this will

still

it

be moving about the

have further

on the

effects

Earth-Moon system, but never mind that now.

Even

if

there were

no oceans on the Earth, there would

friction, for the solid

be

tidal

substance of the Earth does yield a bit to the

Moon. This bulge

differential pull of the

around the Earth

still

of solid material traveling

also contributes to internal friction

and

to the slowing

of the Earth's rotation.

We the

work on the Moon, which has no oceans. Just as produces tides on the Earth, so the Earth produces tides on

can see

Moon

Moon. Moon, but the

this at

Since the mass of the Earth the distance

is

is

the same one

eighty-one times that of the

way

as the other,

you might

suspect that the tidal effect of Earth-on-Moon would be eighty-one times that of Moon-on-Earth. Actually, a smaller

body than the Earth

ence over

its

its

I

can give you the

Earth-on-Moon the

Moon

is

is

in the case of the larger Earth.

its

all,

I

must

results

considered to be i.oo, then the

32.5.

affected 32.5 times as

mass, and therefore

is

so there's a smaller gravitational differ-

the effect of Moon-on-Earth

With

The Moon

into the details of the mathematics (after

spare you something)

effect of

not quite that high.

width than there would be

Without going If

it's

much

as the

Earth

is,

and with

rotational energy, considerably less than that

:

TIME AND TIDE ample time

of the Earth, there has been

tem is

to dissipate

its

frozen into the

toward the Earth.

We As

Moon, and where

I’his

Moon

Moon would

face

fact, there are six

that are Moon-sized or a

the

one side to

its

Moon

little larger,

we continue to be i.oo, we have Table

225

Jiipiter-on-Callisto

225

upiter-on-Ganymede

on lo

What

about the

satellites

Of

its

on

145

all

these satellites have

reverse,

their

of these satellites

though?

What

about the

effect of the various

their primaries?

and Triton

is

remaining four, which are

we

is

262,000 miles from

cube of the distance, we can suspect that these

have considerably more

will

and Triton. lo

219,000 miles from Neptune. Because the effect

varies inversely with the

all

effect

much

on

farther

their primaries

away from

than

drop

is

far larger

across the lesser width of

conclude that of the

gest. Let’s see

how much

that

this

six planet/satellite

considering, the tidal effect of lo

we

width of Jupiter than

Neptune. Since the extent of

we have been Again,

we note

than Neptune and that there will therefore be a

in the gravitational field across the

cial, it is fair to

will the

their primaries.

consider the Jupiter/ lo and Neptune/Triton pairs, then

that Jupiter larger

Each

had

the six Moon-sized satellites just mentioned, the two which are

Jupiter

If

are therefore

primary constantly.

closest to their primaries are lo

two

They

945

rotations with respect to their primaries stopped.

one side to

attached to a

5^650

There seems no question but that faces

is

i

Saturn-on-Titan

Jupiter

times.

i

720

on Europa

all

consider the effect of the

Neptune-on-T riton

Jupiter

were very

primary at

and each of them

Table

J

it

other satellites in the solar system

affected tidally. If

on the Earth to

even

a tidal effect

(unless

planet considerably more massive than the Earth.

much more

bulge

tidal

one side only

faces

which receives

satellite

Moon) also

larger than the

matter of

where the

actually the situation.

is

can suspect that any

a

in the history of the solar sys-

rotational energy to the point

greater than that received by the

much

11

drop

is

cru-

combinations

on Jupiter

is

the stron-

Moon on

the Earth

is.

are considering the tidal effect of the

ASIMOV ON ASTRONOMY

12 to

be

lo

on Jupiter

you

1.00. In that case (if

lliis

is

is

amount, surprising to anyone who would assume,

a sizable

Jupiter.

on Jupiter that our Moon roughly the effect on Jupiter that the Earth

has. It has thirty times the effect

has on the Earth. lo exerts exerts

on the Moon.

Naturally, although the Earth’s effect rotation relative to ter to in

much

could scarcely have

satellite like lo

on giant

of a gravitational effect it

calculations) the effect of

equal to 30.

without analysis, that a small ‘Well,

my

will trust

we wouldn’t

itself,

sufficient to stop the

is

expect

lo’s similar effect

slow Jupiter’s rotation significantly. After

mass than the

Moon

is

and packs

more

far

all,

Jupiter

is

Moon’s on Jupi-

far larger

rotational energy into

its

structure. Jupiter can dissipate this rotational energy for billions of years

without slowing

tional energy at the still

much

rotation

its

same

rate

while the

brought to

is

Moon,

a halt.

dissipating

rota-

its

And, indeed, Jupiter

rotates with a period of only ten hours.

However, there are

effects other

tidal

than rotation-slowing.

It

has

recently been discovered that Jupiter’s emission of radio waves varies in

time with the rotation of

number I

lo.

of theories to explain

exactly understand. (I

am

This seems to puzzle astronomers and a it

have been proposed which

not, after

all, a

I

am

not sure

professional astronomer.)

suspect that these explanations must surely take into account lo’s tidal action on Jupiter’s huge atmosphere. This tidal action must affect I

the turbulence of that atmosphere and therefore

its

radio emission. In

the extremely unlikely case that this has not been considered, the suggestion to

That

all

leaves only

fect of the

comers

I

offer

free of charge.

one more thing to consider.

Sun upon Earth’s

tides.

This

is

I

have discussed the

ef-

not terribly large (o.q6) and

one can expect that since the Sun’s tide-producing effect drops off as the cube of the distance, the effect on planets more distant than the Earth would prove to be insignificant.

What

about the

closer to the

effect

Sun than

Well, according to 1.06

and

its

effect

is

my

on Venus and Mercury, however, which are

the Earth? calculations, the Sun’s tidal effect

on Mercury

is

is

is

3.77.

lliese are intermediate figures.

on Earth, which

on Venus

They

are

more than the Moon’s

effect

not sufficient to stop Earth’s rotation altogether; but

TIME AND TIDE they are

less

stop the

Moon.

than the Eartli’s effect on the

One might wliile slowed,

suppose, then,

tliat

13

Moon, which was enough

the rotations of

would not yet have slowed to

single side to the guess, for

as

Venus and Mercury,

a stop.

Nevertheless, for a long time, the rotations of

were considered

to

Venus and Mercury

having been stopped, so that both planets faced a

Sun

Venus,

at all times. In the case of

no one had ever seen the

this

was

surface, but in the case of

a

pure

Mercurv, ^

where surface markings could be made out (though obscurely) the ing seemed to check with observations. In the last }ear or two, however, this view has

feel-

to be revised in the

Venus and Mercury are each rotating slowly with the Sun as (with the wisdom of hindsight) one might have

case of both planets. respect to

had

'

suspected from the figures on tidal

effects.

THE ROCKS OF DAMOCLES

some ways,

In

science fiction writers aren’t doing too well these days.

In late 1962, Mariner

II

seemed to

temperature of Venus, placing

With

it

far

settle the question of the surface

above the boiling point of water.

some of the most beautiful settings for s.f. stories. Old-timers may remember with nostalgia, as I do, the moist, swampy world of Weinbaum’s 'Tarasite Planet.” Well, it’s gone! For that matter, I wrote a short novel under a pseudonym, a number of years ago, that was set on a Venus that was one huge ocean, with Earth-cities that, there vanished

underwater

built

Now

in the shallower regions.*

along comes Mariner

IV and

.

.

.

All gone!

discovers craters (but

no

canals)

on Mars.

No that *

one expected

had ever placed

that!

I

craters

don’t

know

on Mars.

.

of a single science fiction story .

.

Canals, yes, but craters, no!

This was Lucky Starr and the Oceans of Venus (Doubleday, 1954). Since this article first appeared it has appeared as a paperback under my own name, with a preliminary note explaining the outdated astronomy.

THE ROCKS OF DAMOCLES I

have written several

the canals

stories set

placed no water in

(I

15

on Mars and I have always mentioned them; I knew enough for that) but Tve

never had craters.

And

yet,

going by the photographs sent back by various Mariner

probes, the Martian surface

Moon

craters as the

is,

some

in

places anyway, at least as rich in

is.

Fortunately for science-fictional self-respect, astronomers themselves

much

didn’t do

Not one

better.

of them, as far as

I

know, suggested

Venus might be as hot as Mercury until after the first microwave observations came to be analyzed in detail. And very few even speculated on the possibility of a cratered Mars. In the

first

flush of the

Martian pictures, the newspapers announced

meant there was no

that this

life

on Mars.

To be

sure, the pictures didn’t

give us life-enthusiasts anything to cheer about but, as

things aren’t

The mere in

numerous

turns out,

that bad.

all

fact that the pictures

Some

of course.

itself,

it

of our

show no

signs of life

own weather

means nothing have taken

satellites

pictures of the Earth under conditions comparable to those

IV and Mars, and the Earth pictures show no signs of life on our planet, either. I don’t mean no signs of man; I mean no signs of any life at all. And, mind you, we know where to look for signs of life of Mariner

on Earth, and what

A

more

subtle

existence of

all

to look for.

argument

upon the mere Mars had ever had an ample

for the anti-life view rests

those craters on Mars.

If

ocean and atmosphere those craters would have been eroded away. Since the craters are there, goes the argument.

Mars has always been

cated and almost airless and, therefore, the chances of

veloped in the

first

life

desic-

having de-

place are extremely small.

Quickly, however, the pro-life forces shot back. Since Mars closer to the asteroid

zone than the

Moon

is,

and since the

is

much

asteroids are

very likely to be the source of the large bodies that collide with planets

and form times as

sizable craters.

many

Mars ought

to have

something

like twenty-five

Moon does. It appears many. What happened to the

craters per unit surface area as the

to have considerably fewer than that

other five-sixths?

Eroded away! If so,

then the craters we do see represent the youngest ones, the ones

that have not

had time

erosion proceeds at

away

Assuming that the process of uniform speed, then the marks we see only tell us to erode

yet.

ASIMOV ON ASTRONOMY

i6

about the

last sixth or so of

Martian history

—say

seven hundred

six to

million years.

What happened

before that

we

still

can’t

present in larger quantities before then and

tell.

life

Water may have been

might have started on a

comparatively water-rich and water-comfortable Mars.

may have been hardy enough

life

to survive even

If

now on

Martian

so,

the gradually

bleaching bones of the planet.

Maybe not, but we can’t tell from only the pictures we have. We will need much more detailed photographs or, better yet, a manned expedition to

Mars.*

Still,

craters

on Mars do

set

one thinking. In the course of the history

of the solar system (its inner regions, at least) the major worlds

must

have undergone a continuous peppering bombardment from smaller bodies.

The Moon and Mars

bear the visible scars of this and

it is

quite

out of the question to suppose that the Earth could possibly have

caped

its

share of the

bombardment.

Although the Earth it

is

is

as far

from the asteroid zone

eighty-one times as massive as the

Moon

as the

Moon

and, at equivalent

tances, pulls with eighty-one times the force. Eurthermore, Earth larger target, with fourteen times the cross-sectional area of the

so

it

should have endured

though Earth 3.5

is

es-

much

many

farther

times as

many

is

is,

dis-

the

Moon,

collisions. In fact, al-

from the asteroid zone than Mars

is, it

has

times the cross-sectional area and ten times the mass of Mars, and

suspect

it

has suffered more of a peppering than Mars has.

Moon

Tlie

I

is

supposed to have 300,000 craters with diameters of one

kilometer or more. Even allowing for the fact that the Earth

is

seventy

percent ocean, which can absorb collisions without being marked up as a result, the

remaining thirty percent— Earth’s land surface— might well

have suffered at

Where

least a million collisions in

are they all? Erased!

things quickly wipes

The

them out and

its

billions of years of history.

effect of

wind, water, and living

every last trace of

more than

ninety-

nine percent of the craters formed on Earth must have vanished by now.

But

surely there are

remnants of the more recent ones. All one need

*

Mars probes, since this article was written, show conditions on Mars to be even harsher than had been thought colder, for instance. They also showed a very variegated surface, with volcanic areas and possible erosion effects. The question of life

—

on Mars

open question. There are even suggestions based on recent data that Mars enjoys periodic eras of mild conditions with both air and water in is still

rather an

considerable quantities.

THE ROCKS OF DAMOCLES do

is

17

look for depressions in the Earth that are more or

less eircular.

These are easy

to find, as a matter of faet, espeeially sinee they

only be roughly

eireiilar.

After

all,

unevennesses ean be put

down

need

to the

effeets of erosion, slippage, further

bombardment, and so on. A niee near-eircular depression might fill with water and form a neareireular sea. The Aral Sea is an example. Then, too, the northern boundary of the Blaek Sea forms a near-eireular are. Tlie Gulf of Mexieo ean almost be made to fit a semieirele. For that matter the Indian Oeean, and even the Paeifie Oeean, have eoastlines that ean be

made

to

fit

\\ hether

want

with surprising eloseness.

eireles

you find sueh

to find

eireles or

how

not depends on

them and how ready you

eagerly

are to dismiss departure

you

from

striet eireularity.

One

of those

most anxious to

find

possible eraters

Daehille of Pennsylvania State University. At just sent

no

less

me

a diseussion of his views,

least,

is

Dr. Frank

the university has

whieh ineludes

a

map on whieh

than forty-two '‘probable and putative’’ meteorite craters or

groups of craters are listed in the United States and near-vicinity alone. Of these, the largest in the United States proper is what is marked

down

"Michigan Basin.” This is the near-circle formed by Lakes Michigan and Huron, a circle some three hundred miles in diameter. According to Dachille’s estimate a crater this large would have to be as the

caused by a meteorite thirty or forty miles in diameter.

On

the

map

a

still

larger crater

is

indicated as "Kelly Crater,”

and

marks out the Atlantic coastline of the United States along the continental shelf. This forms an arc of about one-third of a circle which this

would be over twelve hundred miles in diameter if it were complete. I suppose that would require a meteorite a hundred miles in diameter or so — in other words, one of the larger asteroids.* One might dismiss such catastrophic events as having been confined to a highly specialized period of planetary history.

At the very beginning

of the formation of the solar system, planetesimals were gathering to-

gether to form the planets and the left

the huge scars that

last

few

really large

ones could have

mark major formations on the Earth and Moon.

*

Since this article was written, geologists have grown convinced that, through continental drift, the Atlantic Ocean has been formed quite recently (as geologic history goes). Its borders fit together quite closely and could not possibly have been

hammered out by large meteor strikes. I doubt gained many adherents, but that doesn't mean place at various sites and times.

that Dr. Dachille’s enthusiasm has that meteorite strikes haven’t taken

i8

A

S

I

M OV ON ASTRONOMY

photo: national aeronautics and space administration

THEROCKSOFDAM OGLES Or

else there

was a period

in planetary history

ig

during which a possible

planet between Mars and Jupiter exploded (or underwent a series of explosions) leaving the asteroid belt behind and riddling the solar sys-

tem from Mercury

to Jupiter with flying shrapnel in bits

up

to a

hun-

dred miles across. In either case^ one might argue, this specialized period

damage

is

Tliere are

done, the craters are formed, and

no more

planet-busters,

no more

we can

is

over, the

dismiss the matter.

one hundred

asteriods of

CRATERS ON MARS When

Alariner

photographs,

JV made

its

pass at Aiars

and sent back a

series of

happened, by sheerest circumstance, to send us shots of craters very much like those on the Aloon. Off went the canals, on it

went the craters. Gone was the thought of Aiars as a world, somewhat like Earth, with a dying civilization [well, astronomers hadn’t ever really thought that, except for one writers had),

or two,

and bang, came the thought of Aiars

but science-fiction as a

dead world

like

the Aloon.

But then

in 1972, Alariner 9 was orbiting Aiars and taking photographs of virtually its entire surface and it turned out that there was

a great deal more to Aiars than craters. There was one huge volcano that was larger than any volcano on Earth. There was a canyon longer

and deeper than any canyon on Earth. What’s more, there was dust everywhere [as we thought there might be on the Aloon before the first landings and as it turned out there wasn’t).

The

whipped on by the thin atmosphere,

dust was

covering and uncovering various features. In this photograph three large craters partly obscured by drifting sand.

lower

As to

left

has had part of

its

we

see

The one on

the

sand cover removed by the wind.

happens, there are also signs of meandering rifts that seem have been made by flowing water not too long ago [geologiit

cally speaking). It

portion of

may be

that

we have caught

Aiars in the

‘‘ice

age”

most of its atmosphere frozen into polar caps of mingled water and carbon dioxide. In the other part of its cycle, its

cycle, with

may have no

and a fairly thick atmosphere covering some liquid water. There would be no free oxygen, of course, but free oxygen is not essential to life and so it may be that some life it

polar caps at

all,

forms may yet be found on Aiars.

ASIMOVON ASTRONOMY

20 miles in diameter or

more

around within reaching distance of

floating

the Earth.

Indeed, that

is

so.

There

no body that ever approaches within 25

is

million miles of the Earth that

over, let us say, twenty-five miles in

is

diameter, except for the

Moon

itself,

Moon

Are we

safe,

to leave

its

orbit.

CRATERS ON THE

and there

is

no reason

to expect the

then?

MOON

Almost everything about the Moon is of significance to Man, his environment, and to the growth of his science, since it is so near to us. It is the only object in the sky, for instance, except for the Sun and an occasional comet, which

is

more than a dot

object in the sky that steadily

of light,

and

and consistently changes

it is

in

the only

shape in a

regular cycle.

Even

in ancient times,

reflected light clearly

To

it

could be seen that the

from the Sun,

was not self-luminous.

so that It

it

{who

shone only by

was the only heavenly body that

was a dark world

the Greek theoreticians

Moon

called

like

the Earth.

themselves philosophers)

seemed that the heavenly bodies were so different in properties from the Earth they had to be formed of some fundamentally different it

substance. Aristotle called that substance “ether.”

were supposed

be

to

perfect,

The heavenly

unchanging, glowing;

not

bodies

imperfect,

changing, and decaying like the Earth.

The Moon, with

its

darkness (except where lighted by the Sun)

and with the mysterious smudges on its shining face, alone seemed to cast doubt on this generalization. But, after all, the Moon was the closest of the heavenly bodies to Earth and it might partake some-

what of Earth’s imperfection. Then, however, in 1609, Galileo constructed a telescope and turned mountains clearly it first on the Moon. And what he saw were mountains and dark patches that looked like seas. The Moon was a

—

—

world very

much

the heavens and

Earth and the notion of the perfection of essential difference from Earth went smashing.

like the

its

For three and a half centuries

after Galileo, the craters of the

Moon

could only be seen with some fuzziness owing to Earth’s atmosphere, but in the ig 6 os photographs were taken from space and from

Moon. This photo was taken by Apollo about the Moon.

tances close to the

was orbiting

1 1

dis-

when

it

THE ROCKS OF DAMOCLES

21

photo: national aeronautics and space administration

o ''.L'JjL L A

I

CADILLAC,

-

'jAU,

^

:

6

I

49601

AUy

ASIMOV ON ASTRONOMY

22

No,

vve are not!

Space

is

loaded with dust particles and pebbles that

burst into our atmosphere, glow, and vaporize harmlessly. In addition,

however, larger chunks— not large enough to gouge out an ocean, per-

damage— are moving

haps, but large enough to do horrendous

W

in

e can after all find craters that are indubitably craters

such good condition that they must be quite new.

lar of

these craters

small lunar crater;

is

it

is

and that are

The most

located near Winslow, Arizona.

It

near us.

spectacu-

looks just like a

roughly circular, with an average diameter of

about 4,150 feet or four-fifths of a mile. It is 570 feet deep and the bottom is filled with a layer of mashed and broken debris about 600 feet thick. It

is

surrounded by a wall which

is

from 130

to 160 feet higher

than the surrounding plain.

The

to demonstrate that the crater

first

was caused by a meteorite

(and was not an extinct volcano) was the American mining engineer Daniel Moreau Barringer, and the

site

is

therefore called the Barringer

more dramatic name Meteor Crater. I have even seen it called the Great Barringer Meteor Crater, a name which honors, at one stroke, the size, the man, and the Crater. Because of

its

origin

it

also bears the

origin.

The

fact that the strike took place in

an arid region where the

effects

of water and living things are minimal has kept the Barringer Crater in better preservation than

would have been the case

most places on Earth. Even to is

be more than

fifty

so, its

condition

is

if it

were located in

such that

it is

not likely

thousand years old and, geologically speaking, that

yesterday. If

the meteorite that formed

struck now, largest city

Other

and

in

(some millions of tons in mass) had the proper spot, it could, at one stroke, wipe out the it

on Earth, and more or

strikes (not as large to

toric times,

destroy vast tracts of

its

suburbs.

be sure) have taken place even

two respectable ones

place in central Siberia in 1908.

less

It

in the twentieth century.

in his-

One

took

involved a meteor of only a few dozen

tons of mass, perhaps, but that was enough to gouge out craters up to

one hundred and

fifty feet

in

diameter and to knock

down

trees for

twenty to thirty miles around.’^

No

meteoric matter has been located on the site so far. Could it have been a colwith a small comet made up of substances that vaporize after impact? Could it have been a small piece of anti-matter which explodes on contact with ordinary matter and leaves nothing behind? Alas, no one knows.

lision

THE ROCKS OF DAMOCLES That

will do,

and how!

A

23

that in the middle of

fall like

Manhattan

would probably knock down every building on the island and large numbers across the rivers on either side, killing several million people within minutes of impact. In fact, that 1908

been calculated that actual path, but

did

fall

come

close to wiping out a

the meteor had

if

had been displaced

Earth to rotate for

moved

just

city. It

an orbit parallel to

in

enough

has its

space to allow the

in

more hours before impact,

five

major

would have

it

Petersburg (then the capital of the Russian Empire and

hit St.

now Lenin-

grad) right on the nose.

Then,

in 1947, there

was another such

fall,

but a smaller one,

in far-

eastern Siberia.

Two

then, both in Siberia, and both doing virtually no

falls,

except to trees and wild animals.

Mankind has

damage

had an unusual

clearly

run of luck.

There are some astronomers who estimate that there may be two such Aity-busters” hitting Earth per century.

If so,

we can make some

calcu-

The area of New York City is about kn 70.000 of the Earth’s surwe assume that a city-buster can strike any place on Earth, at

lations.

face. If

random, then

a given city-buster has

1

chance

in

670,000 of hitting

New

York. If it

hitting 1

comes, on the average, once every

New

York

in

fifty years,

any one particular year

is

1

in

then

its

chances of

670,000 times 50, or

in 33,000,000.

But

New

York City

is

only one city out of many.

we

consider that

330 times the area of New spur-of-the-moment estimate) then the chance of some

the total densely urbanized area on Earth

York City (my

If

is

urban center being flattened by a city-buster

in

any given year

is

1

in

100,000.

To put

it

another way,

next hundred thousand years, will

be wiped out by

even

money

sometime within the some good-sized city somewhere on Earth

it’s

a city-buster.

And

that

that

is

probably an overopti-

mistic prediction since the urbanized area on Earth

may

is

increasing

continue to increase for quite a while, presenting a

much

and

better

target.

ITiis

makes

it

clear, too,

why no urbanized

area in the past has been

destroyed. Cities have only existed for say seven thousand years and until

the last couple of centuries, the really large ones have been few and

widely scattered.

ITe chances

for a

major disaster of

this sort

having

ASIMOV ON ASTRONOMY

24

taken place at any time in recorded history 1

100— and

in

But must

On

it

vve

hasn’t happened.

have

striking land fifty miles

after If

But what

all,

ought

may be

Even

tolerable.

the meteorite strikes the ocean? Three out of four,

is

not inordinately large and

damage may be

a large meteorite

landlocked arm of the

may

sea. It

cities of

if

strikes far

it

But there

small.

strike near a coastline,

enough

always a

is

even perhaps

might then do severe damage, and

since the appropriate regions of the sea are

the

1908-type meteorite

a

from a populated center may leave that center

a coastline, the

chance that

about the dangers of a near-miss.

to.

the meteorite

away from in a

if

Wh^t

a direct hit?

land, a near-miss

intaet.

probably not better than

is

much

larger in area than are

the world, the chances of a catastrophic oceanic near-miss

on

are correspondingly greater than the chances of a dead-center hit

a

city.

The a

oceanic near-miss might be expected, on the average, not once in

hundred thousand

years,

but once

in ten

some record

In short, there should be

thousand years or even

less.

of such disasters in historical

and I think that perhaps there are. Noah’s Flood did happen. Or, put it

times,

and

disastrous flood

ago. Concerning this,

—

way There was a vast in the Tigris-Euphrates some six thousand years Babylonian stories were handed down which, with this

the generations, were interlarded with mythological detail.

Noah’s Flood

Nor

is

as given in the Bible

this speculation.

is

Archaeological probings on the

there are

come

The

ficient to

me.

I

is

that since

may have

it

flooded

on one occasion. This has always seemed

don’t see

how any

river flood

the observed sediment or do the kind of

some

artifacts.

usual suggestion

the Tigris-Euphrates complex floods occasionally, particularly disastrously

sites of

across thick layers of sedi-

no human remains or

these sediments?

tale of

a version of those stories.

of the ancient cities of Babylonia have

ment within which What laid down

The

can possibly lay

insuf-

down

damage dramatized

all

later in

the tales of universal flood, death, and destruction— even allowing for

the natural exaggeration of storytellers. I

have an alternative suggestion which,

with me.

What

I

if

have seen nothing about

it

The

I

know,

is

original

anywhere.

a city-buster meteorite had,

landed in the Persian Gulf.

as far as

some

Persian Gulf

six is

thousand years ago,

nearly landlocked

and

such an impact might well have heaved up a wall of water that would

THEROCKSOFDAM OGLES move northwestward and would

burst

25

with absolutely devastating

in,

impact, upon the low plain of the I’igris-Euphrates valley. It

would be

a super-tsunami, a tidal

would scour much of the was indeed “all the world” it

numbers

wave

to

end

all tidal

The water would

valley clean.

waves, and cover what

and drown countless

to the inhabitants

in its path.

In support of this notion,

more than

speaks of

of heaven were

day were

all

I

would

point out that the Bible

like to

Genesis 7:11 says not only that “the windows

rain.

opened” meaning that

the fountains of the great deep broken up.”

what? Meaning,

seems to me, that the water came

it

in

drifted.

Where

did

it

drift? It

came

to rest

Meaning

from the

Furthermore, Noah’s ark lacked any motive power, either

and simply

same

rained, but also that “the

it

sails

sea.

or oars,

on the mountains

of Ararat (the ancient Urartu) in the Caucasian foothills northwest of

the Tigris-Euphrates. But an ordinary river flood would have washed

boats southeastward out to sea. Only a tidal wave of unprecedented

scope would have carried the ark northwestward.*

Nor need Noah’s Flood be near-miss.

Many

the only tale of a

peoples (but not

have flood legends, and

all)

have been such a flood legend that gave dramatized

finally

When

A thousand Of

years?

course,

Tomorrow?

we might

the

in

next disaster come?

will the

rise to

in his story of Atlantis.**

deed have happened more than once

A

Tliere’s

remembered oceanic

the

dim

it

may

tales that Plato

Such catastrophes may

memory

of

in-

man.

hundred thousand years hence?

no way of

telling.

scan near-space and see what’s floating around

there. Till 1898, the

answer was Nothing! Between the orbits of Mars and

Venus was nothing all

anyone could

at

any

rate, that

of

tell.

any consequence save the Earth and Moon, for

Nothing worse than pebbles and small boulders,

could produce the effect of a shooting star and oc-

casionally reach Earth’s surface.

man *

or demolish a house, but the

Two

Guide

A

years after this article

Volume

first

meteorite might conceivably

damage

appeared,

I

to

kill

a

be expected of meteorites

repeated this conjecture in Asimov’s

The Old Testament

(Doiibleday, 1968). So far, no archaeologists have written to say what an inspired guess this is. It’s a shame the way we geniuses go unrecognized. ** I was wrong here. Since this article was written, archaeologists have discovered to the Bible,

I,

that a small island in the southern Aegean exploded in a gigantic volcanic explosion

up a tsunami that destroyed Cretan civilization. It was this exploding island about 1400 b.c. that left vague memories that turned up as Plato’s Atlantis a thousand years later. No meteorite, but anyway a catastrophe. that set in

ASIMOV ON ASTRONOMY

26

was only the stance,

and man has managed to

Then

Nothing unusual,

elliptical, of course,

and

as did all asteroids

till

with the thunderstorm.

Witt

calculated

its

discovered Asorbit.

This was

that part of the elliptical path that was

then discovered.

In the remainder of

of

in

until

bolts, for in-

from the Sun, the asteroid traveled between Mars and Jupiter

farthest

of

live

German astronomer Gustav Witt

in 1898, the

teroid 433.

done by lightning

tiniest fraction of that

its

orbit,

however,

it

traveled

between the

orbits

Mars and Earth. Its orbit approached within 14 million miles of that Earth and at set intervals, when both objects were at just the right

place in their respective orbits, that approach would be realized and

the asteroid would be at only half the distance from us that

Venus is. Witt named the asteroid Eros after the son of Mars and Venus in the classical myths. That began the practice of masculine names for all asteroids with unusual orbits.

Eros's close approach scientists.

was

a

could be used to determine the dimensions of the solar

It

system with unprecedented accuracy, 1931,

when

it

flickering of irregular,

light, it

And this was done in miles. From the periodic

close.

was decided that Eros was not a sphere but an

roughly brick-shaped object which brightened

dimmed when we saw

estimated to be fifteen miles and

There was no all,

when

approached within ly million

its

long-ways and

After

among

matter of self-congratulations

its

it

end-ways.

Its

when we saw

it

longest diameter

is

shortest five miles.

particular nervousness felt about Eros's close approach.

14 million miles

isn't exactly close,

As time went on, however,

you know.

several additional objects

were discovered

with orbits that came closer to Earth's than that of Venus does, and such objects came to be called “Earth-grazers." Several of them passed

Sun than Venus does and one of them, Icarus, actually moves in closer to the Sun than Mercury does. The climax came in 1937, when the asteroid Hermes was discovered by an astronomer named Reinmuth. On October 30, it passed within 487,000 miles of the Earth and the calculation of its orbit seems to show that it might possibly come within 200,000 miles — closer than the Moon! closer to the

(For information on some Earth-grazers, see Table

Even

so,

why

worry?

A

2.)

miss of two hundred thousand miles

is

still

a

fair-sized miss, isn't it?

No,

it

isn't,

and

for three reasons. In the

asteroids aren't necessarily fixed.

The

first

place, the orbits of

Earth-grazers have small masses,

astronomically speaking, and a close approach to a larger world can

THE ROCKS OF DAMOCLES Table

2

THE EARTH-GRAZERS*

NAME

YEAR OF

LENGTH OF

estimated

DISCOVERY

ORBITAL PERIOD

maximum diameter

(years)

(miles)

Albert

1911

Eros

1898

1.76

Amor

1932

2.67

10

Apollo

1932

1.81

Icarus

estimated mass

CLOSEST

APPROACH TO EARTH

(trillions OF tons)

(millions OF miles)

300

20

1

5,000

M

1

2,000

10

2

100

7

1949

1.12

1

12

4

Adonis

1936

2.76

1

12

1-5

Hermes

1937

1-47

1

12

0.2

*

c.

4

3

1964, two years before this article appeared, another Earth-grazer was discovered and was named Toro. It never approaches closer to us than 9 million miles, but its orbit is locked into that of Earth’s, so that it never recedes past a certain In

Sun 5 times in 8 years in a complex dance about the Earth. “quasl-moon” and seems to present Earth with no danger.

limit. It circles the is

a kind of

introduce changes in that orbit. Comets, for instance, which have teroidal masses,

It

as-

have been observed on more than one occasion to un-

dergo radical orbital changes as a result of close approaches to Jupiter.

Hermes doesn’t approach Jupiter at all skim the Earth-Moon system, and, on

closely to

be

sure,

but

does

it

and

occasion. Mercury,

it

is

subject to orbital change for that reason.

As

ing in 1937, though so.

This

that at

Hermes

a matter of fact,

we

all,

may don’t

it

should have come

mean that know the right

well

its

first

sight-

fairly close every three years or

orbit has already

place to look for

change

in

Hermes’ orbit

away from Earth than toward than toward. it

Still,

to zero in,

ing to think of. be. If

its

it

changed somewhat so

and

will rediscover

it, if

only by accident.

A random

cause

hasn’t been located since

there

and

is

it,

since

a finite

much more likely to move it there is much more room away

is

chance that such

a direct collision of

The Earth won’t be

a

change might

Hermes and Earth

perceptibly damaged, but

you think that a meteorite a few million tons

in

is

horrify-

we could

mass could gouge

out a hole nearly a mile across, you can see that Hermes, with a few trillons

of tons of mass, could excavate a good-sized portion

American

state or of a

European nation.

of an

ASIMOV ON ASTRONOMY

28

Secondly, these Earth-grazers probably haven’t been in their orbit

through

the history of the solar system.

all

Some

some

collision,

pertur-

bation, has shifted their orbits— origittally within the asteroid belt— and

caused them to

might

join

in

fall

toward the Sun.

them. Naturally,

it is

On

occasion, then,

new ones

the smaller asteroids that stand the best

chance of serious orbital changes but there are thousands of Hermessized asteroids in the asteroid belt

of

unwelcome new

and Earth can

still

witness the

coming

strangers.

we can

Thirdly, the only Earth-grazers

detect are those big enough to

see at hundreds of thousands of miles. Tliere are

ones in greater profusion.

If

bound

to be smaller

there are half a dozen objects a mile in

diameter and more that come wandering into near-space on occasion, there

may be

diameter or

wandered

No,

I

more which

half a thousand or

so,

and which could

still

are

one hundred

do tremendous damage

feet in if

they

in too closelv.

see

no way,

at present, of either predicting or avoiding the

occurrence of a very occasional major catastrophe.

We

spin through

space with the possibility of collision with these rocks of Damocles ever present.

In the future, perhaps, things

may be

space stations that will eventually be set themselves,

something

among

the find

other things, on the watch for the Earth-grazers,

watch conducted

like the iceberg

the sinking of the Titanic (but

The

The men in up about the Earth may different.

much more

in

northern waters since

difficult of course).

and mountains of space may be painstakingly tagged and numbered. Their changing orbits may be kept under steady watch. Then, a hundred years from now, perhaps, or a thousand, some computer on such a station will sound the alarm: “Collision orbit!” rocks, boulders,

Then in

a counterattack, kept in waiting for all that

motion.

The dangerous

rock would be

time would be

met with an H-bomb

(or,

set

by

more appropriate) designed to trigger off on eolThe rock would glow and vaporize and ehange from a boulder to

that time, something lision.

a

conglomeration of pebbles.

Even if they continued on would merely be treated to shooting

course, the threat a spectacular

would be

lifted.

Earth

(and harmless) shower of

stars.

Until then, however, the Rocks of Damocles remain suspended, and eternity for millions of us may, at

any time, be an hour away.

3

HARMONY

IN

HEAVEN

I

never actually took any courses in astronomy, which

I

regret for, looking

did take which

I

back on

in a while,

which gladdens

I

come

my

and

I

across a little item in

field,

I

number

which

my

1961) which has delighted

me, therefore, recommend

it

is

me

B.

all

in

have been old

astronomy. InMiff-

Let

you without reservation.

me

comments on Kepler’s harmonic law that, in my more thought to it than I had ever done before, and I who

had had

McLaughlin* (Houghton

his

Professor McLaughlin,

If I

delight.

As an example. Professor McLaughlin intrigued

*

astronomy.

in this fashion in several places.

to all of

I

that now, every

something new.

then these items would

Dean

of courses

astronomical reading

have come across a recent text

troduction to Astronomy by lin,

me

would have missed rny moments of

For instance,

a

sacrificed for a bit of

at the bright side,

heart by teaching

formal training in the stuff

now, there were

might cheerfully have

However, one must look once

it

something

is

has died, alas, since this article father of a well-known sciencc-fir*;ion fan of the same name.

so

much

ecstasy,

see first

I

with

devoted

no reason why appeared,

is

the

— ASIMOV ON ASTRONOMY

30 I

should not share the results of that thinking with you. In

upon

your minds:

all

In 1619, the relationship

and

insist

I

it.

might begin,

I

fact,

I

suppose, by answering the question that

What

Kepler’s

is

German

between the

two thousand

for

know

is

in

harmonic law? Well

astronomer, Johannes Kepler, discovered a neat

from the Sun,

relative distances of the planets

their periods of revolution

Now

I

about the Sun.

had

years, philosophers

felt

spaced at such distances that their movements gave

that planets were

sounds that

rise to

harmony (the ‘'music of the spheres”). This was in the manner in which strings of certain different lengths gave

united in heavenly

analogy to

sound that united

forth

in

harmony when simultaneously

pleasing

struck.

For that reason, Kepler’s relationship of distances and periods, which is

usually called, with scientific dullness, “Kepler’s third law” (since he

had

earlier discovered

tary orbits)

two other important generalizations about plane-

much more

also called,

is

romantically, “Kepler’s harmonic

law.”

The law may be

stated thus:

“The

squares of the periods of the

mean

planets are proportional to the cubes of their

distances from the

Sun.”

To

follow up the consequences of

(as slightly as

possible,

I

get slightly mathematical

this, let’s

promise). Let’s begin by considering two

planets of the solar system, planet-i and planet-2. Planet-i

Di from the Sun and planet-2 (“Mean” means “average.”) Their periods

distance

is

Pi^/P2^=Di^/D 2^

at

mean

distance

D2.

of revolution are, respec-

Pi and P2. Then, by Kepler’s harmonic law,

tively,

mean

at

is

we can

(Equation

say that:

1)

not a very complicated equation, but any equation that can be simplified should be simplified, and that’s what I’m going to do next. Tliis

is

Let’s pretend that planet-2

ure

all

is

the Earth and that

periods of revolution in years and

all

we

are going to meas-

distances in astronomical

units (A.U.).

The

period of revolution of the Earth, by definition,

therefore Po

and P2^ both equal

1.

Then,

is

one

year;

too, the astronomical unit

is

defined as the

mean

the Earth

A.U. from the Sun, which means that D2 and D2^ both

equal

1.

is

1

distance of the Earth from the Sun. Consequently,

HARMONY

HEAVEN

IN

3I

Tlie denominators of both fractions in Equation

and disappear. With only one

set of P’s

eliminate subscripts and write Equation

P2 — provided

we remember

and D’s to i

become unity worry about, we can i

simply as follows:

(Equation 2)

to express

P

in years

D

and

in

astronomical

units.

show how this works let’s consider the nine major planets of the solar system, and list for each the period of revolution in years and the distance from the Sun in astronomical units {see Table 3). If, for Just to

each planet, you take the square of the value under P and the cube of Table 3

PLANET

p

D

(period of revolution IN years)

(mean distance IN A.U.)

Mercury

0.241

6

Venus

0.615

0.723

Earth

1.000

1.000

Mars

1.881

1.524

1.86

5.203

00

Jupiter

1

Saturn

29.46

9-54

Uranus

00

Tq

19.18

Neptune

164.8

30.06

Pluto

248.4

39.52

the value under

D you will

find, indeed, that the

two

results are virtually

identical.

Of

course, the period

and distance of any given planet can be

termined separately and

independently by actual

connection between the two, therefore,

However, what

if

we

is

observation.

interesting but

not

de-

The vital.

can’t determine both quantities independently.

Suppose, for instance, you imagined a planet between Mars and Jupiter at a distance of just 4

olution be?

Or

from the Sun.

if

you imagined

What

From Equation

A.U. from the Sun.

2,

would

we

its

What would

its

period of rev-

a far, far distant planet, 6,000

A.U.

period of revolution be?

see that:

P=\/D'^

(Equation 3)

and therefore we can answer the questions

easily.

In the case of the

planet between Mars and Jupiter, the period of revolution would be the

ASIMOV ON ASTRONOMY

32

square root of the cube of four, or just eight years. As for the far distant planet,

comes

its

period would be the square root of the cube of 6,000 and that

to 465,000 years.

You can work

the other

it

way round,

too,

by converting Equation

2 into:

D=>5/PYou can then have

how

find out

(Equation 4)

distant from the

a period of revolution of just

In the former case, you

twenty and

in the latter,

twenty years, of

A.U.

for the second.

how The

far

out can a

Any

planet which

what the plane

its

sider

it

An year

orbit. It

of the square of

first

and about 10,000

case

is

is

is

Alpha Centauri^ which

is

4.3

Sun

as close as 2 light-years to the

Sun than

to

any other

safely in the Sun’s grip

no matter

star

and

let’s

con-

the “farthest reasonable planet.”

astronomical unit

is

equal to about 93,ooo,ocxd miles while a light-

equal to about 5,860,000,000,000 miles. Therefore, one light-

is is

vears.

fun,

closer to the

of

one million

just

to

now, by seeking extremes. For instance, planet be and still be a member of the solar system? little

must therefore be

year

for the

nearest star system to ourselves

light-years away.

must be

the cube root of the square of one million. Tliis

you an answer of 7.35 A.U.

can have a

a planet

must take the cube root

gives

We

Sun

equal to about 63,000 A.U. and our farthest reasonable planet

at a distance of

about 126,000 A.U. From Equation

that the period of the farthest reasonable planet

3, is

then,

we can

is

see

about 45,000,000

years.

Let’s ask next,

how

close a planet can

be to the Sun? Let’s ignore

temperature and gas resistance and suppose that a planet can

Sun

at

its

skimming

equator, just

its

surface.

We can

circle the

call this a

“surface

planet.”

The

distance of a planet from the

center. If

we

Sun

is

always measured center to

consider the surface planet to be of negligible

distance from the

Sun

is

equal to the radius of the Sun, which

miles or 0.00465 A.U. Again using Equation

period of such a body

Next

let’s

find out

is

size,

3,

w'e

is

then

its

432,300

can show that the

0.00031 years or 2.73 hours.

how

fast a planet

miles per second (relative to the Sun).

is

moving, on the average,

To do

so, let’s first figure

in

out

HARMONY how many

seconds

We

orbit.

HEAVEN

IN

make

takes the planet to

it

33

complete turn

a

in its

already have that period in years (P). In each year, there

are about 31,557,000 seconds. Therefore the period of the planet in

seconds

An

31,557,000 P.

is

astronomical unit

have the distance of tance in miles ever,

is,

as

I

the length of the orbit

is

exact circle (which

What we itself.

If

really

we assume

we

the planet in miles,

and that

To

6.2832. If

is

we

its

is

equal to

value of "pi”

its is

multiply that by the distance of

get the length of the planetary orbit in miles,

find the average velocity of a planet in miles per second,

D) by

we must

the duration

period of revolution in seconds (31,557,000 P). This gives us the

We

D/P,

for the

can simplify

Equation

D/VD^

mean

this

orbital velocity of a planet.

by remembering that Pz=>yD‘^, according to

we can write the velocity of a moving planet as 18.5 Since \/D^ is equal to \/D“XD, or D\/D, we can write the so that

3,

velocity of a planet in orbit as equal to 18.5

V

plification, letting

Remember

that

D/D\/D

or, in a final

sim-

represents the distance of a planet from the

Sun

stand for velocity:

V=:i 8.5/\/D

D

(Equation

5)

astronomical units. Eor the Earth the value of

the square root of its

how-

584,000,000 D.

is

value 18.5

in

dis-

the orbit to be an

The

divide the length of the orbit in miles (584,000,000 of

this point,

length

its

distance from the Sun, multiplied by twice "pi.”

3.1416 and twice that

(D), so that the

need at

approximately true) then

is

We

about 93,000,000 miles.

a planet in astronomical units

93,000,000 D.

is

said before,

D

is

also equal to

1.

D

is

equal to

1

and

Therefore, the Earth moves in

orbit at the average rate of 18.5 miles per second.

Since

D

known

is

for the other planets, the

mean

orbital velocity

can be calculated without trouble by taking the square root of dividing

it

into 18.5.

Nowadays,

Mach

1

is

The

velocity

is

result

is

Table

At 0°

often spoken of in

C

along at

Mach

Mach

Our

is

1,090 feet per second,

fastest airplanes are

and more, while an astronaut

in orbit

now moving

moves

at

about

25 with respect to the Earth.

Compare at a

2

"Mach numbers,” where air, Mach 2 to twice that

the speed of sound

or just about 0.2 miles per second.

and

4.

equivalent to the speed of sound in

speed, and so on.

D

this

mere Mach

with Pluto, which moves (with respect to the Sun) 14.5,

only half the velocity of an astronaut.

The Earth

ASIMOV ON ASTRONOMY

34

Table 4

MEAN

on the other hand

Mach

a zippy

But

let’s try

The

ORBITAL VELOCITY (miles per second)

Mercury

29.8

Venus

21.7

Earth

18.5

Mars

15.0

Jupiter

8.2

Saturn

6.0

Uranus

4.2

Neptune

3-4

Pluto

2.9

moving

is

Mach

at a respectable

our extremes again.

farthest reasonable planet, at 126,000 A.U.,

would have an

Mach

about 0.052 miles per second, or about

rather impressive that even at a distance of

is still

93 and Mercury at

149.

bital velocity of just It’s

PLANET

two

or-

0.26.

light-years, the

Sun

capable of lashing a planet into traveling at one-quarter the speed

of sound.

As

0.00465 A.U.,

for the surface planet at a distance of

velocity

must be 271 miles per second, or Mach

1,355.

Mach

Mach

orbital

(Incidentally,

the fastest conceivable velocity, that of light in a vacuum,

about

its

is

equal to

who talks casually about Mach 1,000,000 and you’ll

930,000, so watch out for anyone

1,000,000. Bet

him you

can’t reach

win.)

Actuallv, a planet orbits about the

Sun not

with the Sun at one focus (Kepler’s

first

an

ellipse

you imagine

a line

in a circle

law).

If

but

in

connecting the Sun and the planet (a “radius vector”), that line would

sweep out equal areas

When

the planet

sweep out angle.

is

in

the planet

it is

must move through far

Kepler’s second law.) is

short and, to

a comparatively large

from the Sun, the radius vector

and, to sweep out the same area, needs to gle.

is

close to the Sun, the radius vector

a given area,

When

equal times. (Tliis

move through

is

long

a smaller an-

HARMONY

HEAVEN

IN

35

Thus, Kepler’s second law describes the manner orbital velocity speeds

recedes from

it.

I

up

would

as

it

The

orbit.

a planet effect

like

to point out

from the Sun. velocity,

come

It

a planet’s

down

one consequence of

as

it

this

detail.

suddenly increasing

upon

which

approaches the Sun and slows

without going into mathematical

Imagine

in

its

some point

velocity at

in its

would be analogous to that of throwing it away would move away from the Sun at a steadily decreasing it

to a halt,

and then

start falling

toward the Sun again.

1 his resembles the situation where one throw's a stone into the air here on Earth, but since the planet is also revolving about the Sun, the effect

not a simple up-and-down motion, as

is

in the case of the

it is

stone.

Instead, the planet revolves as velocity decreasing until

it

it

recedes from the Sun,

reaches a point that

is

precisely

its

orbital

on the other

Sun from the point at w'hich its velocity had suddenly increased. At this point on the other side of the Sun, its distance from the Sun has increased to a maximum (aphelion), and its orbital velocity

side of the

has slow'ed to a

As the

minimum.

planet' continues past the aphelion,

Sun again and

its

orbital velocity increases

to the place at wdiich at that point in

and

its

The

its

it

begins to approach the

once more.

When

it

returns

had suddenly increased its velocity, it w'ould be new orbit which was nearest the Sun (perihelion) it

orbital velocity

would then be

at a

maximum.

greater the velocity at a given perihelion distance, the

more

dis-

tant the aphelion and the

more elongated the elliptical orbit. The elongreater and greater rate w'ith equal increments of

gation increases at a

speed because as the aphelion recedes, the strength of the Sun’s gravity weakens and it can do less and less to prevent a still further recession. Eventually, at

some

particular velocity at a given perihelion distance,

the ellipse elongates to infinity

— that

is,

it

becomes

a parabola.

The

planet continues along the parabolic orbit, receding from the Sun forever and never returning. This velocity is the “escape velocity” and it

can be determined for any given planet by multiplying the

Table 4 by the square root of two; that shown in Table 5.

velocity in sult

is

Tims,

if

the Earth, for any reason, ever

second or more,

it

would leave the

don’t lose sleep over

another

star,

this,

lliere

is

is,

moved

solar system

mean orbital by 1.414. The re-

at 26.2 forever.

miles per

(However,

nothing, short of the invasion of

that can bring this about.)

ASIMOV ON ASTRONOMY

36

Escape velocity

would be 0.073 the surface planet would be 385 miles

for the farthest reasonable planet

miles per second while that for

per second.

Table 5

PLANET

ESCAPE VELOCITY (miles per second)

Mercury

42.1

Venus

30.7

Earth

26.2

Mars

21.2

Jupiter

1

1.6

Saturn

8.5

Uranus

5-9

Neptune

4.8

Pluto

4

‘

Newton used Kepler’s three laws as a guide in the working out of his own theory of gravitation. Once gravitation was worked out, Newton showed that Kepler’s three laws could be deduced from it. In Isaac

fact,

he showed that Kepler’s harmonic law

Equation

1)

as originally stated

was only an approximation. In order to make

exact, the masses of the

count. Equation

1

Sun and the planets had

would have to be written

in this

(M-l-mi)Pi2/(M-fm2)P2^=E)i^/D2^

it

{see really

to be taken into ac-

way: (Equation 6)

where, as before. Pi and P2 are the periods of revolution of planet-i and planet-2, Di and D2 are their respective distances, and where the

new symbols mi and m2

are their respective masses.

The symbol

M

represents the mass of the Sun.

As

it

happens, the mass of the Sun

is

mass of any of the planets. Even the

overwhelmingly greater than the largest planet, Jupiter, has only

M

and nii or of Yiooo the mass of the Sun. Consequently, the sum of and m2 can be taken, without significant inaccuracy, to be equal to

M M

itself.

Equation 6 can therefore be written

MP12/MP2—D1VD2"

as follows:

(Equation 7)

The M’s cancel and we have Equation 1. Of course, you may decide that since Newton’s out to be stick

just

correct

form works

about exactly that of Kepler’s approximate form, why not

with Kepler,

who

is

simpler?

HARMONY

IN

HEAVEN

37

Ah, but Newton’s form can be applied more broadly.

had been discovered nine years before Kepler had harmonic law. Kepler had worked out that law entirely

Jupiter’s satellites

announced

his

from the planets, yet when he studied

found

it

he

Jupiter’s satellite system,

applied to that, too.

Newton was

able to

two or more

different svstems at once.

show from his theory of gravitation that all three of Kepler’s laws would apply to any system of bodies moving about some central body and his form of the harmonic law could be applied to

Suppose, for instance, that planet-i circling Sun-2.

You can

is

circling Sun-i

Mi and AE

is

say that:

(Mi-fnii)Pr/(M2+m2)P2“=Di''^/n2-'^

where

and planet-2

(Equation 8)

and Sun-2, where mj.

are the masses of Sun-i

Pi,

and Di are the mass, period, and distance of planet-i, and where m2, and D2 are the mass, period, and distance of

P2,

Now the

much

simplify that rather formidable assemblage of symbols. In

let’s

first

place,

we can

take

eliminate nii and

it’s

it

for granted that the planet

Sun that

smaller than the

not always true but

its

Secondly,

will

as the

e will 1.

Also

we

own Sun equal to

will

all

write Equation 8 as:

consider

it

to

(Equation 9)

be the planet-2/Sun-2 system.

all

Suns

in

That means that M2, the mass of the Sun, Equation 9 becomes (dropping all subscripts)

where the symbols

1.

terms of the mass of our

1.

MP-r=rD^

refer to the system other

(The Earth can

serve as a central

the

than the Earth/Sun system.

we chose

body

the Earth

body around which smaller

bodies, satellites, can revolve.) Suppose, further, that

culate the period of revolution of a

is

(Equation 10)

Suppose, for instance, that for the other Sun, itself.

We

periods of revolution in years so that P2^ will equal

measure the mass of

taken as 1.

we can

distances in astronomical units so that D2^ will equal

measure

is

take the situation of the Earth revolving about the

norm and all

(Tliis

true in the solar system.) In other words,

m2 and

let’s

measure

always so

is

mass can be neglected.

MiPi“/M2P2^=Di^/D 2^

Sun

planet-2.

circling the

we wanted

to cal-

Earth at a mean

photo: the bettmann archive

NEWTON

ISAAC

In the opinion of many, and in mine, too, Isaac greatest

scientist

who

ever lived.

He

Christmas Day, 16^2, prematurely, and

would

live.

After he

managed

to

was born at first

hang on

to

it

life,

in

Newton was

the

Lincolnshire,

on

was not thought he it

was not thought

he would amount to anything, for he showed no signs of any great brilliance as a boy.

was only by a stroke of fortune that he was sent to college, since an uncle, who was himself a college man, seemed to detect something unusual in the young man. It

And

then

Newton

flowered.

After graduating from Cambridge in

166^ without any particular distinction, he retired to his mothers farm in 1666 to escape the plague which was then devastating London

and

in

that year he

experienced the most remarkable series of

spirations in his history of science.

in-

HARMONY distance of 237,000 miles. Since ing, let us rewrite

Equation 10

IN

it is

M

value of

D

is

39

a period of revolution

we

are seek-

The

value of

as:

P=\/D''^/M

The

HEAVEN

(Equation 11)

equal to 237,000 miles or 0.00255 A.U.

equal to the mass of the Earth expressed in Sun-masses.

is

Earth’s mass

The

^32,500 of fke Sun or 0.000003 Sun-masses. Substituting these values into Equation 11, we find that P, the period of revolution, comes out to 0.0745 years, or 27.3 days. It

is

Moon

happens that the

from the Earth, and the stars) rected by

at

is

an average distance of 237,000 miles

happens that

it

its

period of revolution (relative to

27.3 days. Consequently, Kepler’s

is

Newton,

much

applies as

to the

harmonic law,

as cor-

Earth-Moon system

as

to

the Sun-planet system.

Moon

Furthermore, since the distance of the the

Moon’s period

from the Earth and

known; and since the distance

of revolution are both

of the Earth

from the Sun and the Earth’s period of revolution are also both known; then if the mass of the Earth is known, the mass of the Sun can be calculated from Equation q. Or, if the mass of the Sun is

known, that

The mass

of the Earth can be calculated. of the Earth was

worked out by

a

method independent

of

the harmonic law^ in 1798. After that, the mass of any astronomical

body which

known

is

itself at a

distance and in

He watched

an apple

was in the sky and, began the

in

known distance, and has a body circling it a known period (all these quantities being

fall to

the ground one evening

wondering why the

Moon

when

Moon

the

did not likewise

fall,

thought which ended with the theory of universal gravitation which, for the first time, served to explain the workings of the Universe with something approaching adequacy. That same year train of

he performed experiments on light which showed that white light was composed of various colors that could be spread out into a rainbow by a prism. He also worked out the binomial theorem and began the train of thought that ended with the invention of the calculus.

Two

years later, he invented the reflecting telescope

which

in

some

was a great improvement over the refracting telescope used by Galileo. He spent all his long life working out the consequences of

respects

his youthful inspiration,

venerated in his

own

and no

scientist before his

lifetime; nor

coming of Einstein. He died

was there

in iy2y.

time was ever so

to he such

another

till

the

at a

easy

ASIMOV ON ASTRONOMY

40

to determine within the this reason, the

with

all

masses of Mars, Jupiter, Saturn, Uranus, and Neptune,

satellites, are

The masses

plar system) can quickly be determined. For

known with

considerable accuracy.

Mercury, Venus, and

of

Pluto,

which lack known

can only be worked out by more indirect means and are

satellites,

known with considerably less accuracy. (It seems unreasonable that the mass of Venus is less well known than that of Neptune when the latter * is a hundred times farther from us, but now you see why.) *

Since this was written, the effect of Venus on artificial probes passing near has given astronomers the chance to calculate its mass with considerable precision.

it

ASTRONAUTS While space

by no means the only subject of concern to science-fiction writers, it has certainly been involved in a majority of the s.f‘ stories written. Nothing has been so exotic, and yet has seemed travel

is

so inevitable, as reaching

seemed

beyond Earth

and

so intrinsically adventurous

Stories of flights to the

Moon

to other worlds.

Nothing has

exciting.

were written in ancient times, though,

of course, no one then understood that there was anything but air be-

tween ourselves and the Moon.

It

was only

that science-fiction writers really

turies

ficulties in

the way

—

that the

Moon

in the last couple of cen-

came

to understand the

dif-

could not be reached simply by

hitching a few dozen geese to a chariot.

One

of the

most inescapable

facts

about the

Moon

was that

it

had no atmosphere. Any daring astronaut who wished to explore the Moon had to see to it that he was entirely enclosed in an airtight suit, with

air

on the

inside. Science-fiction stories

throughout the twentieth

century dealt with space suits whenever they dealt with flights to the Moon or beyond. The space suit was as well known an adjunct of the space traveler as the horse was of the cowboy, and there were many

hundreds of illustrations in the science-fiction magazines of the ig^os and 1 940s showing men in space suits.

And

then came

reality in the ig6os.

In

many

ways, what turned out

be true was vastly different and more complex than the dreams of the science-fiction writer but not the space suits. In the illustration we see L. Gordon Cooper, Jr., in training for to

—

eventual spaceflight, and the space suit he that existed

and more.

on the pages of

science-fiction

is

wearing

is

the space suit

magazines thirty years ago

photo: national aeronautics and space administration

ASIMOV ON ASTRONOMY

42

Tlie masses of the various satellites (except for the is

a special case)

monic law

Moon

itself,

which

The hardrowned by the much

are hard to determine for similar reasons.

can’t be used because their masses are

larger

mass of their primary, and no other method of mass determina-

tion

as

is

convenient or as accurate.

Periods, distances,

and

orbital velocities of satellites, real or imagined,

can be worked out for any planet is

known, exactly

(real or

imaginary) for which the mass

can be worked out for the planets

as these quantities

with respect to the Sun.

Without going 5,

on the surface

into arithmetical details,

satellite for

I

in the case of Pluto that

If

this,

will leave

out. In

it

its

6,

you

use must be

Table

place, for

made

so uncertain

comparison

is

light

moved along

and

its

minimum

will see that the period of a

can be long for either of two reasons. As

the planet is

I

in

include the Sun.

you consider Table

satellite

For

and radius of the planet and these values are

of both mass

I

some data

each planet; the theoretical situation where

a satellite just skims the planetary equator.

purposes,

will list

gravitational force

is

Mercury,

in the case of

weak that the

so

satellite

slowly and takes several hours to negotiate even the

small length of the planetary equator.

On

the other hand, as in the case of the Sun or of Jupiter, the grav-

and the surface satellite whizzes along at high speed, but the central body is so large that even at high speed, several itational force

is

great

hours must elapse before the circuit

The

is

period of the surface satellite

completed. is

shortest

when

the planet packs

Table 6

planet

SURFACE SATELLITE PERIOD (hours)

PERIOD (minutes)

ORBITAL VELOCITY (miles per SEC.)

Mercury

3-13

Venus

1.44

861/2

4.58

Earth

1.41

8 4 1/2

4-95

Mars

1.65

Jupiter

188

1.87

2.27

2.96

99 177

26.4

Saturn

4.23

T34

15.6

Uranus

2.62

157

00

137

2-73

165

Neptune Sun

9.85 11.2

271

HARMONY

IN

HEAVEN

43

mucli mass as possible into as small a volume as possible. In other words, the greater the density of the eentral body, the shorter the as

period of the surfaee

satellite.

bodies listed in Table

6, it is

Since Saturn

the least dense of the

is

not surprising that

its

surface satellite has

the longest period.

As

it

happens, of

planet. Earth,

is

all

the sizable bodies of the solar system, our

the densest.

The

period of

its

surface satellite

is

own

there-

fore the shortest.

An

astronaut in orbit about the Earth, a hundred miles or so above

the surface,

is

virtually a surface satellite

the earth in just under ninety minutes.

body

in the solar

How’s that company?

and he completes

An

his circuit of

astronaut of no other sizable

system could perform so speedy a circumnavigation.

for a

system-wide distinction for Gagarin, Glenn, and

THE TROJAN HEARSE

The

very

first

ever

I

had published (never mind how long ago

concerned a spaceship that had come to

that was)

zone. In

story

it,

I

had

a character

comment on

grief in the asteroid

the foolhardiness of the

captain in not moving out of the plane of the ecliptic

the earth’s orbit, which

close to that in

is

ponents of the solar system move)

zone and avoid almost certain TTie picture

I

had

in

mind

in order to

fiction writers

exists,

at that

and

*

day,

Oh,

mind

of almost

all

is

the

science-

one imagines, can

easily

of rubble to the next in search of valuable minerals.

\^acationers can pitch their tents tioners

com-

go over or under the

was with pebbles. This

readers. Individual miners,

hop from one piece

virtually all the

time was of an asteroidal zone as

believe, in the

I

the plane of

collision.

thickly strewn with asteroids as a beach

same picture that

which

(i.e.

on one world and wave

at the vaca-

on neighboring worlds. And so on.

well,

why be

tells

1972) Off Vesta” and

it

all

coy.

my

My

recently published

secrets

appeared

anyway.

in the

My

first

book The Early Asimov (Doublepublished story was “Marooned

March 1939 Amazing

Stories.

THE TROJAN HEARSE

How is

just

true

is

The number

this picture?

about 1,800, but, of course,

have seen estimates that place the

Most

tlie

total

of the asteroids are to be

45

of asteroids so far discovered

number

actual

number

is

far higher.

I

at 100,000.

found between the

orbits of

Mars

and Jupiter and within 30° of the plane of the ecliptic. Now, the total volume of space betw een those orbits and within that tilt to the ecliptic

now — mumble, mumble, mumble) something like 000,000,000,000,000,000,000 cubic miles. If wc allow' for a total is

(let’s

see

of 200,000 asteroids, to be

on the

safe side, then there

is

200,000,-

quantity

one asteroid

for every 1,000,000,000,000,000,000,000 cubic miles.

This means that the average distance bctw'ccn asteroids is 10,000,000 miles. Perhaps we can cut that down to 1,000,000 miles for the more densely populated regions. Considering that the size of most asteroids is

under a mile

will in all will

you can see that from any one asteroid you probability see no others with the naked eye. The vacationer in diameter,

be lonely and the miner

heck of

will hav'e a

a

problem reaching

the next bit of rubble. In fact, astronauts of the future will in

all

probability routinely pass

through the asteroid zone on their way to the outer planets and never see a thing. Far from being a dreaded sign of danger, the occasional cry of “asteroid in view” should bring

all

the tourists rushing to the

portholes.*

Actually,

we mustn’t

think of the asteroidal zone as evenly strewn

with asteroids. There are such things as clusters and there are also bands within the zone that are virtually empty of matter.

The and

its

responsibility for

both situations

rests

with the planet Jupiter

strong pull on the other components of the solar system.

As an asteroid

in the course of its

to Jupiter (in the course of asteroid reaches a

which an asteroid also at a

its

its

closest

approach

motion), the pull of Jupiter on that

maximum. Under is

motion makes

pulled out of

this

its

maximum

normal orbit

pull, the extent (is

“perturbed”)

by is

maximum.

Under ordinary circumstances, however,

this

approach of the asteroid

to Jupiter occurs at different points in their orbits. Because of the rather elliptical

and

tilted orbits of

most

asteroids, the closest

fore takes place at varying angles, so that * In

approach there-

sometimes the asteroid

1973, Pioneer 10 passed tlirougli the asteroid zone on the way to Jupiter. had no trouble and found fewer particles than had been expected.

is

It

ASIMOV ON ASTRONOMY

46

pulled forward at the time of

closest

its

approach and sometimes back-

ward, sometimes downward and sometimes upward.

The

net result

is

that the effect of the perturbations cancels out and that, in the long

move

run, the asteroids will

manent average-orbit. But suppose an asteroid about 300,000,000 miles?

about

circled

compared with

six years as

about the sun at

would then have

It

twelve years

later, Jupiter

asteroid just

two

circuits.

a

mean

distance of

a period of revolution of

Jupiter’s period of twelve years.

the asteroid were close to Jupiter at a given

If

about some per-

orbits that oscillate

in

would have made

moment

just

one

Both would occupy the same

of time; then

circuit

and the

relative positions

would repeat every twelve years. Every other revolution, the would find itself yanked in the same direction. The perturba-

again. This asteroid

tions, instead of canceling out,

become

its

close approach,

the asteroid was con-

it

would then no longer match

longer. Its period of revolution

Jupiter’s,

If

would gradually be moved more distant from the sun, and its year would

stantly pulled fonvard at

into an orbit slightly

would build up.

and the perturbations would cease building up.

on the other hand, the asteroid were pulled backward each time, would gradually be forced into an orbit that was closer to the sun. Its If,

it

become

year would

shorter;

would no longer match

it

Jupiter’s;

and

again the perturbations would cease building up.

The

general effect

is

that no asteroid

where the period of revolution

it

that portion of the zone

left in

just half that of Jupiter.

is

originally in that portion of the

is

Any

asteroid

zone moves either outward or inward;

does not stay put. Idle

same

would have repeat

had

its

is

true of that region of the zone in

a period of revolution equal to four years, for then

it

would

position with respect to Jupiter every three revolutions.

a period of revolution equal to 4.8 years,

with respect to Jupiter every

The

which an asteroid

five revolutions,

it

would repeat

its

If it

position

and so on.

regions of the asteroid zone which have thus been swept clear of

asteroids

by Jupiter are known

as

“Kirkwood’s gaps.” They are so named

because the American astronomer Daniel Kirkwood called attention to these gaps in 1876

An

and explained them properly.

exactly analogous situation

Saturn’s rings— which

is

also to be

found

in

the case of

why we

speak of “rings” rather than

discovered by the

Dutch astronomer Christian

is,

in fact,

ring.

The

rings

were

first

THE TROJAN HEARSE Huyghens

seemed

1655. ^ ®

in

Saturn but touching

47

a simple ring of light circling

nowhere. In 1675, however, the Italian-born French astronomer Giovanni Domenico Cassini noticed that a dark gap divided the ring into a thick and bright inner portion and a thinner

and some\\’hat

less

it

bright outer portion. Ibis gap, which

is

3,000 miles

wide, has been called the “Cassini division" ever since. In 1850, a third, quite ^^as spied

the

dim ring, closer to Saturn than are the others, by the American astronomer George Phillips Bond. It is called

crape ring" because

it is

so dim.

The

crape ring

is

separated from

the inner bright ring by a gap of 1,000 miles. In 1859, the Scottisli mathematician Clerk

Maxwell showed that from

gravitational considerations, the rings could not be

one piece of material

but had to consist of numerous light-reflecting fragments that seemed one piece only because of their distance. The fragments of the crape

much more sparsely distributed than those of the bright rings, which is why the crape ring seems so dim. Tliis theoretical prediction was verified when the period of revolution of the rings was measured ring are

and found to vary from point to point. After all, if were one piece, the period of revolution would be everywhere

spectroscopically

the rings the same.

The innermost

portion of the crape ring

Saturn’s surface. Tliose particles

mere 7,000 miles above move most rapidly and have the short-

est distance to cover. Tliey revolve

As one moves outward

and must cover greater tion

is

a

about Saturn

in

about 3I4 hours.

in the rings, the particles

move more

slowly

which means that the period of revolu-

distances,

mounts. Particles at the outermost edge of the rings have a period

of revolution of about 13I/2 hours. If particles

were to be found

Saturn in a period of a

little

in that region of the rings,

in Cassini’s division,

they would circle

over 11 hours. But particles are not found

which

is

why

it

stands out black against the

brightness on either side.

Why? Well, outside the ring system, Saturn possesses a family of nine satellites, each of which has a gravitational field that produces perturbation in the motion of the particles of the rings. Saturn’s innermost satellite,

* In late

Mimas,* which

lies

only 35,000 miles beyond the outer edge

1967, some four years after this article first appeared, another satellite of Saturn was discovered which was even closer to the planet than Mimas. Tliis new satellite, Janus, has a period of revolution of 18 hours. Its existence does not affect the line of argument here.

ASIMOV ON ASTRONOMY

48

of the rings, has a period of revolution of 22V2 hours. Enceladus, the

second

has a period of 33 hours; and Tethys, the third a period of 44 hours.

Any

satellite,

particles in Cassini’s division

would have

satellite,

a period of revolution

Mimas, a third that of Enceladus, and a fourth that of Tethys. No wonder the region is swept clean. (Actually, the satellites are small bodies and their perturbing effect is insignificant on anything half that of

larger than the gravel that

themselves would by

satellites

too-closely

As

makes up the

matching

rings. If this

now have been

were not

so,

forced out of their

the

own

orbits.)

gap between the crape ring and the inner bright one, particles within it would circle Saturn in a little over seven hours, onethird the period of revolution of Mimas and one-sixth that of Tethys. There are other smaller divisions in the ring system which can be exfor the

plained in similar fashion.

must stop here and point out a curiosity that I have never seen mentioned. Books on astronomy always point out that Phobos, the inner satellite of Mars, revolves about Mars in less time than it takes I

SATURN It

is

usually stated that Saturn

heavens and

judgment it

to

is

if

you look

hard to argue with. There

systems, but one

ever lay eyes

The Sun in

is

may be

entitled to

is

will find that

such a

absolutely nothing else like

other ringed planets, in other

doubt whether any

man

will

on them.

rings don’t always look like this.

As Saturn moves around the

a twenty -nine-year cycle, the rings are seen at a progressively

changing angle. In to

the most beautiful sight in the

photograph you

at this

be seen anywhere. There

stellar

is

our sight,

photograph, we see them at the greatest angle and this sort of view only reaches its extreme every 14I/2 this

years.

In between

we

see

the rings

more and more

nearly edge-on

every 14 1/2 years {in between greatest-angle views) actly edge-on. sibly only a

So thin are the

rings

{at

we

see

most a few miles

them

and ex-

thick, pos-

few yards) that they then disappear completely from view

even in the best telescopes.

photo:

It

so happens that Galileo,

when he

first

mount wilson and palomar observatories

looked at Saturn with his

primitive telescope, observed the planet at a time fairly close to

edge-on.

He

couldn’t

make them out

looked at another time, the rings were gone.

had thought that Saturn was

a triple body,

who,

all,

is

the

Roman

the rings were

clearly.

He was

When

its

equivalent

he

frustrated for he

two small orbs on either

of a large one. ''Has Saturn indeed swallowed

Saturn, after

when

side

children?” he asked. of

the

in the myths, did swallow his childreri to keep

up and overthrowing him. This has caused some

Greek Kronos,

any from growing

wonder whether the ancients gave Saturn the name because they had somehow observed the disappearing rings. The answer is: No way. That part of it is pure coincidence. Goincidences do happen after all. to

50

ASIMOV ON ASTRONOMY

photo; the bettmann archive

THE TROJAN HEARSE

51

GIOVANNI DOMENICO CASSINI Cassini was born near Nice on June 8, 162^. He made his first reputation as an astronomer in Italy, where he taught astronomy at

the University of Bologna. In i66~j he was invited to Paris by Louis

XIV, then

the most powerful

rest of his life in

While still and Alars by

monarch

in

Europe, and he spent the

France.

he determined the periods of rotation of Jupiter carefully following unevennesses on their surface, as they in Italy,

disappeared round the eastern edge and reappeared on the western.

He

improved astronomical knowledge of Saturn. Huygens had discovered the ring about Saturn in 16^6, but Cassini noted that the ring was actually a double one and that there w’as a dark gap between also

a larger and brighter inner portion and a smaller

The gap

portion.

Cassini also

is

and dimmer outer

called ''Cassini’s division” to this day.

enlarged the

matter of Saturn’s

satellite

accompani-

ment. Huygens had discovered Saturn’s largest satellite. Titan, and with that had brought the total number of satellites {including Jupiter’s four large satellites and our large

known

Moon)

to six.

'made an even dozen bodies that completed the list.

planets,

Huygens

felt

This, plus the six

circling

Sun and

the

showed there was more than numerical symmetry the Solar system when he discovered four more satellites of Saturn the i6yos and 1680s.

Cassini, however, to in

Cassini’s

most remarkable

bit of work,

however, was his determina-

Mars with reasonable

tion of the distance of

accuracy. It was the

time the distance of any heavenly body other than the been determined, and the determination, in i6j2, gave first

inkling of the scale of the Solar system.

Mars

to rotate about

axis.

its

is

is

is

Moon had men

their

died in iyi2.

Mars’ period of rotation

while Phobos’ period of revolution

go on to say that Phobos

He

first

only yVi hours.

is

24V2 hours

The books then

the only satellite in the system of which this

true.

Well, that size.

is

correct

However, each

they are counted

Saturn about

its

we

consider natural satellites of appreciable

particle in Saturn’s rings

in,

axis

if

the situation changes. is

is

really a satellite,

The

and

if

period of rotation of

10V2 hours, and every particle in the crape ring

)

ASIMOV ON ASTRONOMY

52

and

in the inner bright ring revolves

about Saturn

from there being only one there are uncounted millions pf them.

that. Therefore, far

type,

in less

satellite of

time than

the Phobos

In addition, almost every artificial satellite sent up by the United

and the Soviet Union revolves about the earth four hours. They, too, are of the Phobos type. States

in less

than twenty-

Gravitational perturbations act not only to sweep regions clear of particles

where

To

but also to collect them.

The most remarkable

particles are collected not in a zone,

explain that,

I

have to begin

will

but actually

case

is

one

in a point.

at the beginning.

Newton’s law

of universal gravitation

lem

(at least in

was a complete solution of the ‘‘two-body probclassical physics, where the modern innovations of

and quantum theory- are ignored). That is, if tile universe cononly two bodies, and the position and motion of each are know n.

relativity

tains

JAMES CLERK

MAXWELL

Maxwell was born in Edinburgh, Scotland, November 31, 18^1. He was a mathematical prodigy as a young boy and his brilliance was easily mistaken for foolishness so that he was nicknamed *‘Daffy” by his classmates. {The reverse does not hold true, I am sorry to say. The mere fact that a boy is considered daffy by his friends is no sure sign he

is brilliant.

Usually he

is

daffy

.

Maxwells major contributions were

in physics.

kinetic theory of gases, showing, in the

bouncing

off

out the

1860s, that the behavior of

gases could be quite adequately explained

a vast collection of tiny particles

He worked

moving

if

they were assumed to be

in

random

directions

and

each other {and the walls of the container) with perfect

elasticity.

He

also

produced a

set

of simple

equations that adequately de-

scribed the behavior of electrically charged particles fields

and showed that

phenomena but were also

showed that a

electricity

and of magnetic

and magnetism were not independent

aspects of a unified “electromagnetic field.”

He

fluctuating electromagnetic field produced radiation

that traveled at the speed of a light traveled at that speed,

it

little

over 186,000 miles an hour. Since

was shown

of “electromagnetic radiation.”

to

be the best-known example

photo: the bettmann archive Ilis

greatest

contribution

to

astronomy came

in

i

8 ^j,

when he

demonstrated that the rings of Saturn could not be solid objects.

If

they were, grayntational forces, of different strength in different places,

would break them up. They had to consist of swarms of small which seemed solid only because we saw them from a great

particles

distance.

In later generations, spectroscopic evidence has amply demonstrated the truth of

Maxwells contention. Maxwell died

in

1879

.

ASIMOV ON ASTRONOMY

54

then the law of gravitation sitions of the

is

sufficient to predict the exact relative po-

two bodies through

all

of time, past

Hov^’ever, the universe doesn’t contain only

uncounted

them

all

trillions,

d’he next step then

into account by solving

three bodies in the universe, with will their relative positions

be

is

and

future.

two bodies.

to build

It

contains

up toward taking

the “three-body problem.”

known

position

Given

and motion, what

at all given times?

photo: the bettmann archive

THE TROJAN HEARSE And

right there, astronomers are stymied.

55

There

tion to such a problem. For that reason there

is

is

no general

solu-

no use going on to

the ‘'octillion-body problem” represented by the actual universe.

The

Fortunately, astronomers arc not halted in a practical sense.

theory

may be

that

was necessary to calculate the orbit of the earth about the sun so

it

lacking but they can get along. Suppose, for instance,

that the relative positions could be calculated for the next million years.

CHRISTIAN HUYGENS Huygens

He

born in the Hague, Netherlands on April 14, 1629. was the son of a Dutch government official and in early life

seemed

ivds

heading for a career

to be

in

mathematics. While he was

still

hook on probability, the

first

in his twenties, for instance, he published a

mathematics hook

to appear that

was devoted entirely to that subject.

At the time, though, he was helping his brother devise an improved telescope and hit upon a new and better method for grinding lenses. {He had the help here of the Dutch-Jewish philosopher, Benedict Spinoza.)

He

His most

began to use

his

improved lenses

in telescopes.

startling astronomical discovery involved Saturn. In 1610,

Galileo with his

first

primitive telescope had noted something peculiar

about the shape of Saturn but couldn’t quite make out what

it

was.

The planet was too distant for him to see it clearly. In 16^6, Huygens made out the fact that it was surrounded by a thin ring that nowhere touched

it.

That same year he also discovered a satellite circling Saturn, the first body found to be circling the planet. He named it Titan, because Saturn {Kronos, to the Greeks) had been the leader of the Titans. Titan also means '‘giant” and the name proved appropriate since it M’as a giant satellite, larger

two of the four

Huygens

than our Moon, and larger than at

satellites of Jupiter

also discovered the

nous dust and

gas,

and was the

least

discovered by Galileo.

Orion Nebula, a huge cloud of lumifirst

to

make

a reasonable estimate as

to the distance of the stars.

Nor was he behindhand

in physics.

form of wave motion. This was that

it

He

thought of

in opposition to Isaac

light as being a

Newton

s belief

consisted of a stream of particles, hi the eighteenth century,

ton’s great reputation

swung matters

his way; in the nineteenth, science

inclined in the direction of Huygens; in the twentieth,

both were, to an extent,

right.

New-

Huygens died

in i6g^.

it

turned out

ASIMOV ON ASTRONOMY

56

the sun and the earth were

If

a trivial

one to

But the

solve.

that existed, the problem

all

and of Mars and the other planets, and, the

moon must be

gravity of the

for

would be

considered,

complete exactness, even

stars.

Fortunately, the sun vicinity

and so much

is

so

much

any other

closer than

drowns out

gravitational field

bigger than any other

in the

massive body, that

really

The

others.

all

body

orbit obtained for the

earth by calculating a simple two-body situation

almost

is

You

right.

then calculate the minor effect of the closer bodies and rections.

The

closer

its

you want to pinpoint the exact

orbit,

make the more

corcor-

you must make, covering smaller and smaller perturbations. The principle is clear but the practice can become tedious, to be sure. The equation that gives the motion of the moon with reasonable rections

exactness covers

many hundreds

predict the time

and position of

of pages.

But that

is

good enough to

with great correctness for long

eclipses

periods of time into the future. Nevertheless, astronomers are not satisfied.

It is all

very well to

out orbits on the basis of successive approximations, but

and elegant

it

would be

to prepare

The man who most

closely

beautiful

an equation that would interrelate

bodies in a simple and grand way.

all

how

work

Or

three bodies, anyway.

approached

this

ideal

was the French

astronomer Joseph Louis Lagrange. In 1772, he actually found certain very specialized cases in which the three-body problem could be solved. Imagine two bodies in space with the mass of body A at least 25.8 times that of body B, so that

B can be

said to revolve about a virtually

motionless A, as Jupiter, for instance, revolves about the sun. Next imagine a third body, C, of comparatively insignificant mass, so that it

A

does not disturb the gravitational relationship of

found that

was possible to place body

it

ship to bodies

A

and B, so that

C

C

at certain points in relation-

would revolve about

with B. In that way the relative positions of

known

all

A

in perfect step

three bodies

would be

for all times.

There are

five

points at which

body

C

can be placed; and they are,

naturally enough, called “Lagrangian points.’'

and

and B. Lagrange

La, are

small body

on the

C

line

A and of A in

between

but on the other side

conneeting B.

the

A

Three of them,

and B. Tlie

first

Both Lo and La first

case

lie

Li,

Lo

point, Li, places

on the

and on the other

line also,

side of

B

in the next.

These three Lagrangian points are not important.

If

any body located

THE TROJAN HEARSE at

one of those points, moves ever so

slightly of? position

some body outside the system, the

perturbation of

A

the gravitational

fields of

point. It

long stick balanced on edge.

is

so slightly,

like a it

tips

However, the

A

bodies

and B

is

more and more and

final

C

resulting effect of still

Once

farther off the

that stick tips ever

falls.

equilateral triangles with

As B revolves about A, L4

is

stant angle of 60°, while

moves behind

last

throw

to

due to the

Lagrangian points are not on the line connecting

and B. Instead, they form

These

57

L.^,

two points are

B

the point that moves before

stable. If

it

A

and B.

at a con-

at a constant 60°.

an object at either point moves

slightly off position,

through outside perturbations, the effect of the

gravitational fields of

A

and B

them

to bring

is

back. In this way, ob-

L4 and L5 oscillate about the true Lagrangian point, like a long stick balanced at the end of a finger which adjusts its position conjects at

stantly to prevent falling.

Of

course,

if

the stick tips too far out of vertical

the balancing efforts of the finger.

And

from the Lagrangian point

lost.

it

will

At the time Lagrange worked at Lagrangian points were in 1906, a

German

be

this out,

no examples

known anywhere

astronomer,

Max Wolf,

An

analysis of

its

fact, it

orbit

was

as far

away

However,

discovered an asteroid, which Iliad. It

from the sun

showed that

far

of objects located

in the universe.

he named Achilles after the Greek hero of the out for an asteroid. In

will fall despite

body moves too

a

if

it

it

was unusually

far

as Jupiter was.

always remained near the

Lagrangian point, L4, of the sun-Jupiter system. Thus, it stayed a fairly constant 480,000,000 miles ahead of Jupiter in its motion about the sun.

Some

years later, another asteroid

was discovered

in the

and was named Patroclus, after moves about the sun in a position that is

L5 position

of the sun-Jupiter system,

Achilles’ be-

loved friend.

a fairly con-

It

stant 480,000,000 miles behind Jupiter.

Other

asteroids were in time located at both points; at the present

time, fifteen of these asteroids are

Following the precedent of Achilles, in the Iliad.

And

known: ten all

in

L4 and

five

in

L5.

have been named for characters

since the Iliad deals with the Trojan

War,

all

the

bodies in both positions are lumped together as the ‘Trojan asteroids.”

Since the asteroids at position L4 include leader, they are

sometimes distinguished

as

Agamemnon,

the Greek

the “Greek group.”

The

ASIMOV ON ASTRONOMY

58

asteroids at position L5 include the

one named

Priamus (usually known

in

and

'Triam”

as

are referred to as the '"pure

It

would be neat and

tidy

Tro^n

Trojan king

for the

English versions of the Iliad),

group.”

the Greek group contained only Greeks

if

and the pure Trojan group only Trojans. Unfortunately, thought

The

of.

result

is

that the Trojan hero Hector

Greek group and the Greek hero Patroclus group.

It

a situation that

is

makes even myself

would

strike

classicist

uncomfortable, and

feel a little

was not

part of the

is

part of the pure Trojan

is

any

this

I

with apoplexy.

am

It

only the very

mildest of classicists indeed.

The Trojan

known examples

asteroids remain the only

of objects at

Lagrangian points. They are so well known, however, that L4 and L5 are

commonly known

as ‘'Trojan positions.”

External perturbing forces, particularly that of the planet Saturn,

keep the asteroids oscillating about the central points. Sometimes the oscillations are wide; a particular asteroid

may be

as

much

as 100,000,000

miles from the Lagrangian point.

Eventually, a particular asteroid

may be

would then adopt

a non-Trojan orbit.

now

may happen

independent,

to

On

pulled too far outward, and

the other hand,

some

asteroid

be perturbed into a spot close to the

Lagrangian points and be trapped. In the long run, the Trojan asteroids

may change

identities,

but there

will always

be some there.

Undoubtedly, there are many more than Their distance from us to one

hundred miles

is

fifteen

Trojan asteroids.

so great that only fairly large asteroids, close

in diameter,

can be seen.

Still,

there are certainly

dozens and even hundreds of smaller chunks, invisible to Jupiter or are chased in an eternal race that

There must be many Trojan if

chase

nobody wins.

situations in the universe.

I

wouldn’t be

met the 25.8 to requirement was accompanied by rubble of some sort

surprised ratio

us, that

every pair of associated bodies which

1

mass-

at

the

Trojan positions.

Knowing

that the rubble exists doesn’t

however; certainly

it

mean

that

it

can be spotted,

can be detected nowhere outside the solar system.

TTiree related stars could be spotted, of course, but for a true Trojan situation,

one body must be of

seen by any technique

now

insignificant mass,

and

it

could not be

at our disposal.

Within the solar system, by far the largest pair of bodies and Jupiter. The bodies trapped at the Lagrangian points of

are the sun

that system

THETROJAN HEARSE could be

and

fairly large

59

yet remain negligible in

mass

in

eomparison to

Jupiter.

The Saturn

situation with respect to Saturn

would be

far less favorable. Since

smaller than Jupiter, the asteroids at the Trojan position as-

is

would be smaller on the average. They would be twiee as far from us as those of Jupiter are, so that they would also be dimmer. They would thus be very difheult to see; and the fact of the sociated with Saturn

matter

no Saturnian Trojans have been found. The ease Uranus, Neptune and Pluto. that

is

worse for

As must

for the small inner planets, there

any rubble

if

Trojan position

they existed. In addition, particularly in

Venus and Mercury, they would be

the case of

even

That alone would make them

consist of small objects indeed.

nearly impossible to see, even

in the

is

lost in the glare of

the

sun. In faet, astronomers

do not

really expeet to find the equivalent of

Trojan asteroids for any planet of the solar system other than Jupiter, until such time as an astronomical laboratory is set up outside the earth or, better .yet, until spaceships actually explore the various La-

grangian points.

Yet there

is

one exception

one place where observation from

to this,

the earth’s surfaee ean turn up something and, in so.

That

is

a Lagrangian point that

faet,

may have done

not associated with a sun-planet

is

system, but with a planet-satellite system. Undoubtedly you are ahead of

me and know

that

am

I

referring to the earth

and the moon.

The fact that the earth has a single satellite was known as soon as man grew intelligent enough to beeome a purposeful observer. Modern man with all his instruments has never been able to find a second one.* Not a natural one, anyway. In fact, astronomers are quite certain that, other than the moon itself, no body that is more than, say, half a mile in diameter, revolves about the earth.

This does not preclude the presence of any number of very small

Data brought back by

partieles.

dicate that the earth

is

artificial

surrounded by

after the fashion of Saturn,

though on

satellites

would seem to

a ring of dust partieles a

much more tenuous

in-

something

seale.

Visual observation eould not detect such a ring except in plaees

where the *

27-

There

is

partieles

might be eoneentrated

of course the “quasi-moon” Toro,

in

unusually high densities.

mentioned

in the footnote

on page

ASIMOV ON ASTRONOMY

6o

Tlie only spots where the concentration could be great enough would be at the Lagrangian points, L4

the earth as

is

more than

and

earth-moon system. (Since

L5, of the

moon— it

25.8 times as massive as the

is

81 times

massive in point of fact— objects at those points vould occupy a

stable position.)

Sure enough, in 1961, a Polish astronomer, K. Kordylewski, reported

two very

actually spotting

luminous patches

faintly

in these positions.

Presumably, they represent dust clouds trapped there.

And

in

connection with these ‘Aloud

satellites,”

have thought up a

I

practical application of Lagrangian points which, as far as

I

know,

is

original.

As we

all

know, one of the great problems brought upon us by the

technologv of the space age \\’astes.

Many

the disposal

that of

is

radioactive

of

solutions have been tried or have been suggested.

wastes are sealed in strong containers or, as

They may be buried underground,

The

suggested, fused in glass.

is

stored in salt-mines or dropped into

an abyss.

No

solution that leaves the radioactivity

upon the

earth, however,

is

wholly satisfactory; so some bold souls have suggested that eventually

measures

The

will

safest

be taken to

fire

the wastes into space.

procedure one can possibly imagine

into the sun. This, howe\er,

is

them

mers would veto

would be

an orbit about the sun, and

to shoot these wastes

not an easy thing to do at

take less energy to shoot that. It

is

to the

moon, but

still

Pm

all.

would

sure that astrono-

easier simply to shoot

easiest of all to

It

them

into

shoot them into an orbit

about the earth. In either of these latter cases, however, run, of cluttering

up the inner portions

we run

the

risk, in

the long

of the solar system, particularly

the neighborhood of the earth, with gobs of radioactive material.

would be

living in the

Granted that space is

own refuse, and the amount

midst of our is

large

small, so that collisions or near-collisions

radioactive debris

We

so to speak.

of refuse, in comparison,

between spaceships and

would be highly improbable,

it

could

still

lead to

trouble in the long run.

Consider the analogy of our atmosphere. All through historv, freely

that

poured gaseous wastes and smoky particles into

all

come

a

it

let’s

not pollute space.

has

in the certaintv

\\ould be diluted far past harm; yet air pollution has

major problem. WAll,

man now

be-

THE TROJAN HEARSE One way space and

marked

out

make

is

sure

off-limits

To do

to concentrate it

onr wastes into one small portion of

Those regions

stays there.

and everything

one would have

6l

else will

of space can then be

be free of trouble.

one or the other of the Trojan positions assoeiated with the earth-moon system in sueh a this,

to fire the wastes to

way as to leave it trapped there. Properly done, the wastes would remain at those points, a quarter-million miles from the moon and a quarter-million miles from the earth, for indefinite periods, certainly

long enough for the radiation to die Naturally, the areas

would be

down

to

nondangerons

levels.

death trap for any ship passing

a

through

—a

priee to

pay for soh ing the radioactive ash disposal problem,

pun

is

kind of '‘IVojan hearse,”

a small priee to

pay

for giving

in fact. Still,

me

it

would be

a small

just as this

a title for the ehapter.

BY JOVE!

Suppose we ask ourselves

On

what world of the solar system (other than earth itself, of course) are we most likely to discover life? I imagine I can plainly hear the unanimous answering shout, ‘'Mars!’' Tlie argument goes, and I know it by heart, because I have used it

myself a

number

cold and a

little

a question:

of times, that

short on

air,

but

Mars may be it

remaining

life.

are definitely too hot, the

satellites of

and

a little

too small, too cold, or too airless

isn’t

to support the equivalent of primitive plant

Venus and Mercury

a little small

On

moon

the other hand, is

airless,

and the

the solar system, and the planetoids as well (to

say nothing of Pluto), are too cold, too small, or both.

And

then

we

include a phrase which

Saturn, Uranus and Neptune,

we can

may go leave

like this:

them out

“As for Jupiter,

of consideration

altogether.”

However, Carl Sagan, an astronomer take this attitude at

all,

and

into doing a bit of thinking

a

at Cornell University, doesn’t

paper of his on the subject has lured

on the subject of the outer

me

planets.

Before Galileo’s time, there was nothing to distinguish Jupiter and

!

BY

J

OVE

63

Saturn (Uranus and Neptune not having yet been discovered) from the other planets, except for the fact that they moved more slowly against the starry background than did the other planets therefore, presumably farther

7 ’he

from the

earth.

showed Jupiter and Saturn

telescope, however,

and were,

angular widths that could be measured.

When

as

with

discs

the distances of the

planets were determined, those angular widths could be converted into

and the

miles,

was

result

a shocker.

As compared with an earthly equa-

diameter of 7,950 miles, Jupiter’s diameter across 88,800, while Saturn’s was 75,100.

torial

The The more

its

equator was

outer planets were giants! discovery of Uranus in 1781 and

Neptune

in

1846 added two

not-quite-so-giants, for the equatorial diameter of

000 miles and that of Neptune,

at latest

measurement,

Uranus is

is

31,-

about 28,000

miles.

The world

disparity in size

even greater

is

if

between these planets and our own tight little one considers volume, because that varies as the

cube of the diameter. In other words,

if

the diameter of

Body

A

is

ten

volume of Body A is ten times ten times ten, or a thousand times the volume of Body B. Tims, if we set the volume of the earth equal to 1, the volumes of the giants are given in Table 7. times the diameter of

Body

B, then the

Table 7

VOLUME (earths 1

PLANET Jupiter

1,300

Saturn

750 60

Uranus

Neptune

Each

40

of the giants has satellites. It

of the various satellites

is

easy to determine the distance

from the center of the primary planet by measur-

ing the angular separation. tion of the satellite.

)

From

It is also

easy to time the period of revolu-

those two pieces of datum, one can quickly

obtain the mass of the primary (see Chapter 3). In terms of mass, the giants remain giants, naturally. earth

is

The

taken as

1,

If

the mass of

the masses of the giants are as given in Table

four giants contain virtually

all

8.

the planetary mass of the solar

system, Jupiter alone possessing about 70 per cent of the total.

The

)

ASIMOV ON ASTRONOMY

64

Table 8

MASS ( EARTH =

PLANET Jupiter

318

Saturn

95

Uranus

15

Neptune

17

remaining planets, plus

all

the

satellites,

1

planetoids, comets and, for

that matter, meteoroids, contain well under

per cent of the total plane-

1

Outside intelligences, exploring the solar system with true

tary mass.

impartiality,

would be quite

sun

in their records thus:

4 planets plus debris. But take another look at the figures on mass.

Compare them with

Star X, spectral class

likely to enter the

GO,

those on volume and you will see that the mass

is

consistently low. In

much room as much matter. The matter

other words, Jupiter takes up 1,300 times as

the earth

does, but contains only 318 times as

in Jupiter

must therefore be spread out more formal language, that Jupiter's density If

we

by the

is

set the earth's density equal

densities of the giants

by

less

to

in

The

volume.

more

than that of the earth.

1,

then

we can

obtain the

just dividing the figure for the relative

figure for the relative

given in Table

which means,

loosely,

mass

densities of the giants are

9.

Table 9

DENSITY

PLANET

On

see, then,

as

same

this

(eARTH=i)

Jupiter

0.280

Saturn

0.125

Uranus

0.250

Neptune

0.425

scale of densities, the densitv of water

Neptune, the densest of the

giants,

is

is

0.182.

only about

As you

2 14

times

dense as water, while Jupiter and Uranus are only 1V2 times as dense,

and Saturn I

is

actually less dense than water.

remember

by stating that

seeing an astronomy book that dramatized this last fact if

one could

find

an ocean large enough, Saturn would

— BY float in

less

it,

J

OVE

65

!

And

than three-fourtlis submerged.

pressive illustration,

showing Saturn,

and

rings

there was a very im-

all,

choppy

floating in a

sea.

But don’t misinterpret one naturally might have than that of water, however,

is

not

Jupiter has

upon

a

visible

its

I

matter of density.

The

first

thought any-

that because Saturn’s overall density

is

must be made

it

so, as

this

can explain

some

of

less

corklike material. This,

easily.

banded appearance, and certain surface move around the planet at a steady striped

is

or

features rate.

By

following those features, the period of rotation can be determined with a high degree of precision;

it

turns out to be

9 hours, 50 minutes, the period of rotation can

and 30 seconds. (With increasing difficulty, be determined for the more distant giants as well.) But here a surprising fact is to be noted. The period of rotation have given

is

I

that of Jupiter’s equatorial surface. Other portions of the

more

surface rotate a bit

slowly. In fact, Jupiter’s period of rotation in-

creases steadily as the poles are approached. This alone indicates

we

are

not looking at a'solid surface, for that would have to rotate

in

one

all

piece.

Tlie conclusion

and of the other

is

quite clear.

What we

giants, are the clouds of

see as the surface of Jupiter,

its

atmosphere. Beneath those

clouds must be a great depth of atmosphere, far denser than our own,

and yet

far less

dense than rock and metal.

phere of the giant planets density appears so low.

counted

is

we took

If

But how deep

is

most

is

with their volume that their

we could

find a density as great

likely, greater.

the atmosphere?

Consider that, fundamentally, the giant planets chiefly in that, being further

their history, they retain a

compounds but remains

differ

from the earth

from the sun and therefore colder through

much

larger quantity of the light elements

hydrogen, helium, carbon, nitrogen

remains

because the atmos-

into account only the core of the

planet, underlying the atmosphere, as that of earth’s, or,

in

It

as a gas.

and oxygen. Helium forms no

Hydrogen

is

present in large excess so

compounds with carbon, nitrogen and oxygen, to form methane, ammonia and water, respectively. Methane is a gas, and, at the earth’s temperature, so is ammonia, but it

as a gas, too,

but

it

also forms

ASIMOV ON ASTRONOMY

66

JUPITER *

Through most of human think that the Golden Age

«k

history,

there has been the tendency to

lay in the past,

that our ancestors were

we were, stronger, more heroic, more honest, better. Even when the present age has surpassed everything in the past in

wiser than today,

knowledge and in an understanding of the physical universe, we feel uneasy about the past. Could it be that the ancients knew more about astronomy than we give them credit for? scientific

For instance, how

is it

the ruler of the gods the

after

telescope

that they

and then

it

was invented,

named

a certain planet Jupiter after

turns out, thousands of years later, to

be by far the largest of the

planets?

Well, that’s not so mysterious. The fact that Jupiter is so large makes it appear brighter in our skies than it would otherwise be; brighter even than a closer but smaller planet it is

at its closest

having only

mer than

%oo

Jupiter

briefly brighter

is

is,

would

be.

Mars when

than Jupiter, but most of the time,

^^ surface area of Jupiter, even though closer.

1

it

is

considerably dim-

Venus is considerably brighter than Jupiter much of the time, but because Venus is closer to the Sun than we are, it appears in the sky only for a few hours after sunset, or for a few hours before sunrise and, then, never near zenith. You can’t name a planet that is slave of

Sun

the

after the king of the gods.

In the midnight sky on a clear and moonless night, Jupiter, ally,

night.

is

Why,

is

the brightest object.

then, shouldn’t

a liquid. If

is

below, both

be a

it

Only the Moon and, very occasionMars outshine the steady white gleam of Jupiter in the deep

in the sky,

water

if

it

be named for the king of the gods?

the earth’s temperature were to drop to

ammonia and water would be

solid,

—100° C.

but methane would

or

still

gas.

As

a matter of fact, all this

is

not merely guesswork. Spectroscopic

evidence does indeed show that Jupiter’s atmosphere

helium

in a three-to-one ratio

methane. (Water

is

is

hydrogen and

with liberal admixtures of

ammonia and

not detected, but that

may be assumed

to

be frozen

out.)

Now,

the structure of the earth can be portrayed as a central solid

.

photo;

body

of rock

mount wilson and palomar observatories

and metal (the lithosphere), surrounded by

water (the hydrosphere), which

is

in turn

a layer of

surrounded by a layer of gas

(the atmosphere)

The

light elements in

which the giant planets are particularly

would add to the atmosphere and

to the hydrosphere, but not so

rich

much

to the lithosphere. ITie picture w^ould therefore be of a central litho-

sphere larger than that of the earth, but not necessarily enormously larger,

surrounded by

atmosphere.

a gigantic

hydrosphere and an equally gigantic

ASIMOV ON ASTRONOMY

68

But how gigantic is gigantic? Here we can take into consideration the polar Tlius, although Jupiter it is

88,800

is

nailes in

flattening of the giants.

diameter along the equator,

only 82,800 miles in diameter from pole to pole. This

is

a flattening

of 7 per cent, compared to a flattening of about .33 per cent for the earth. Jupiter has a visibly elliptical appearance for that reason. Saturn’s

aspect

while cent.

is

even more extreme, for

its

polar diameter

is

its

equatorial diameter

is

75,100 miles

66,200 miles, a flattening of nearly 12 per

(Uranus and Neptune are

than are the two larger

less flattened

giants.)

The amount

of flattening depends partly on the velocity of rotation

and the centrifugal

effect

which

is

set up. Jupiter

and Saturn, although

than the earth, have periods of rotation of about 10 hours as compared with our own 24. Thus, the Jovian surface, at its equator, is

far larger

moving

at a rate of 25,000 miles

surface

moves only

surface

is

an hour, while the earth’s equatorial

at a rate of 1,000 miles

thrown farther outward than

an hour. Naturally, Jupiter’s

earth’s

is

greater gravity), so that the giant planet bulges

more

is

(even against Jupiter’s

more

at the equator

and

flattened at the poles.

However, Saturn

is

distinctly smaller than Jupiter

and has

a period

some twenty minutes longer than that of Jupiter. It exerts smaller centrifugal effect at the equator; and even allowing for its

of rotation a

smaller gravity,

However,

it is

it

should be

more

less flattened at

flattened.

The

the poles than Jupiter

reason for this

is

is.

that the degree of

depends also on the distribution of densitv, and if Saturn’s atmosphere is markedly thicker than Jupiter’s, flattening will be greater. The astronomer Rupert Wildt has estimated what the size of the

flattening

lithosphere, hydrosphere

planet in order to give its

the overall density

polar flattening. (Tliis picture

erally,

but

let’s

cluded in Table

*

it

and atmosphere* would have

work with 10, to

it

which

is

add

be on each

was observ'ed to have plus

not accepted by astronomers gen-

anvvvay.**) I

it

to

The

figures

I

have seen are

figures for the earth as a

in-

comparison:

The atmosphere

would be

might be liquid or even

pressures

As

is not necessarily gaseous. It is made up of compounds which gases under earthly conditions, but which under Jupiter’s temperatures and

solid at certain depths.

matter of fact, a picture currently popular is of a Jupiter made up almost entirely of hydrogen and helium (with fourteen atoms of the former for every one of the latter). Under great pressures, hydrogen solidifies and gains metallic properties, so that Jupiter’s core is of “metallic hydrogen.” Actually, though, all speculations concerning Jupiter’s inner structure remain, as yet, omy intelligent guesses. a

!

BY

J

O VE

69

Table 10

HYDROSPHERE

ATMOSPHERE

(radius in

miles)

(thickness in miles)

(thickness in miles)

Jupiter

18,500

17,000

8,000

Saturn

14,000

8,000

16,000

Uranus

7,000

6,000

3,000

Neptune

6,000

6,000

2,000

Earth

3>975

2

know

I

no

LITHOSPHERE

that the atmosphere

However,

thickness.

fixed

calculate

its

volume

—

is

8’='

thicker tlran eight miles, and that in fact

—

am

taking the earth’s atmosphere and shall later only to the top of its cloud layers, which is what we do for I

the giant planets.

As you a

much

and

though smaller than

see, Saturn,

Jupiter,

which accounts

thicker atmosphere,

pictured as having

low overall density

its

unusual degree of flattening. Neptune has the shallowest

its

mosphere and

is

parison \\ith the giants,

if

earth’s lithosphere equal to

isn’t

too pygmyish in com-

the lithosphere alone

assume that the lithospheres are Table

at-

therefore the densest of the giant planets.

Furthermore, you can see that the earth

in

for

is

1,

all

is

of equal density

considered.

and

set the

If

we

mass of

then the masses of the others are shown

11.

Table 11

MASS OF LITHOSPHERE

(earth= 1

PLANET

It is

emphasize

various

100

Saturn

45

Uranus

5>/2

Neptune

3 1/2

the disparity of the hydrosphere and atmosphere that blows up

the giants to so large a

To

Jupiter

)

size.

this last fact,

components

in

it

would be better

to give the size of the

terms of volume rather than of thickness. In

liable 12 the volumes are therefore given in trillions of cubic miles.

Once

again, the earth

is

included for purposes of comparison:

)

ASIMOV ON ASTRONOMY

70

Table 12

LITHOSPHERE

HYDROSPHERE

ATMOSPHERE

VOLUME

VOLUME

VOLUME

Jupiter

27

Saturn

11.5

161

155

33

185

Uranus

1-4

7.8

8.4

Neptune

0.9

6.4

4.2

Earth

0.26

0.00033

0.001

As you can see

up only

a small part of the total

volume, whereas

up the volume of each component

total

makes

at a glance, the lithosphere of the giant planets it

makes up almost

the volume of the earth. This shows up more plainly set

1

if,

in

Table

as a percentage of

13,

all

we

planet’s

its

volume. Thus: Table 13

LITHOSPHERE HYDROSPHERE ATMOSPHERE (% OF planet’s (% OF planet's (% OF planet’s

VOLUME

VOLUME

)

volume)

Jupiter

7-7

47.0

45-3

Saturn

4.8

14.4

80.8

Uranus

8.0

44-3

47-7

Neptune

8.0

55-5

36.5

Earth

The

0.125

99-45

difference can’t be

made

plainer.

0.425

Whereas

the earth

about

is

99.5 per cent lithosphere, the giant planets are only 8 per cent, or less, lithosphere. About one-third of Neptune’s apparent volume is gas. In

the case of Jupiter and Uranus, the gas volume in the case of Saturn, the least

four-fifths of the total.

The

giants” and, as you see, that

This

is

one-half the total, and

dense of the four, the gas volume

is

a

good name, particularly

we have drawn

are violently poisonous,

manently dark even on the “sunlit

and from what we can

to be beaten into the turmoil of

The

fully

for Saturn.

of the giant planets.

extremely deep and com-

pletely opaque, so that the surface of the planet

gigantic;

is

giant planets are sometimes called the “gas

a completely alien picture

The atmospheres

is

side.” Tlie

is

entirely

and

per-

atmospheric pressure

see of the planets, the

is

atmosphere seems

huge storms.

temperatures of the planets are usually estimated as ranging

!

BY from

— ioo°C.

a

maximum

Neptune, so that even sures

if

J

O VE

71

— 230° C.

for Jupiter to a

we could

minimum and the

survive the buffeting

and the poisons of the atmosphere, we would land on

for

pres-

a gigantic,'

ammoniated ice. land and live on such a planet,

planet-covering, thousands-of-miles-thick layer of

Not only but

inconceivable for

is it

man

seems inconceivable that any

it

bles our

own

could

to

live there.

Are there any loopholes

in this picture?

Yes, a very big one, possibly, and that perature. Jupiter

To it

be

sure,

Of

keeps.

about

five

is

the light

we have

receives

it

the remaining five-ninths

is

as

much

we

radiation

receives

it

absorbed.

The absorbed it

how much

but

from the sun, four-ninths

are, so that

However, the

solar radiation.

penetrate to the planetary surface as light, but

—as

thought.

times as far from the sun as

how much

not

the question of the tem-

is

not be nearly as cold as

one twenty-fifth

receives only

crucial point

may

it is

that even remotely resem-

life at all

is

reflected

it

and

portion does not

gets there just the

same

heat.

The

planet would ordinarily reradiate this heat as long-wave in-

frared,

but the components of Jupiter’s atmosphere, notably the am-

monia and methane,

are quite

opaque

tained forcing the temperature to is

to infrared,

rise. It is

only

quite high that enough infrared can force

its

which

is

therefore

re-

when the temperature way out of the atmos-

phere to establish a temperature equilibrium. even possible that the surface temperature of Jupiter, thanks to

It is

this

“greenhouse

The

effect,”

is

other giant planets

as high as that of earth.

may

also

have temperatures higher than those

usually estimated, but the final equilibrium

would very

likely

be lower

than that of Jupiter’s since the other planets are further from the sun. Perhaps Jupiter above 0° C. Tliis

the only giant planet with a surface temperature

is

means that

one with

Jupiter, of all the giant planets, could

a liquid hydrosphere. Jupiter

would have

be the only

a vast ocean, covering

the entire planet (by the Wildt scheme) and 17,000 miles deep.*

On house

the other hand,

Venus

also has an

effect, raising its surface

atmosphere that exerts a green-

temperature to a higher level than had

been supposed. Radio-wave emission from Venus indicates temperature to be Of ment is *

course,

if

seriously

much

its

surface

higher than the boiling point of water, so that

the hydrogen-helium picture of Jupiter

compromised.

is

correct, this

whole argu-

ASIMOV ON ASTRONOMY

72 the surface of

Venus

is

powdery dry with

water supply in the cloud

all its

layer overhead.

A

strange picture.

The

planetary ocean that has been so time-honored

a science-fictional picture of

along. It

all

is

Venus has been pinned

to the

Jupiter that has the world-wide ocean, by Jove!

whom

Considering the Jovian ocean, Professor Sagan (to at the beginning of this chapter) says: sibility of life life

wrong planet

referred

I

“At the present writing, the pos-

on Jupiter seems somewhat better than the

possibility of

on Venus.’’

Tliis

is

a

commendably

cautious statement, and as far as a scientist

can be expected to go in a learned journal. However,

I,

particular soapbox, don’t have to be cautious at

I

much more

all,

so

picture,

it is

large as earth’s ocean and, in fact, earth. This ocean

is

this

can afford to be

sanguine about the Jovian ocean. Let’s consider

moment. If we accept Wildt’s

on

myself,

it

for a

a big ocean, nearly 500,000 times as

620 times

as

voluminous

under the same type of atmosphere

as all the

that, according

to current belief, surrounded the earth at the time life developed

our planet. All the simple compounds

— methane,

ammonia, water,

on dis-

solved salts— would be present in unbelievable plenty by earthly standards.

Some

source of energy

is

required for the building up of these organic

compounds, and the most obvious one

is

the ultraviolet radiation of the

sun. Tlie quantity of ultraviolet rays that reaches Jupiter

is,

as aforesaid,

only one twenty-fifth that which reaches the earth, and none of

it

can

get very far into the thick atmosphere.

Nevertheless, the ultraviolet rays must have

some

effect,

because the

colored bands in the Jovian atmosphere are very likely to consist of free radicals (that nar}'

is,

energetic molecular fragments) produced out of ordi-

molecules by the

ultra\’iolet.

The constant writhing of the atmosphere would carry the free radicals downward where they could transfer their energy by reacting with simple molecules to build up complex ones.

Even

if

ultraviolet light

sources remain. There

that

is

called a Jovian

tinuous than radioactivity.

it

ever

is

is

is

discounted as an energy source, two other

first,

lightning. Lightning in the thick soup

atmosphere may be or was

on

far

more

energetic

earth. Secondly, there

is

and con-

always natural

BY \\ is

why ean

then,

ell,

The raw

right.

J

OVE

!

73

the Jovian ocean breed

t

material

is

life?

there. I’he energy supply

Tlie temperature is

present. All the

requirements that were sufficient to produce life in earth’s primordial ocean are present also on Jupiter (if the picture drawn in this article is correct), only more and better.

One might wonder whether

could withstand the Jovian atmospheric pressures and storms, to say nothing of the Jovian gravity. But the life

storms, however violent, could only

mile-deep ocean. if

you

As

like,

A

few hundred feet

up the outer skin of a 17,000below the surface, or a mile below',

roil

there w'ould be nothing but the slow^ ocean currents.

for gravity, forget

it.

Life within the ocean can ignore gravity

buoyancy neutralizes

altogether, for

its effects,

or almost neutralizes

it.

No, none of the objections stand up. I’o be sure, life must originate and develop on Jupiter in the absence of gaseous oxygen, but that is exactly one of the conditions under w'hich life originated and developed on earth. There are living creatures on earth right now that can live without oxygen.

So once again

ask the question;

let’s

system (other than earth cover

of course)

are

world of the solar

we most

likely to dis-

life?

And now, Of

itself,

On what

it

seems

course, life

to

me, the answ'er must be:

on Jupiter w'ould be

On

Jupiter,

by Jove!

pitifully isolated. It w^ould

have a

vast ocean to live in, but the far, far vaster outside universe w'ould

be

closed forever to them.

Even

if

some forms

parable to our

of Jovian life developed an

own (and

com-

intelligence

there are reasonable arguments to suggest

that true sea life— and before you bring

up the

point, dolphins are

descendants of land-living creatures— w'ould not develop such gence), they could do nothing to break the isolation.

intelli-

highly unlikely that even a manlike intelligence could devise methods that would carry itself out of the ocean, through thousands of It is

miles of violent, souplike atmosphere, against Jupiter’s colossal gravity, in order to reach Jupiter’s inmost satellite and, from its alien surface,

observe the universe.

And

as

long as

no indication heat,

and

life

remained

in the Jovian ocean,

it

w'ould receiv^e

of an outside universe, except for a non-directed flow^ of

excessively feeble

microwave radiation from the sun and

a

few

other spots. Considering the lack of supporting information, the micro-

ASIMOV ON ASTRONOMY

74

waves would be as indecipherable a phenomenon even

if it

But If

as

one could imagine,

is,

then

were sensed.

let’s

not be sad;

the Jovian ocean

mass would be

let’s is

end on*a cheerful note.

as rich in life as

our

own

living matter. In other words, the total

% 0,000

mass of sea

on Jupiter would be one-eighth the mass of our moon, and of mass for a mess of

What

of

ils

life

that’s a lot

fish.

fishing-grounds Jupiter

would make

if it

could be reached some-

how!

And,

in

der over.

.

view of our population explosion, just one question to pon.

.

Do

you suppose that Jovian

life

might be edible?

6

SUPERFICIALLY SPEAKING

For the

last century, serious science-fiction writers,

from Edgar Allan

Poe onward, have been trying to reach the moon; and now governments are trying to get into the act.* It kills some of the romance of the deal to have the project

up the other

side,

but

if

become that’s

a '‘space spectacular’^ designed to

what

it

takes to get there,

I

show

suppose

we

can only sigh and push on.

So

far,

however, governments are only interested

in

reaching the

moon, and as science-fiction fans we ought to remain one step ahead of them and keep our eyes firmly fixed on populating the moon. Naturally,

we can

ignore such

little

problems

missing on the moon. Perhaps

as the fact that air

we can bake water out

and water are

of the deep-lying

rock and figure out ways of chipping oxygen out of silicates.** *

And

We

can

As we all know, American astronauts first reached the Moon 1969, some years after this article was first published and have revisited it several times since then. ** Rocks brought back from the Moon since 1969 show no traces of having ever had any water content, which is disheartening for the prospects of Moon colonizain

tion.

not

just trying.

photo: national aeronautics and space administration

superficially SPEAKING live

underground

away from the heat

to get

yy

of the day

and the cold of

down from

a cloudless sky

the night. In fact, with the sun shining powerfully

two weeks at

for

a

time,

solar

might be able

batteries

moon colonists with tremendous quantities of energy. Maybe the land with the high standard of living of up there enough

in

the sky. Etched into

some

to

supply

the future will be

of the craters, perhaps, large

to be clearly seen through a small telescope, could

be a message

MAN ON THE MOON One

many

of the

simply that

IS

it

is

reasons close

why

Moon

the

enough

to

have

so important to Earth

is

distance determined hy

its

naked-eye observation. There is no way of judging the distance of any of the heavenly bodies in the easy way ii’c can judge the distance of objects on Earth. cant take a tape measure to the sky, or pace

We

off

a distance. In primitive times,

it

was reasonable

to judge the heavenly

bodies to be as far off as they looked and the sky was judged to be not far above the mountain tops. In fact, if the clouds are considered to be just under the sky, that s a pretty reasonable conclusion, since big

mountains are sometimes masked in clouds. Trigonometry can be used, however. The position of a heavenly object can be sighted (^in comparison to a dimmer and therefore more distant object) from each of two widely separated obsery’atories. The change in position ( parallax can then be related to the distance of

)

the body. As long ago as the second century

Hipparchus of Nicaea determined the

Moon

to thirty times the diameter of the Earth. In

b.c.,

Greek astronomer,

to be at a distance equal

modern terms

this

means

Moon is 240,000 miles away. The Moon is the only object

the

whose distance can be determined by was enough to give a notion of the im-

such naked-eye objects but it mensity of the Universe. The sky was not near at hand. After

Moon

was astonishingly distant and yet

it

was the

all

the

closest of all the

heavenly bodies.

And

it

was close enough for man

to

dream of reaching

a quarter of a million miles away.

If it

enly body would be

at best,

the

Moon.

months

to

It

Venus which,

did not is

a

exist,

it.

It

was only

the closest heav-

hundred times

as far as

takes three days of rocketing to reach the

reach Venus.

Man

has

now

set foot

on the

Moon; six Moon. If it

were not there as our steppingstone, would we ever have ventured to reach out into space at all?

ASIMOV ON ASTRONOMY

78

me

that starts: ‘‘Send

ing to breathe free

.

your

your poor, your huddled masses yearn-

tired,

.

Who knows? But

if

moon

the

is

ever to be a seeond earth

of our population, there

we ought

to

know. That

question

TTie

first

The

size of the

is,

it

that

size.

is, its

what do we mean by

“size”?

most often given

is

some

to siphon off

is

a certain significant statistic about

is

moon

and

in

terms of

its

because once the moon’s distance has been determined,

diameter,

its

diameter

can be obtained by direct measurement. Since the moon’s diameter miles, is

most people cannot

resist

2,160 miles and the earth’s

is

7,914

the temptation of saying that the

moon

is

one-quarter the size of the earth, or that the earth

moon. (The exact

size of the

size of the earth,

is

that the

this viewpoint, or that

moon

is

the earth

0.273 times the

is

3.66 times the

moon.)

size of the

makes the moon appear quite

All this

But,

from

figure

four times the

is

a respectably-sized world.

consider size from a different standpoint. Next to diameter,

let’s

the most interesting statistic about a body of the solar system

mass, for upon that depends the gravitational force

Now, mass

varies as the

cube of the diameter,

it

all

can

its

is

exert.

things being equal.

moon, diameterwise, it could be 3.66X3-66X3-66 or qq times the size of the moon, masswise. (Hmm, there’s something to be said for this Madison Avenue speech monstrosity, conveniencewise.) But that is only if the densities of the two If

the earth

is

3.66 times the size of the

bodies being compared are the same.

As

it

happens, the earth

the discrepancy in mass

is

lliis

is

is

distressing because

pygmyish on say. Is the

us,

moon

moon,

so that

even greater than a simple cubing would

is

indicate. Actually, the earth

1.67 times as dense as the

81 times as massive as the

moon.

now, suddenly, the moon has grown a

and the question

arises as to

which we ought

one-quarter the size of the earth or

is

it

bit

really to

only

the

size of the earth?

Actually,

we ought

to use

whichever comparison

a particular set of circumstances,

concerned, neither

is

directly

and

meaningful.

face area, the superficial size of the

On

as far as

is

meaningful under

populating the

What

counts

is

moon

is

the sur-

moon.

any sizable world, under ordinary circumstances,

human

beings

SUPERFICIALLY SPEAKING on the

will live

surface.

Even

if

pleasant environment, they will

79

they dig underground to escape an un-

do so only very

when compared

slightly,

with the total diameter, on any world the size of the earth or even that of the moon. Therefore, the question that ought to agitate us with respect to the size of the moon is: What is its surface area in comparison with that of the earth? In other words, what is its size, superficially speaking? Tliis

easy to calculate because surface area varies as the square of the diameter. Here density has no effect and need not be considered. is

the earth has a diameter 3.66 times that of the area 3.66X3.66 or 13.45 times that of the moon. If

But

M3.45 that of the earth

how

large

is

it

has a surface

me. The picture of a surface that

this doesn't satisfy

exactly? Just

moon,

dramatic enough.

isn’t

What

is

does

equal to it

mean

such a surface?

thought of an alternate way of dramatizing the moon’s surface, and that of other areas, and it depends on the fact that a good many Americans these days have been jetting freely about the United States. I’ve

This gives them a good conceptual feeling of what the area of the United States is like, and we can use that as a unit. The area of all fifty states is

To

see

3,628,000 square miles and

how

this works, look at

Table

we can 14,

call

that

1

which includes

USA unit. a

sampling of

geographic divisions of our planet with their areas given in USAs.

Now, you see, when know at once that the

I

say that the moon’s surface

colonization of the

moon

will

is

4.03

make

USAs, you available to

Table

GEOGRAPHIC

AREA

geographic

area

DIVISION

(in USAS)

DIVISION

(in USAS)

Australia

0.82

North America

Brazil

0.91

Africa

2.50

Canada

0.95

Asia

3.20

United States

1.00

Indian Ocean

4.70

Europe

1.07

Atlantic

China

1.19

Total land surface

1.50

Pacific

Antarctica

1.65

Total water surface

17.60

South America

1.90

Total surface

36.80

Arctic

Soviet

Ocean

Union

2.32

Ocean

Ocean

54.30

7.80 8.80

17.50

ASIMOV ON ASTRONOMY

8o

of land equal to four times that of the

humanity an area

To

times that of the Soviet Union.

or

area of the

But all

moon

let’s

is

another way, the

it still

about halfway between that of Africa and Asia.

just

go further and assume that mankind

is

going to colonize

can colonize or that

is

worth colonizing.

the solar system that

When we

put

United States

it

we

say 'Tan colonize,”

future, the “gas giants,” that

eliminate, at least for the foreseeable

Jupiter, Saturn,

is

Uranus and Neptune.

(For some comments on them, however, see the previous chapter.) That still leaves four planets: NIercury, Venus, Mars, and (just to

be complete

— and

extreme) Pluto. In addition, there are a number of

sizable satellites, aside

from our own moon, that are large enough to

seem worth colonizing. These include the four large satellites of Jupiter (lo, Europa, Ganymede and Callisto), the two large satellites of Saturn (Titan and Rhea), and Neptune’s large

The

satellite

(Triton).

surface areas of these bodies are easily calculated; the results are

moon

given in Table 15, with the earth and

included for comparison.

Table 15

PLANET OR SATELLITE

As you can

see,

SURFACE AREA (uSAS)

Earth

54-3

Pluto

(34)??

Venus Mars

49.6 15-4

Callisto

9.0

Ganymede

8.85

Mereury

8.30

Titan

7.30

Triton

6.80

lo

4.65

Moon

4.03

Europa

3-30

Rhea

0.86

if

we exclude

round dozen bodies

the sun and the gas giants, there are a

in the solar

system with a surface area in excess of

USA, and a thirteenth with an area just short of that figure. The total surface area available on this baker’s dozen of worlds is roughly equal to 225 USAs. Of this, the earth itself represents fully one 1

quarter,

and the earth

is

already colonized, so to speak, by mankind.

superficially speaking Another quarter

8i

represented by Pluto, the eolonization of whieh, with the best wiW in the world, must be eonsidered as rather far off.*

Of what

is

(about 118 USAs), Venus, Mars and the moon make up some five-ninths. Since these represent the worlds that are closest is

left

and therefore the most

easily

reached and colonized, there

may be

quite

a pause before

humanity dares the sun’s neighborhood to reach Mercury, sweeps outward to the large outer satellites. It might seem that the

or

extra pickings are too slim.

However, there are other alternatives, as I shall explain. So far, I have not considered objects of the solar system that are less than 1,000 miles in diameter (which is the diameter of Rhea). At first glance, these might be considered as falling under the heading of ‘not worth colonizing” simply because of the small quantity of surface area they might be expected to contribute. In addition, gravity would be so small as to give rise to physiological and technological difficulties, perhaps.

However,

let’s

ignore the gravitational objection, and concentrate on

the surface area instead.

Are we correct ‘in assuming that the surface area of the minor bodies small enough to ignore? There are, after all, twenty-three satellites in

is

the solar system with diameters of respectable number.

On

than 1,000 miles, and that’s a the other hand, some of these satellites are less

quite small. Deimos, the smaller satellite of Mars, has a diameter that isn’t more than y.5 miles.**

To

handle the areas of smaller worlds,

The American

make

use of another unit.

We

Los Angeles covers 450 square miles. can LA. This is convenient because it means there are

city of

set that equal to just

let’s

1

about 8,000 LAs

in

1

USA.

A

comparison of the surface areas of the minor satellites of the solar system is presented in Table 16. (I’ll have to point out that the diameters of all

these satellites are quite uncertain and that the surface areas as given are equally uncertain. However, they are based on the best information available to me.)

Tlie total area of the minor satellites of the solar system thus comes to just under 20,000 LAs or, dividing by 8,000, to about USAs. All 2.5

*

Actually, Pluto is now thought to be only the size of Mars, rather than the size of Earth, as had been assumed when this article first appeared. ** In 1971 a Mars-probe photographed Phobos, the larger satellite,

to

be potato-like in appearance (honest!) and 16 miles in

its

and showed

longest diameter.

it

ASIMOV ON ASTRONOMY

82

twenty-three worlds put together have area of the of

moon,

or, to

North America. This would seem

put

little

more than

another way, have

it

just

.

.

.

about the area

minor

to confirm the notion that the

not worth bothering about, but

half the surface

satellites are

think again. All these

let’s

satellites,

lumped together, have just a trifle over one-sixth the volume moon, and yet they have more than half its surface area.

of the

This should remind us that the smaller a body, the larger its surface area in proportion to its volume. The surface area of any sphere is equal to 47rr%

where

radius.

r is its

This means that the earth, with a radius

of roughly 4,000 miles, has a surface area of roughly 200,000,000 square miles.

But suppose the material

of the earth

is

make up a earth. Volume

used to

smaller worlds each with half the radius of the

Table 16

SATELLITE

(

PRIMARY)

SURFACE AREA (lAS)

lapetus (Saturn)

T 450

Tethys (Saturn)

3,400

Dione (Saturn)

3,400

Titania (Uranus)

P*

Oberon (Uranus)

2,500

Mimas

(Saturn)

Enceladus (Saturn) Ariel

(Uranus)

Umbriel (Uranus) Hyperion (Saturn)

00

630

630

630 440 280

Phoebe (Saturn) Nereid (Neptune)

280

Amalthea (Jupiter)

70

Miranda (Uranus)

45

yi

35

(Jupiter)

120

VII (Jupiter)

6.5

VIII (Jupiter)

6.5

IX XI

(Jupiter)

1-5

(Jupiter)

1-5

XII (Jupiter)

1-5

Phobos (Mars)

1-5

X

0.7

(Jupiter)

Deimos (Mars)

0.4

series of

varies as

superficially speaking

83

the cube of the radius, so the material of the earth can than eight half-earths, each with a radius of roughly surface area of each

and the

miles,

total

make up no 2,000 miles.

less

The

would be roughly 50,000,000 scjuare surface area of all eight '‘half-earths’’ would be half-earth

400,000,000 square miles, or twice the area of the original earth. If e consider a fixed volume of matter, then, the smaller the bodies into which it is broken up, the larger the total surface area it exposes.

You may three

minor

feel this analysis satellites

do

accomplishes nothing, since the twenty-

any

not, in

case,

much

have

area.

Small though

they are, the total area comes to that of North America and no more. Ah, but we are not through. Tliere are still the minor planets, or

as-

teroids. It IS

estimated that

the asteroids put together have a mass about

all

per cent that of the earth.

If all

them were somehow combined

of

1

into

a single sphere, with

an average density equal to that of the earth, the radius of that sphere would be 860 miles and the diameter, naturally,

1,720 miles. It would be almost the size of one of Jupiter’s satellites, Europa, and its surface area would be 2.6 USAs, or just about equal to that of all the minor satellites put together.

But the large

area

asteroids

number comes

as 100,000;

in.

do not

exist as this single fictitious

of smaller pieces,

The

and

if

total

and here

number

that figure

is

where the increase

is

of asteroids

correct,

sphere but as a

is

in surface

estimated to be as high

then the average asteroid has a

diameter of 35 miles, and the total surface area of all 100,000 would then come to as much as 130 USAs. This means that the total surface area of the asteroids is equal

to

slightly

more than that

of the earth,

lumped together. It is 7.5 times the Here is an unexpected bonanza.

Venus, Mars, and the moon

area of the earth’s land surface.

Furthermore, we can go beyond that. the surface of the worlds? Surely of the interior materials otherwise

all

Why

we can

restrict ourselves

dig into

only to

them and make use

beyond our reach.

On

large worlds,

with their powerful gravitational forces, only the outermost skin can be penetrated, and the true interior seems far beyond our reach. On an asteroid, however, gravity is virtually nil and it would be comparatively easy to hollow I

made

it

out altogether.

use of this notion in a story

was called Strikebreaker and you Other Stories (Doubleday, 1969). It

11

I

once wrote* which was

find

it

in

my

set

collection Nipfitfall

on and

MA .\flAADfe-:OB^Ct) photo;

mount wilson and palomar observatories

ICARUS Asteroids by the thousands are moving against the background of designed the stars. If a telescope is so geared as to turn in a fashion to neutralize the

motion of the Earth’s

surface^ the stars will all turn

photograph. The brighter stars overexpose the film and seem to be larger points, but at least they remain

out to be points of

light, as in this

in place.

Asteroids

move

against

the

background of the

stars

and

so

they

—

show up as streaks of light dim streaks because their light doesn’t pile up in one place. In general, an asteroid that is close to us has a greater apparent motion and leaves a longer streak. Any long streak against the starfield that is very thin (so it can’t be a planet) and very

dim

(so

it

astronomers for

can’t be a meteorite) it

must

is

bound

surely be the track of

an

to fix the attention of ‘

Earth-grazer.

SUPERFICIALLY SPEAKING

A

a fictitious asteroid called Elsevere.

visitor

85

from earth

is

being

lec-

tured by one of the natives, as follows:

“We

are not a small world, Dr.

dimensional standards. ters that of

The

can occupy,

if

we

surface area of Elsevere

New

the state of

Lamorak; you judge us by two-

Remember we

York, but that's irrelevant.

wish, the entire interior of Elsevere.

volume of well over

miles’ radius has a

only three-quar-

is

of Elsevere were oceupied by levels

A

sphere of

half a million eubic miles. If

fifty feet

And none

all

apart, the total surface area

within the planetoid would be 56,000,000 square miles, and that to the total land area of earth.

fifty

is

equal

of these square miles, doctor,

w ould be unproductive.” Well,

that’s for

miles in diameter.

only about

an asteroid 50 miles

An

asteroid that

fbe volume, and

its

only 2,000,000 square miles, wdiich area of the United States (0.55

One

in radius

and, consequently, 100

35 miles in diameter would have levels w'ould offer a surface area of

is

is

nevertheless over half the total

USAs, to be exact).

small 35-mile-diameter asteroid would then offer as

space as the moderately large Saturnian satellite lapetus, ease, only surface'area

The It

if,

much

living

in the latter

were considered.

material hollowed out of an asteroid would not be w^aste, either.

could be utilized as a source of metal, and of

silicates.

The

only im-

portant elements missing would be hydrogen, carbon and nitrogen.

The Icarus.

but at

it

thin track indicated here by the arrow

is

that of the asteroid

This does not approach us as closely as some Earth-grazers do, has its own special point of interest. Its orbit is so lopsided that

one end

it

comes

closer to the

Sun than Mercury

does; closer than

any chunk of matter except an occasional comet. That, in

fact, is

what

gives

it

its

name. In the Greek myths, Icarus

was escaping from Crete by the use of homemade wings consisting of feathers stuck to a light framework by wax and propelled by armpower. With him was Daedalus, Icarus’ father, who had designed those wings. Against his father’s advice, Icarus vaingloriously

high in the his feathers,

air,

approached the Sun too

and plummeted

melted the wax,

into the sea to his death.

Icarus certainly goes nearer the

have.

closely,

Sun than

mounted

The

lost

asteroid

the mythological Icarus could

.

.

ASIMOV ON ASTRONOMY

86

and these could be picked up (remember we’re viewing the future through rose-colored glasses) in virtually limitless quantities from the atmosphere of the gas If

giants, particirlarly Jupiter.

we imagine 100,000

asteroids, all

more

or less hollowed out,

we

could end with a living space of 200,000,000,000 square miles or 55,000

USAs.

would be more than 150 times

I’his

able on

all

as

much

area as

was

avail-

the surfaces of the solar system (excluding the gas giants,

but even including the asteroids).

Suppose the

levels

within an asteroid could be as densely populated

We

United States today.

as the

might then average 100,000,000

population of an asteroid, and the total population of

would come

The

as the

the asteroids

all

to 10,000,000,000,000 (ten trillion).

question

whether such

is

can be supported.

a population

can visualize each asteroid a self-sufhcient unit, with

all

One

matter vigor-

ously and efficiently cycled. (This, indeed, was the background of the story

from which

I

quoted

earlier.)

bound to be the energy supply, since energy the one thing consumed despite the efficiency with which all else

The

bottleneck

is

is

is

cycled.

At the present moment,

virtually all our energy supply

derived

is

from the sun. (Exceptions are nuclear energy, of course, and energy drawn from tides or hot springs.) The utilization of solar energy, almost entirely by way of the green plant,

not

is

efficient, since

the

green plant makes use of only 2 per cent or so of all the solar energy that falls upon the earth. The unutilized 98 per cent is not the major loss,

however.

Solar radiation streams out in it

reaches the earth’s orbit,

it

all

directions

from the sun, and when

has spread out over a sphere 93,000,000

miles in radius. Tlie surface area of such a sphere

is

110,000,000,000,-

000,000 (a hundred and ten quadrillion) square miles, while the crosssectional area presented by the earth is only 50,000,000 square miles.

The

fraction of solar radiation

stopped by the earth

is

therefore

50,000,000/110,000,000,000,000,000, or just about 1/2,000,000,000 (one two-billionth If

all

)

the solar radiation could be trapped and utilized with no

greater efficiency than ible

it is

now on

earth, then the population support-

(assuming energy to be the bottleneck)

billion times the

000,000

would mount

to

two

population of the earth or about 6,000,000,000,000,-

(six quintillion)

SUPERFICIALLY SPEAKING To

be

sure, the

87

energy requirement per individual

is

bound

to in-

but then efficiency of utilization of solar energy may increase also and, for that matter, energy can be rationed. Let’s keep the six

crease,

quintillion figure as a talking point.

To

utilize all of solar radiation,

space in staggered orbits at

all

power

stations

would be

inclinations to the ecliptic.

more energy was required, the station would present there would be more of them, until eventually the be encased. Every bit of the radiation would the stations before

it

had

a

This would create an

strike

set

up

in

As more and

larger surfaces, or

would

entire sun

one or another of

chance to escape from the solar system. interesting

studying the sun from another

star.

effect

The

to

any intelligent being

sun’s

visible

light

would,

over a very short period, astronomically speaking, blank out. Radiation wouldn’t cease altogether, but it would be degraded. The sun would

begin to radiate only in the infrared. Perhaps this always happens when an intelligent race becomes

in-

enough, and we ought to keep half an eye peeled out for any that disappears without going through the supernova stage any

telligent star

—

that just blanks out.

Who knows? An

even more

consideration,

I

thought can be expounded. From an energy said that a human population of six quintillion might grisly

be possible.

On

the other hand, the total population of the asteroids, at an Ameri-

can population density, was calculated at a mere ten trillion. Population could still increase 6oo,ooo-fold, but where would they find the

room?

An

increase in the density of the population

might seem undesirable and, instead, the men of the asteroids might cast envious eyes on other worlds. Suppose they considered a satellite like Saturn’s Phoebe, with

its

about

estimated diameter of 200 miles.

tw'O

hundred small

It

could be broken up into

asteroids with a diameter of

miles each.

Instead of one satellite with a surface area of 120,000 square miles, there w'ould be numerous asteroids with a total internal area of 400,000,000 square miles.

The

gain might not be great

\^•ith

out might be carried on upon that Still,

what about the moon,

to the

outermost skin?

wffiere

Phoebe, for considerable hollowing satellite

even wdiile

it

was

intact.

hollowing w^ould have to be confined

ASIMOV ON ASTRONOMY

88

has a greater mass than

It

were broken up,

it

the asteroids put together, and

all

would form 200,000

asteroids of 35-mile diameter.

At

a stroke, the seating capacity, so to speak, of the

be

tripled.

solar system will

be broken into fragments

for the use of

But, of course, earth would be in a special original

home

Once

all

of the

human

class.

race

would

would be the

It

it

intact.

the bodies of the solar system, except for the gas giants and

are broken

then reach the

up,

the

maximum

But, and here

total

number

is

it

the crucial point, Pluto

isn’t suitable for

quite distant.

Is it

would be

asteroids

of

human

in-

population can

that the energy supply will allow.

may

is

breaking up into asteroids. Then, too,

it

possible that

its

it

is

nature. Perhaps

its

too far away for energy to be

transmitted efficiently from the solar stations to asteroids that can be created

For

offer difficulties.

makeup

one thing, we aren’t too certain of such that

mankind.

and sentiment might keep

race,

creased roughly ten-million-fold, and the total

is

human

can envisage a future in which, one by one, the worlds of the

One

earth,

it

if

all

from Pluto, out four

the millions of

billion miles

from

the sun?

Pluto

If

is

ignored, then there

is

only one way in which mankind can

and that would be to use the earth. I can see a long drawn-out campaign between the Traditionalists and the Progressives. The former would demand that the earth be kept as a museum of the past and would point out that it was not important reach

its full

reach

to

potential,

full

potential population,

that a

few

trillion

more

or less

people didn’t matter.

The

Progressives

would

insist that

the earth was

made

for

man and

not vice versa, that mankind had a right to proliferate to the maximum, and that in any case, the earth was in complete darkness because the solar stations bet\\’een itself tion, so that I

it

and the sun soaked up

could scarcely serve as a

realistic

virtually all radia-

museum

have a feeling that the Progressives would,

of the past.

in the end, win,

and

I

the curtain as the advancing work-fleet, complete with force beams, prepares to make the preliminary incision that will allow the pull

down

earth’s internal heat to tion.

blow

it

apart as the

first

step in asteroid forma-

7

ROUND AND ROUND AND

Anyone who tually

writes a

book on astronomy

eomes up against the problem of

Moon always presents one face to To the average reader who has fore

and who

is

trying

come up

is

to

explain

is

that

the

nevertheless rotating.

against this problem be-

impatient with involved subtleties, this

tradiction in terms. It

.

for the general publie even-

the earth, but

not

.

.

easy to accept the fact that the

is

a clear con-

Moon

always

presents one face to the Earth because even to the naked eye, the

shadowy blotches on the Moon’s surface are alwavs found in the same position. But in that case it seems clear that the Moon is not rotating, for

if

it

were rotating we would,

bit

by

bit,

see every portion of

its

surface.

Now

it

is

no use smiling gently

at the lack of sophistication of the

average reader, because he happens to be right. tating with respect to the observer

astronomer says that the

Moon

is

The Moon

on the Earth’s rotating,

surface.

is

not

When

ro-

the

he means with respect to

other observers altogether.

Eor instance,

if

one watches the

Moon

over a period of time, one

ASIMOV ON ASTRONOMY

90

can see that the line marking gresses steadily

the

Moon

around the Moon; the Sun shines on every portion of

means that

in steady progression. TKi's

surface of the

seem to be

from the shadow pro-

off the sunlight

Sun (and there

an observer on the

to

are very few of those), the

rotating, for the observer would, little

portion of the Moon’s surface as

it

by

Moon would

little,

see every

turned to be exposed to the sunlight.

But our average reader may reason to himself as follows: “I see only one face of the Moon and I say it is not rotating. An observer on the Sun sees all parts of the Moon and he says it is rotating. Clearly, I am

more important than the Sun observer doesn’t, and, secondly, even fore, I insist

on having

little

more

rotation of the Earth

One

thing

the Earth

is

far as

since that

itself,

not rotating.

is

you stay

in

civilized

There-

to

do

so, let’s start

with the

a point nearer to all our hearts.

To an

observer on the Earth,

one place from now

till

dooms-

one portion of the Earth’s surface and no other. As

you are concerned, the planet

most of

I

And

to begin with:

If

and he

exist

I

of this confusion, so let’s think things

systematically.

we can admit

day, you will see but

existed,

my way.

way out

ITiere has to be a

through a

it

firstly,

am me and he isn’t. The Moon does not rotate!”

he

if

since,

human

“reality” (whatever that

standing

still.

even the wisest of

history,

may

is

Indeed, through

men

insisted that

be) exactly matched the appearance and

CIRCLING STARS To and

a casual observer,

set in the west,

see that this

is

but

might seem that the

it

doesn't take

much

stars rise in the east

intentness of study to

an oversimplification.

There are some

New

it

{as seen

stars

York City) that neither

from the latitudes

rise

of, say,

Athens or

nor set but are always in the sky.

The most prominent stars that do so are the seven second-magnitude stars that make up what we call the Big Dipper. They are always to be seen in the northern half of the sky, sometimes farther above the horizon, sometimes closer to the horizon, but never below the horizon. Closer study circling

still

some point

will

show

that the stars of the Big

in the sky

—and

that

all

other stars are doing so

as well. Stars close to that central point

move

do not intersect the horizon;

away move

that do, so that they

rise

and

stars farther

set.

Dipper are

in small circles that in large circles

photo; lick observatory

There are

stars that are closer to the central point

Big Dipper. The closest bright star a degree circle is

it

is

Polaris,

which

than those of the

is

only a

little

over

from the central point and makes small circles indeed. The makes is only a little over twice the width of the Moon. This

so small a circle that to the ordinary observer without instruments

to back

up

his

unaided eye,

it

seems to be motionless

in the sky

—

to

be always in the same position.

The Phoenicians may have been

the

first

Big Dipper always marked the north, and

by

to notice the fact that the

may have been encouraged

open sea instead of hugging the shore, possessing sure knowledge of this compass in the sky. The photograph is a time exposure which shows the stars circling this to venture out into

the Celestial North Pole. Polaris. Naturally, this

rotation about

its axis.

The

bright arc quite close to the center

motion of the

stars

is

is

the result of the Earth’s

)

ASIMOV ON ASTRONOMY

92

that the Earth ‘heally” did not rotate. As late as 1633, Galileo found V

himself in a spot of trouble for maintaining otherwise.

But suppose we had an observer on

(for simplicity’s

a star situated

sake) in the plane of the Earth’s equator; or, to put

it

another way, on

the celestial equator. Let us further suppose that the observer was

equipped with a device that made Earth’s surface in detail.

by

for little

By

eyes.

and

To

him,

it

possible for

it

taking note of

to study the

would seem that the Earth

he would see every part of

little

him

its

surface pass before his

you

particular small feature (for example,

some

standing on some point on the equator) and timing

I

rotated,

its

return,

could even determine the exact period of the Earth’s rotation-^that as far as

We

he

is

he is,

concerned.

can duplicate his

feat, for

when

the observer on the star sees us

exactly in the center of that part of Earth’s surface visible to himself,

we

in turn see the observer’s star directly overhead.

we could time

just as

he

of ourselves to that centrally located

would time the periodic return position, so

And

the return of his star to the overhead point.

The

period determined will be the same in either case. (Let’s measure

this

time

in

A

minutes, by the way.

seconds, where

second

1

minute can be defined

as

60

equal to 1/31,556,925.9747 of the tropical

is

year.

The

period of Earth’s rotation with respect to the star

1,436 minutes.

motion of the Earth, as

is

word

doesn’t matter which star

we

just

use, for the

about

apparent

with respect to one another, as viewed from the

stars

so vanishingly small that the constellations can be considered

moving

The

It

is

all in

one

piece.

period of 1,436 minutes

“sidereal”

comes from

is

called Earth’s “sidereal day.”

a Latin

word

for “star,”

The

and the phrase

therefore m^ans, roughlv speaking, “the star-based day.”

Suppose, though, that If

we were

considering an observer on the Sun.

he were watching the Earth, he,

too,

would observe

the period of rotation would not seem the same to server

on the

star.

Our

solar observer

close enough, in fact, for Earth’s

new

would be much

it

him

the star’s observer), the Earth would have

but

as to the ob-

closer to the Earth;

motion about the Sun

factor. In the course of a single rotation of the

rotating,

to introduce a

Earth (judging by

moved an

appreciable

dis-

tance through space, and the solar observer would find himself viewing the planet from a ditfc'"

more minutes before

it

it

angle. Tlie Earth

adjusted

itself to

the

would have

new

to turn for four

angle of view.

ROUND AND ROUND AND

We

.

93

.

could interpret these results from the point of view of an ob-

on the Earth.

server

server, w’G

To

duplicate the measurements of the solar ob-

on Earth would have

to

one passage of the Sun overhead

measure the period of time from (from noon to noon, in

to the next

other words). Because of the revolution of the Earth about the Sun, the Sun seems to stars.

move from west

background of the

to east against the

After the passage of one sidereal day, a particular star would have

returned to the overhead position, but the Sun would have drifted

eastward to a point where four more minutes would be required to it

pass overhead.

The

solar

day

make

therefore 1,440 minutes long, 4 min-

is

utes longer than the sidereal day.

Next, suppose to the Earth

some

we have an

observer on the

and the apparent motion

Moon. He

is

even closer

of the Earth against the stars

is

him than for an observer on the Sun. Therefore, the discrepancy between what he sees and what the star thirteen times greater for

observer sees

about thirteen times greater than

is

is

the Sun/star dis-

crepancy. If

we

we would be

consider this same situation from the Earth,

measuring the time between successive passages of the overhead.

The Moon

drifts

thirteen times the rate the

Moon

exactly

eastward against the starry background at

Sun

does. After

one

sidereal

day

is

com-

we have

to wait a total of 54 additional minutes for the Moon to pass overhead again. The Earth’s “lunar day” is therefore 1,490 min-

pleted,

utes long.

We could

also figure out the periods of Earth’s rotation with respect

to an observer satellite,

on Venus, on

and so on, but

summarize the

little

Jupiter,

shall

I

on Halley’s Comet, on an

have mercy and

we do have

in

Table

refrain.

We

artificial

can instead

17.

liable ly

day

sidereal solar

minutes

day

minutes

lunar day

By now real

it

may seem

1,490 minutes

reasonable to ask; But which

is

the day?

The

day?

The answer

to that question

that the question

is

but quite unreasonable; and that there

is

is

not a reasonable

one

at all,

real

period of rotation, lliere are only different apparent periods, the

lengths of which depend

unon the position

no

real day,

of the observer.

To

no

use a

ASIMOV ON ASTRONOMY

94

prettier-sounding phrase, the length of the period of the earth’s rota-

depends on the frame of reference, and

tion

all

frames of reference are

equally valid.

But

Not

if all

we

frames of reference are equally valid, are

Frames of reference may be equally

at all!

one

usually not equally useful. In

respect,

left

valid,

nowhere? but they are

one particular frame of

ref-

may be most useful; in another respect, another frame of reference may be most useful. We are free to pick and choose, using now one, now another, exactly as suits our dear little hearts. erence

For instance,

I

actually that’s a its

orbit

lie.

Because the Earth’s

and because the Earth

times farther (so that

day

solar

is

sometimes

a little shorter. If

apart

Sun

all

times

it

is

when

you mark

will

axis

tipped to the plane of

is

faster,

now

slower in

orbit), the

its

than 1,440 minutes and sometimes ‘‘noons” that are exactly 1,440 minutes

a little longer

overhead

it

1,440 minutes long but

is

sometimes closer to the Sun and some-

moves now

off

through the year, there

will pass

day

said that the solar

will

be times during the year when the

minutes ahead of schedule, and other

fully 16

pass overhead fully 16 minutes behind schedule.

Fortunately, the errors cancel out and by the end of the year

all is

even

again.

For that reason

it is

not the solar day

but the average length of

all

itself

that

is

1,440 minutes long,

the solar days of the year; this average

is

“mean solar day.” And at noon of all but four days a year, it is not the real Sun that crosses the overhead point but a fictitious body called the “mean Sun.” The mean Sun is located where the real Sun would be if the real Sun moved perfectly evenly. Tlie lunar day is even more uneven than the solar day, but the sidethe

real

day

is

really steady affair.

A

particular star passes overhead every

1,436 minutes virtually on the dot. If

we’re going to measure time, then,

day

is

day

is

the most useful, since

it is

it

seems obvious that the sidereal

the most constant.

Where

the sidereal

used as the basis for checking the clocks of the world by the

passage of a star across the hairline of a telescope eyepiece, then the

Earth

itself, as it

erence clock.

rotates with respect to the stars,

The second can then be

day. (Actually, the length of the year of the sidereal day,

which

is

why

as a fraction of the tropical year.)

is

serving as the ref-

defined as 1/1436.09 of a sidereal is

even more constant than that

the second

is

now

officially

defined

ROUND AND ROUND AND The

uneven

solar day,

as

it

.

95

one important advantage.

carries

is,

.

It

based on the position of the Sun, and the position of the Sun deter-

is

mines whether

a particular portion of the

In short, the solar day

is

in light or in

The

man

average

throughout history has

unmoved by the position of the stars, and when one of them moved overhead; but he

to remain

have cared

less

shadow.

equal to one period of light (daytime) plus one

is

period of darkness (night).

managed

Earth

couldn't certainly

couldn’t help noticing, and even being deeply concerned, by the fact that

it

might be day or night

noon or

moment;

important day of

was the original

all. It

by

is

far the

basis of

minutes (and 24 times 60

On

day).

lunar day

number

1,440, the

this basis, the sidereal

day

is

is

divided into 60

of minutes in a solar

23 hours 56 minutes long and the

24 hours 50 minutes long.

is

So useful

tomed

is

most useful and

time measurement and

divided into exactly 24 hours, each of which

is

sunrise or sunset;

twilight.

the solar day, therefore, which

It is

it

at a particular

is

mankind has become

the solar day, in fact, that

to thinking of

accus-

and of thinking that the Earth 'Veally" rotates in exactly 24 hours, where actually this is only its apparent rotation with respect to the Sun, no more ‘heal" or “unreal" than

its

as the day,

it

apparent rotation with respect to any other body.

It is

no more

“real" or “unreal," in fact, than the apparent rotation of the Earth with

respect to an observer

on the Earth— that

is,

to the apparent lack of ro-

tation altogether.

Tlie lunar day has

minutes

day

its

seconds every hour,

5

basis. In that case,

exactly twice a day

hour

it

we would

and

intervals (with

And

uses, too. If

at the

minor

extremely useful

is

we

adjusted our watches to lose 2

would then be running on

find that high tide (or

low

a lunar

tide)

same times every day — indeed,

came

at twelve-

variations).

the frame of reference of the Earth

to wit, the assumption that the Earth

is

not rotating at

all.

itself;

In judging

a billiard shot, in throwing a baseball, in planning a trip cross-country,

we

never take into account any rotation of the Earth.

the Earth

is

Now we I

standing

still.

can pass on to the Moon. For the viewer from the Earth, as

said earlier,

finite length.

shift

We always assume

it

does not rotate at

Nevertheless,

all

so that

we can maintain

its

“terrestrial

that the

Moon

day"

is

rotates

of inif

we

our frame of reference (usually without warning or explanation so

ASIMOV ON ASTRONOMY

96

photo:

mount wilson and palomar

observatorii

\^ENUS Venus was the most before the telescope.

beautiful dot of light in the sky in the days

When

it

shone alone,

western sky as the twilight deepened, Star” to which any

Once

number

like

a diamond, in

was the spectacular ‘'Evening

it

of songs have been written.

the telescope was invented,

it

quickly

became the most

beautiful. Its beauty rests in

nearer to the

than any planet.

its

brightness,

it

partly because

it

is

is

Sun than any planet but Mercury, and nearer to us When Venus and Earth are on the same side of the

Sun, they are only 25 million miles apart. Moon comes closer to Earth than that.

Venus

which

frustrat-

made

ing of the planets, arid very largely for the very reasons that

is

the

bright,

however, because

it

No

sizable

its

thick

also possesses a

cover that reflects three quarters of the light that

has prevented us from ever seeing

body but the

surface.

falls

on

it,

and that

In the telescope

nothing but a featureless white body, and, because

it

is

cloud

it

is

closer to the

ROUND AND ROUND AND

.

.

97

that the reader has trouble following). As a matter of fact, we can shift our plane of reference to either the Sun or the stars so that not only can

the

Moon

With

be considered to rotate but to do so in either of two periods.

respect to the stars, the period of the

Moon’s

rotation

is

27

days,

7 hours, 43 minutes, 11.5 seconds, or 27.3217 days (where the day referred to is the 24-hour mean solar day). Tliis is the Moon’s sidereal day. It is also the period (with respect to the stars) of its revolution

about the Earth, so

almost invariably called the “sidereal month.”

it is

In one sidereal

month, the Moon moves about 3 of the length of its orbit about the Sun, and to an observer on the Sun the change in angle of viewpoint is considerable. The Moon must rotate for over two more da^s to make up for it. The period of rotation of the Moon with respect to the

Sun

is

the same as

respect to the Sun,

better out,

still,

it is

utes, 2.8

Of

and

this

called neither.)

The

solar

these two months, the solar

Sun.

like

It is

Moon’s

month

is

I

solar

day

shall shortly

point

are,

the

is

Moon depend

far

more

on the

useful to

or from full

Moon

Moon. More

frustration, for

to full

—the

when

closer, the thinner.

it

mankind

relative positions of

is

month, from

Moon.

the astronomy of the situation causes

only as a crescent

Venus’’

month

it

In ancient

show

to

close to us,

When we

see

it

we as

on the other side of the Sun, far away. The photographs you see here are at an intermediate stage, when Venus is at its ‘‘full

it

is

brightest.

Of

course, in the last

two decades, astronomers have begun

to in-

Venus with radar beams and with rocket vessels which have skimmed Venus or even made soft landings. We know much more about it now than we did only twenty years ago. We know that it is vestigate

enough to melt lead), dreadfully dry, with an unbreathable atmosphere of carbon dioxide that is dreadfully thick. There is much about it that resembles traditional descriptions Hell. dreadfully hot (hot

of

It

is

or,

29 days, 12 hours, 44 min-

therefore 29.5306 days, or one solar

new Moon,

to

Sun than we phases

called the

seconds long, or 29.5306 days long.

Moon and new Moon

it

may be

the solar month. (As a matter of fact, as

because the phases of the

see

period of revolution about the Earth with

its

odd, then, that one

given to the devil.

Roman name

for the planet, Lucifer,

was

ASIMOV ON ASTRONOMY

98

when the phases of the Moon were used to mark off solar month became the most important unit of time.

times,

the

the seasons,

Indeed, great pains were taken ’to detect the exact day on which

new Moons appeared

successive

kept.

It

was the place of the

in order that the calendar

and the

priestly easte to take care of this,

very word ''calendar,” for instance, eomes from the Latin

much ceremony. An assembly

word meaning

month was proclaimed

"to proclaim,” because the beginning of each

with

be aceurately

of priestly officials, such as those

might have proclaimed the beginning of each "synod.” Consequently, what I have been calling the

that, in ancient times,

month,

called a

is

month (the logical name) is, actually, called the "synodic month.” The farther a planet is from the Sun and the faster it turns with

solar

respect to the stars, the smaller the discrepancy

between

sidereal

its

day and solar day. For the planets beyond Earth, the discrepancy can be ignored.

Sun than the Earth the diserepaney Both Mercury and Venus turn one face eternally to the

For the two planets is

very great.

Sun and have no and have

They

solar day.

a sidereal

turn with respect to the stars, however,

day which turns out to be

as

long as the period of

about the Sun (again with respect to the stars).*

their revolution If

closer to the

the various satellites of the Solar System keep one face to their

primaries at

times, as

all

is

very likely true, their sidereal day

would be

equal to their period of revolution about their primary.

can prepare Table 18 (not quite like any I have ever the sidereal period of rotation for each of the 32 major

If this is so

seen) listing

I

bodies of the Solar System: the Sun, the Earth, the eight other planets

(even Pluto, which has a rotation figure, albeit an uncertain one), the

Moon, and

21 other satellites.

the period in minutes and satellite

number

I

shall

put the

For the sake of direct comparison

list

name

them

Fll give

in the order of length. After

of the primary in parentheses

each

and give

to represent the position of that satellite, counting

a

outward

from the primary.

These

figures represent the

time

it

takes for stars to

make

a

complete

from the frame of reference of an observer on the surface of the body in question. If you divide each figure by 720, you get circuit of the skies

the

number

of minutes

it

would take

a star (in the region of the body’s

Heavens! Since this article first appeared in January 1964, astronomers have found that both Mercury and Venus rotate with respect to the Sun after all. Mer*

cury’s period of rotation

is

59 days and that of

Venus

is

243 days.

)

ROUND AND ROUND AND

.

.

99

Table i8

ROTATION OF THE BODIES OF THE SOLAR SYSTEM SIDEREAL DAY

BODY

Venus

350,000*

lapetus (Saturn-8)

104,000

Mercury

82,000*

Moon

39,300

(Earth-1

Sun

35,060

Hyperion (Saturn-y)

30,600

Callisto (Jupiter-5)

24,000

Titan (Saturn-6)

23,000

Oberon (Uranus-5)

19,400

Titania (Uranus-4)

12,550

Ganymede

10,300

(Jupiter-4)

Pluto

8,650

Triton (Neptune-i)

8,450

Rhea (Saturn-5)

6,500

Umbriel (Uranus-3) Europa (Jupiter-3)

5^950

Dione (Saturn-4)

5,100 3,950

Ariel (Uranus-2)

3,630

Tethys (Saturn-3)

2,720

lo (Jupiter-2)

2,550

Miranda (Uranus-i)

2,030

Enceladus (Saturn-2)

U 975

Deimos (Mars-2) Mars

1,815

Earth

Mimas

(Saturn-i)

Amaltheia (Jupiter-i) Uranus Saturn Jupiter

Phobos

1,477

1,4^6

Neptune

*

(minutes)

1,350

948 720 645 614 590 460

The

length of the sidereal day for Venus and Mercury is given here correctly, in line with the new knowledge concerning their rotations. The figures given in the article

when

it first

appeared turned out, of course, to be wrong.

ASIMOV ON ASTRONOMY

lOO celestial

equator) to travel the width of the Sun or

Moon

as seen

from

the Earth.

On

Earth

or not.

On

a minute. rate,

that

itself, this

takes about

Phobos (Mars’s inner

The

stars will

'2

minutes and no more, believe

satellite),

it

takes only a

On

the

over half

be whirling by at four times their customary

while a bloated Mars hangs motionless in the sky.

would be

little

it

What

a sight

to see.

Moon, on

would take 55 minutes for a star the Sun. Heavenly bodies could be

the other hand,

to cover the apparent

width of

it

studied over continuous sustained intervals nearly thirty times as long as as an adis possible on the Earth. I have never seen this mentioned

vantage for a Moon-based telescope, but, combined with the absence of clouds or other atmospheric interference, it makes a lunar observatory

something for which astronomers ought to be willing to undergo rocket

On

trips.

would take 485 minutes or 8 hours for a star to travel the apparent width of the Sun as we see it. What a fix astronomers could get on the heavens there— if only there were no clouds. Venus,

it

BEYOND PLUTO

In the last two centuries, the Solar System was drastically enlarged three times; once

when Uranus was

was discovered

discovered in 1781; then

when Neptune

when Pluto was discovered in 1930. Are we all through? Is there no more distant planet to be discovered even yet? We can’t know for sure, but at least we can speculate. Tliat much is our fundamental human right. in 1846; finally

So— What might how

far

ought

it

to

a possible

Tenth Planet be

To

like?

be from the Sun? For the answer,

we’ll

begin with,

go back to

the eighteenth century.

Back a

scheme

He o,

in 1766, a

German

astronomer, Johann Daniel Titius, devised

to express simply the distances of the planets

did this by starting with a series

the next

3

from the Sun. of numbers, of which the first was

and each one following double the one before, thus: o, 3, 6, 12, 24, 48, 96, 192, 384,

768.

.

.

.

Tlien he added 4 to each term in the series to get the following: 4, 7, 10, 16, 28, 52, 100, 196, 388, 772.

.

.

.

ASIMOV ON ASTRONOMY

102

Now

represent Earth’s,

distance from the

mean

Sun

as lo

and

cal-

mean distance of every other planet in proportion. What happens? Well, we can prepare Table 19 listing Titius’s series of numbers and comparing them with the relative mean distances from the Sun of the six planets known in Titius’s time. Here’s how it would look:

culate the

Table 19

RELATIVE DISTANCE

titius’s

SERIES

PLANET

4

3-9

1 )

Mercury

7

7-2

2)

Venus

10

10.0

3)

Earth

16

15.2

4)

Mars

52

52.0

5)

Jupiter

100

95-4

6)

Saturn

28

When

Titius

first

except for another

wrote about

it

in

announced

this,

no one paid attention,

particularly,

German astronomer named Johann Bode. Bode

1772, banging the

Bode was much more famous than

drums hard on

its

behalf. Since

Titius, this relationship of planetary

distances has ever since been referred to as Bode’s law, while Titius re-

mains terity

profound obscurity. (This shows you can’t always trust to posfor appreciation either— a thought which should help sadden us in

moments of depression.) Even with Bode pushing, the series of numbers was greeted as nothing more than a bit of numerology, worth an absent smile and a that-wasfurther in our

fun-what-shall-we-play-next? But then, in 1781, an amazing thing hap-

pened.

German-born English astronomer, named Eriedrich Wilhelm Herschel (he dropped the Eriedrich and changed the Wilhelm to William

A

becoming an Englishman) was engaged that year in a routine sweeping of the skies with one of the telescopes he had built for himself. On March 13, 1781, he came across a peculiar star that seemed to show

after

a visible disc,

which actual

stars

do not do under the

greatest magnifica-

now either, for that matter). He returned and by March 19, he was certain that it was mov-

tion available at that time (or

to

it

night after night

ing with respect to the stars.

Well, anything with a could not be a

star, so it

and movement against the stars be a comet. Herschel announced the

visible disc

had

to

BEYOND PLUTO body

as a

comet to the Royal

Society.

103

But then,

he continued

as

his ob-

he eouldn’t help noting that it wasn’t fuzzy like a comet, but sharply ending dise like a planet. Moreover, after he had observed

servations,

had it

a

few months, he could calculate

for a

be not strongly

orbit,

its

and that turned out

to

elliptical like

And

the orbit of a planet.

the orbit of a comet but nearly eireular like the orbit lay far outside the orbit of Saturn.

So Herschel announced that he had diseovered

new

a

planet.

What

a sensation! Sinee the telescope

had been invented nearly two centuries number of new objects had been discovered; many new stars

before, a

and

several

new

both Jupiter and Saturn; but never, never, never in recorded history had a new planet been discovered. At one bound, Herschel became the most famous astronomer in the satellites for

Within a year he was appointed and six years after that he married

world.

private astronomer to

III,

a wealthy

even a move, ultimately defeated, to ‘‘Herschel.” (It

And

is

now

name

George

widow. There was

the planet he had discovered

ealled Uranus.)

yet the discovery was aeeidental

new. Uranus

is

actually visible to

and hadn’t even been really the naked eye as a very dim ‘‘star,”

was casually seen any number of times.^ Astronomers had seen it through teleseopes and on a number of occasions its position was even so

it

reported.

As

far

back

mer Royal, prepared —as a star.

as

1690 John Flamsteed, the

a star

map

in

first

which he carefully included Uranus

In short, any astronomer could have discovered

looked for

body

it.

to look

British Astrono-

And he would have had a good hint for and how fast he might expeet it

Uranus

as to

to

if

he had

what kind

move

of a

against the

he would have known its distance from the Sun in advance. Bodes law would have told him. The Bode’s law figure for the relative distance of the Seventh Planet (on an Earth-equals-10.0 scale) is 196 and Uranus’s aetual distance is 191.8. stars, for

Obviously, astronomers weren’t going to

Bode

make

this

mistake again.

law was suddenly the guide to fame and new knowledge and they w’ere going to give it all they had. To begin with, there was that s

missing planet between Mars and Jupiter. At least there

must be

the orbits of It

a missing planet, for Bode’s

Mars and

had to be searched

they realized

law had number 28 between

and no planet was known

to exist there.

for.

German astronomers planet. They divided the

In 1800, twenty-four effort to find the

Jupiter

now

set

up

a kind of

community

sky into twenty-four zones

photo; the bettmann archive

WILLIAM HERSCHEL Herschel was born in Hanover on November 15, 173S. Hanover was

which was then ruled by the King of England and in 1757 Herschel emigrated to England and remained there for the rest of his life. He was a musician by profession, and a good one, but he a

German

state

grew interested

in

astronomy and was

filled

with a burning desire to see

the heavens through a telescope.

Since he could not afford to buy a good telescope, he decided to build his own.

He and

his sister,

from Hanover), spent fantastic

Caroline

(whom

he brought over

effort to grind lenses. Since

he dared not

stop for meals (or did not choose to) William kept on grinding while

Caroline fed him.

By

the end the Herschels had the best telescopes in existence, and

with his It

own

was the

telescope Herschel discovered the planet Uranus in 1781.

first

new

planet ever discovered in historic times and at one

BEYOND PLUTO

105

and each member was assigned one zone. But alas for planning, efficiency, and Teutonic thoroughness. While they were making all possible preparations, an Italian astronomer, Giuseppe Piazzi, in Palermo, Sicily,

accidentally discovered the planet.

was named Ceres, after the tutelary goddess of

It

Sicily, and proved to be a small object only 485 miles in diameter. It turned out to be only the first of many hundreds of tiny planets (“planetoids”) discovered in

the region between

Mars and

numbers

by the way, were found by the German team of

2,

3,

and

4,

Jupiter in

the years since. Planetoids

astronomers within a year or two after Piazzi’s initial discovery, so teamwork wasn’t a dead loss after all. Ceres is far the largest of all the planetoids, however, so let’s concentrate on it. Its relative mean distance from the Sun is 27.7; Bode’s law, as I said, calls for 28.

No astronomer was

in the

mood

to question Bode’s law after that.

when Uranus’s motion in its orbit seemed to be a bit irregular, couple of astronomers, John Couch Adams of England and Urbain In fact,

a

J. J.

Leverrier of France, independently decided there

beyond Uranus with

must be

a planet

a gravitational pull

on Uranus that wasn’t being allowed for. In 1845 and 1846 they both calculated where the theoretical Eighth Planet ought to be to account for the deviations in Uranus’s motions.

They

did that by beginning with the assumption that

its dis-

tance from the Sun would be that which was predicted by Bode’s law.

stroke

doubled the size of the Solar system and made Herschel the most famous and successful astronomer in the world. it

Herschel did not

rest

on

Saturn.

He

by any means. He accepted a pension from the King and married a rich widow but financial security was no excuse to retire. He discovered hvo satellites of Uranus and two

new satellites of and found that

his laurels

studied stars that were very close together in a number of cases they were really “binary stars'

circling each other

about a

common

law of gravity held in the distant

center of gravity so that

Newtons

stars as well as in

our Solar system. located globular clusters of stars; huge halls of closely spaced stars; hundreds of thousands of them in one spherical array. He also counted the stars in selected portions of the sky and decided that they were arranged in a grindstone-shaped array. This was the first indication

He

of the existence of the shape of the Milky the concept of the Galaxy. He died in 1822.

Way

and the beginning

of

ASIMOV ON ASTRONOMY

io6

few more assumptions and both pointed to the same general position of the sky. And the Eighth Planet, Neptune, proved to be there, indeed.

A

Tlie only trouble was that

was at

It wasn’t; it

the wrong

made

to have been at relative distance 388

Neptune ought

basic assumption.

from the Sun.

turned out they had

it

was a

relative distance 301. It

little

matter of 800,000,000 miles closer to the Sun than it should have been, and with one blow that killed Bode s law deader than a dried herring. It

numer-

interesting piece of

went back to being nothing more than an

ology.

When, pected

it

Ninth Planet, Pluto, was discovered, no one exthe Bode’s-law distance predicted for the Ninth Planet

in 1931, the

to

be at

PLUTO Pluto proved extraordinarily

star is

even though

was

it

of distinguishing a planet by noting that in a telescope the former expands into a

The

searched for painstakingly.

from a

diificult to discover,

best

way

remains a dot of light and nothing more. so small that it remains a dot in the telescope

visible globe, while the latter

But what also.

if

a planet

In that case,

it

can be distinguished by the fact that

tive to the other stars.

Pluto

is

shows up

so small

is

The

closer

and so

it is

to Earth, the faster

it

moves.

from the Earth, however, that it and it moves very slowly. Because of

combination of properties it is exceedingly difficult from the surrounding stars all the more so because it

—

make

the telescopic magnification required to stars visible in its

rela-

distant

as merely a dot of light

this

moves

it

it

to distinguish is

visible

dim that makes many so

neighborhood.

The American astronomer Clyde Tombaugh

searched

taking two pictures of the same small part of the sky

for

on two

it

by

different

Such pictures might have anywhere from 50,000 to 400,000 stars on it. If the two plates were focused on a given spot on a screen in rapid alternation, the stars, forming the same pattern in both, would days.

be seen as stationary. position, there

If

any of the

would be a

was

stars

flicker as

one

really a planet that shifted

''star' flipped

back and forth.

February 18, iq^o, after almost a year of painstaking comparisons in many such pairs of photographs, Tombaugh found a star in

On

Gemini new and very

the constellation it

to

be a

that flickered. distant planet

A month and

it

of observation

showed

was named Pluto. The

photograph here shows the change in Pluto's position over a space of twenty-four hours. The change looks sizable because it was magnified by the great 200-inch telescope

at

Palomar.

BEYOND PLUTO

107

(the numbers of the planets are selected, by the way, by skipping the planetoids so that Mars is the Fourth and Jupiter the Fifth), and it wasn’t.

But now wait. There are four known bodies lying beyond Uranus and every one of them is odd, in one way or another. The four are Neptune and Pluto, plus Neptune’s

The oddness

two known

satellites,

Triton and Nereid.

Neptune is, of course, that it lies so much closer to the Sun than Bode s law would indicate. The oddness of Pluto is more complicated. In the first place it has the most eccentric orbit of

of

photo:

mount wilson and palomar observatories

ASIMOV ON ASTRONOMY

io8

any of the major planets. At aphelion

it

recedes to a distance of 4,567,-

000,000 miles from the Sun, while at perihelion

it

approaches to

a dis-

tance of a mere 2,766,000,000 miles. At perihelion it is actually an average of about 25,000,000 miles closer the Sun than is Neptune.

Right now, Pluto

approaching perihelion, which

is

will reach

it

in

century, Pluto 1989. For a couple of decades at the end of the twentieth bewill remain closer to the Sun than Neptune, then it will move out

yond Neptune’s

orbit,

heading toward

its

aphelion, which

it

will reach

in 2113.

A

second odd feature about Pluto

sharply to the ecliptic (which is

17 degrees, which

this tilt

is

much

is

is

that the plane of

its

orbit

the plane of the Earth’s orbit).

is

tilted

The

higher than that of any other planet.

tilt

It is

which keeps Pluto from ever colliding with Neptune. Although

their orbits

to cross in the usual two-dimensional representation of

seem

the Solar System, Pluto at the point of

is

millions of miles higher than

many

Neptune

apparent crossing.

Finally, Pluto

is

peculiar in

its size. It is

smaller than the four other outer planets.

3,600 miles in diameter, It

is

also

much

much

denser. In fact,

and mass,

it

resembles an inner planet such as Mars or Mercury

much more than

it

does any of the outer planets.

in size

Now

let’s

consider Neptune’s satellites.

One

of them, Nereid,

is

a

small thing, 200 miles in diameter and not discovered until 1949odd thing about it is the eccentricity of its orbit. At its nearest approach to Neptune,

it

comes

to within 800,000 miles of the planet, then

it

goes

swooping outward to an eventual distance of 6,000,000 miles at the other end of its orbit. Nereid’s orbit is by far the most eccentric orbit of any satellite in the Solar System. No planet or planetoid can compare with

it

either in that respect; only comets equal or exceed that eccentric-

ity.

In contrast to Nereid, Triton cess of 3,000 miles

Moon) and is

that

its

with a

orbit

is

is

a large satellite,

with a diameter in ex-

compared with the 2,160 mile diameter of the nearly circular orbit. The odd thing about it though (as

tilted sharply to

the plane of Neptune’s equator;

quite near to being perpendicular to that plane, in fact. Now, there are other satellites in the system with eccentric orbits tilted orbits.

Jupiter;

They include

the seven outermost satellites

and Phoebe, the ninth and outermost

it is

and

(unnamed)

of

satellite of Saturn. As-

tronomers agree that these outer satellites of Jupiter and Saturn are probably captured planetoids and not original members of the planetary

BEYOND PLUTO family.

The

members (such

original

including the giant

ter,

satellites,

109

as the five inner satellites of Jupi-

Ganymede,

lo, Callisto,

and Europa;

the eight inner satellites of Saturn, including the giant satellite Titan) all revolve in nearly circular orbits and in the plane of their

planet’s

equator. So, for that matter,

do the

the two small satellites of Mars.

small satellites of Uranus and the manner in which satellite sys-

five

From

tems are supposed to have originated, these

circular, untilted orbits

seem

inevitable.

Well, perhaps Nereid represents surprising that a planetoid belt, especially

toids, at

is

to

captured planetoid, although

a

be found so

What

captured too?

would an object

is

beyond the planetoid

far

one so large (there are not more than four or

most, that are as large as Nereid).

it

And

five

as for Triton,

as large as Triton

plane-

was

it

be doing wander-

ing around in the region of Neptune, getting captured?

Some astronomers have ing

some

Pluto,

suggested that a catastrophe took place, dur-

past age, in the neighborhood of Neptune.

which

is

much more

They

suggest that

nearly the size of a satellite than the size of

an outer planet, was originally indeed a satellite of Neptune. However, it was somehow jarred out of position and took up its present wild and eccentric but independently planetary orbit. The shock of that catastro-

phe may

have jarred Triton’s orbit into a strong But what was the catastrophe? That no one says. also

tilt.

The one

obvious sign of a possible catastrophe in the Solar System of course, the asteroid belt. There is no real evidence that there

is,

ever was

a single planet there,

there once

and that

induced by

its

but certainly

it is

tempting to believe that one was

exploded (due to the tidal forces within its crust next-door neighbor, the giant planet, Jupiter, perhaps). it

An

explosion which produced thousands of fragments of rock including Ceres, which is 485 miles in diameter, and three or four others of 100 miles in diameter or more would certainly be a catastrophe.

One

catch, however,

is

that the total mass of

all

the planetoids be-

tween Mars and Jupiter cannot possibly be more than a tenth that of Mars, or more than a fifth that of Mercury. It would still have been far

and away the smallest planet because

in the system.

Why

should that be?

Was

neighbor Jupiter gobbled up most of the raw materials for planet formation, leaving our mythical planet a pygniy. it

its

Or suppose

that just a fraction of the original planet remained in the space between the orbits of Mars and Jupiter after the explosion? What if

the

‘

4T2th planet” (we must

call it this since

Mars

is

the 4th and

ASIMOV ON ASTRONOMY

no

We

Jupiter the 5th) sent a large piece of itself flying far out into space. can imagine such a piece sailing far out beyond Jupiter, Saturn, Uranus;

being caught or seriously deflected by ‘Neptune.

Perhaps the piece was caught by Neptune in an odd orbit and became Triton, while Pluto, as Neptune’s original satellite, was knocked out into an independent but whimsical planetary orbit as a result. Or perhaps the piece of the 4%th planet was deflected into the planetary orbit,

becoming Pluto, while

perhaps

all

its

gravitational pull tilted Triton’s orbit.

three, Pluto, Triton,

and Nereid are fragments of the

Or

41/2^1

planet.

The

chief bother in

much

could send so

all this is

how an

explosion of the 4V2th planet

material far outward,

all in

one

direction.

Could

be that this was balanced by the sending of a roughly equal mass

it

in-

ward, toward the Sun?

own Moon.

This brings up the question of our

Moon

tilted to

is

the plane of

its

Like Triton, the

primary’s equator; not by as

much, but

moderately eccentric as well. Furthermore, the Moon is far too large for us. A planet the size of the Earth has no business with such a huge moon. Of the other inner planets, Mars has

by

a

good 18° and

two peewee

its

orbit

satellites of

have none at

is

no account whatever, while Venus and Mercury

all.

the mass of the Earth and no other satellite in the system even approaches a mass that large in comparison to its primary. qVzth Is it possible then that the inward-speeding fragment of the

The Moon

is

%o

planet was captured by the Earth and

admit, very unlikely— but speculation

became the Moon? is

free.

Suppose the

It

sounds,

Moon

I

frag-

approached Earth and underwent the stresses of our planet’s gravitational field. One piece of the fragment might have been slowed sufficiently to allow capture by the Earth, while the

ment

split

other

moved

up further

as

it

at a speed that allowed

it

to escape

from the Solar System

altogether.

Or

perhaps, to pile improbability

upon improbability,

this last piece

did not escape but was captured by the Sun, so to speak, and became Mercury, which has, next to Pluto, the most eccentric and the most tilted orbit of all If

the

Moon,

the major planets. Triton, Pluto, and Mercury are

with the debris of planetoids that are

is

a

lumped together

the original orbit, you

body which would be nearly twice as massive as Mars. respectable planet that would fit the qVzth position nicely.

would have This

left in

all

a

BEYOND PLUTO Of

course,

I

can't imagine

that Neptune's orbit

but what the

we

devil,

much

so

is

what

in all this

111

would account

closer to the

Sun than

it

for the fact

ought to be,

can't have everything. Let's just leave explana-

tions of the fine points to the astronomers selves

and continue to content ourwith the heady delight of ungoverned speculation. We can sup-

pose that

counted

all

the bodies beyond

Uranus form one complex,

to

be

as a single planet, in

remains what

it

ought to

which the average relationship to the Sun be, but in which the relationship of the indi-

vidual pieces has been confused by catastrophe. If

we

take the

be (thanks 10.0 basis,

Now

mean

to Pluto)

comes out

let s

distance of the whole complex, that turns out to 3,666,000,000 miles which, on the Earth-equals-

be 395.

to

make up

new and more complete

a

Titius series

as

in

Table 20: Table 20 TITIUS'S

RELATIVE DISTANCE

SERIES

4

3-9

1)

Mercury

7 10

7-2

2)

Venus

10.0

3) Earth

16

15.2

4) Mars

28

27.7

52

52.0

5) Jupiter

100

95-4

6) Saturn

196

191.8

388

395

are, then.

of the article, the it

would have

How

a

4V2) Ceres

7) Uranus

To

9) Neptune-Pluto

8,

?

772

There you

PLANET

10)

answer the question

Tenth Planet should be

mean

big would

it

Tenth Planet I

asked at the beginning

at position y’72

which means

distance of 7,200,000,000 miles from the Sun. be? Well, if we ignore the interloping Pluto

and

just consider the other four outer planets,

diameter as we move out from Jupiter.

we find a steady decrease in The diameters are 86,700 (Jupi-

ler),

71,500 (Saturn), 32,000 (Uranus) and 27,600 (Neptune). Carry that through and lets say the Tenth Planet has a diameter of 10,000 miles,

which makes

With

a nice

that diameter

and

round

figure.

at that distance

from the Sun (and from us)

ASIMOV ON ASTRONOMY

112

Tenth Planet ought to have an apparent magnitude of would make it rather brighter than the nearer but smaller the

would show' very

little disc,

13,

which

Pluto. It

but w4 iat disc there was w'ould be larger

than that of the nearer but smaller Pluto. Well, then, since Pluto has

been discovered and the presumably larger and brighter Tenth Planet

mean

has not, does that

Not of

the Tenth Planet does not exist?*

necessarily. Pluto w'as recognized

magnitude or brighter by the

its

Tenth

w'ould the

among

a veritable flood of stars

moved among them. So much slower rate. From Kepler’s period of revolution of the Tenth

fact that

Planet, but at a

third law', w'e can calculate that the

it

Planet w'ould be 680 years, nearly three times the length of Pluto’s period of revolution, and so the Tenth Planet would third the rate at full

year for the

full

Moon.

which Pluto moves against the

Tenth Planet

Tliis

is

to shift

its

The

of times

thing that strikes

utter isolation. It

its

is

me

as

it,

as

conditions, Pluto

a

it

easily

is

observed by a

has already been seen a

Uranus was.

most unusual about the Tenth Planet

is

twice as far from Neptune, at Neptune’s closest

we on Earth Pluto than we are. Once every

approach to

as

one

would take

stars. It

not the kind of motion that

and not noticed,

at only

position over the width of the

casual survey of the heavens. So perhaps

number

move

are.

Most

of the time,

it is

further

from

2,700 years, allowing the most favorable

would approach

w'ithin tw'O

and

a half billion miles

Tenth Planet (the distance from Earth to Neptune). Nothing barring a possible satellite or comet, would ever come within four

of the else,

and a half

billion miles of

it.

The Sun would have no It

would seem completely

discernible disc to the

starlike

and no

planet Mars appears to us at the time of

its

Sirius, If

our

full

Moon, and

eye, of course.

appearance than the

larger in

although the Sun would be but a point of sixty times as bright as

naked

closest approach. How'ever, light,

it

would

still

be over

a million times brighter

than

the next brightest object in the sky.

there were any sentient beings native to the

alone ought to

tell

them there

w'as

Tenth

Planet, that

something different about

this par-

In 1972, actually, calculations have been made from deviations in the orbit of Halley’s comet that aren’t accounted for by the known planets. These calculations *

Tenth Planet may indeed exist with properties rather like those described in this article, which was written in early 1960. The Tenth Planet, however, hasn’t been seen and later calculations make its existence at least on the Halley’s comet basis unlikely. show that

a

—

—

BEYOND PLUTO ticular star.

Furthermore,

the Sun eonstantly,

As

if

if

^13

they watehed elosely, they would see that

slowly, shifted position against the other stars.

to the planets, all the

known members

of the Solar System, as seen

from the Tenth Planet, would seem to hug the Sun. Even Pluto, viewed from so far beyond its own orbit, would never depart more than 40°

from the Sun, even when

maximum Sun

at all

it

happened to be

at aphelion at the time of

elongation. All other planets would remain far eloser to the times.

As seen from the Tenth Planet, Mercury and Venus would never be more distant from the Sun than the diameter of our full moon. The Earth would recede, at times, to a distance that was at most half again the width of the full moon and Mars would periodically recede to a distance twice the width of the

full

moon.

the absence of an obscuring atmosphere, in the brilliance of the point-size

Tenth Planet without

special

all

feel certain that,

I

even in

four planets would be lost

Sun and w^ould never be seen from the

equipment.

Tliat leaves only the five outer planets, Jupiter, Saturn, Uranus, Neptune, and Pluto. They w'ould be best seen when w^ell to one side of the

Sun

at

wTich time they w’ould show up

(in telescopes) as fat crescents.

In that position, Jupiter, Saturn, Uranus, and

Neptune would

all

be at

roughly the same distance from the Tenth Planet. Pluto might, under favorable conditions, be rather closer than the rest. This means that, with the distance factor eliminated, Saturn w^ould be

dimmer than

Jupiter, since Saturn

smaller and

more distant from the Sun, hence less brightly illuminated. By the same reasoning, Uranus would be dimmer than Saturn, Neptune would be dimmer than Uranus, and Pluto dimmer than Neptune. is

In fact, Uranus, Neptune,

and Pluto, although approaching more closely to the Tenth Planet at inferior conjunction, than do Jupiter and Saturn, would be invisible to the naked eye. Jupiter and Saturn w^ould be the only planets visible from the Tenth Planet without special equipment and they would be anything but spectacular.

At

its

brightest, Jupiter

would have

magnitude of something like 1.5, about that of the star Castor. And it would only be for a year or so, every six years, that it would approach that brightness, and then it would be only 4° from the Sun and probably not too easy to observe. As for Saturn, there would be tw^o-year periods every fifteen years

age

when

star.

it

might climb

That’s

all.

to a brightness of 3.5,

a

about that of an aver-

ASIMOV ON ASTRONOMY

114

Undoubtedly, any astronomers stationed on the Tenth Planet would completely ignore the planets.

Any

other world in the system would

them a better view. But they would watch the stars. Tlie Tenth Planet would offer them the largest parallaxes in the system, because of its mighty orbital sweep. (Of course, they would have to wait 340 years to get the full parallax.) Measurements of stellar distance by parallax, give

the most reliable of

all

methods

for the purpose, could

hundred times deeper into space than

One

last point.

What

is

now

be extended one

possible.

ought we to name the Tenth Planet?

got to stick to classical mythology by long and revered custom. the Ninth Planet

named

Pluto, there might be a temptation to

WeVe With name

the Tenth after his consort Proserpina, but that temptation must be sisted.

may

Proserpina

is

the inevitable

name

any

for

re-

satellite of Pluto's that

ever be discovered and should be rigidly reserved for that.

However, consider that the Greeks had souls of the

a

ferryman that carried the

dead across into Hades, the abode of Pluto and Proserpina.

name was Charon. There was also a three-headed dog guarding the entrance of Hades, and its name was Cerberus. My suggestion then is that the Tenth Planet be named Charon and that its first discovered satellite be named Cerberus. And then any interstellar voyager returning home and approaching

His

the Solar System on the plane of the ecliptic would have to cross the orbit of

What

Charon and Cerberus

to reach the orbit of Pluto

could be more neatly symbolic than that?

and Proserpina.

JUST MOONING AROUND

Almost every book on astronomy

have ever seen, large or small, contable of the Solar System. For each planet, there’s given its

tains a little

diameter,

Since

I

distance from the sun,

its

density, the

I

number

am

of

its

time of rotation,

its

its

albedo,

its

moons, and so on.

morbidly fascinated by numbers,

with the perennial hope of finding

new

I

jump on such

tables

items of information. Occasion-

am

rewarded with such things as surface temperature or orbital velocity, but I never really get enough. ally, I

So every once

in a while,

when

the ingenuity-circuits in

my

brain are

purring along with reasonable smoothness, I deduce new types of data for myself out of the material on hand, and while away some idle hours. (At least I did this in the long-gone days when I had idle hours.) I

can

form; so fashion,

still

do

come and

it,

however, provided

join

see

me and we

put the

I

will just

results into

moon around

formal essay-

together in this

what turns up.

Let’s begin this way, for instance

.

.

.

According to Newton, every object

in

the universe attracts every

ASIMOV ON ASTRONOMY

ii6

other object in the universe with a force (f) that

product of the masses

(irii

and m2)

is

proportional to the

two objects divided by the

of the

square of the distance (d) between "them center to center.

We

multiply

by the gravitational constant (g) to convert the proportionality to an

and we have:

equality,

gmima

(Equation 12)

-32

This means, for instance, that there

is

an attraction between the

Moon and

Earth and the Sun, and also between the Earth and the

be-

tween the Earth and each of the various planets and, for that matter, between the Earth and any meteorite or piece of cosmic dust in the heavens.

Sun

Fortunately, the

is

so overwhelmingly massive

compared with

everything else in the Solar System that in calculating the orbit of the Earth, or of any other planet, an excellent tained

in the Universe.

same way, the

In the

supposing that

it is

at this point that

on the

The

effect of other bodies

can be calculated

satellite

be worked out

orbit of a satellite can

alone in the Universe with

more massive than any tion

at-

is

minor refinements.

later for relatively

is

approximation

only the planet and the Sun are considered, as though they

if

were alone

It

first

something

even though

that satellite than the primary

it is

is?

me.

interests

planet, shouldn’t

its

by

first

primary. If

the Sun

is

so

much

exert a considerable attrac-

it

much greater distance from just how considerable is ‘"con-

at a

If so,

siderable”?

To each

put

it

another way, suppose with

satellite,

its

we

picture a tug of war going

for

planet on one side of the gravitational rope and

the Sun on the other. In this tug of war, I

on

how

well

is

the Sun doing?

suppose astronomers have calculated such things, but

I

have never

seen the results reported in any astronomy text, or the subject even discussed, so

Here’s

I’ll

will

primary

be

dg.

would be and

its

for myself.

can go about

it.

Let us

fp

the mass of a satellite m,

The

I

gravitational force

whole business.

I

between the satellite

mean

the planet

it

distance from the satellite

be dp and the distance from the

and that between the

that’s the

chapter.

will

The

call

primary (by which, by the way,

m^, and the mass of the Sun ms.

circles) its

it

how we

the mass of

to

do

satellite to

satellite

and

its

the Sun

primary

and the Sun would be

promise to use no other symbols

fs

—

in this

JUST MOONING AROUND From Equation a satellite

and

its

12,

we can

H-y

say that the force of attraction between

primary would be;

gmnip _ h

(Equation 13)

Up

while that between the same

satellite

and the Sun would be:

gmuis d

What we

(Equation 14)

2

are interested in

is

how

the gravitational force betw'een

and primary compares wdth that between satellite and Sun. In other words we want the ratio fp/fsy which we can call the '‘tug-of-war

satellite

value.’'

To

get that

we must

The

such a division would be:

result of

^p/fs= (nip/nis) In

divide equation 13 by equation 14.

making the

(Equation 15)

(ds/dp )2

division, a

number

of simplifications have taken place.

For one thing the gravitational constant has dropped out, wTich means we won’t have to bother wdth an inconveniently small number and some inconvenient units. For another, the mass of the

satellite

out. (In other words, in obtaining the tug-of-war value, ter how^ big or little a particular satellite

same

in

any

What we

is.

The

it

has dropped doesn’t mat-

result w'ould

be the

case.)

need

for the tug-of-war value (fp/fs),

of the planet to that of the sun (niplms)

the distance from satellite to

Sun

the ratio of the mass

is

and the square of the

to the distance

from

ratio of

satellite to pri-

mary (djdp)-. There are only

six planets that

have

satellites

and

these, in order of

decreasing distance from the Sun, are; Neptune, Uranus, Saturn, Jupiter, Mars, and Earth. (I place Earth at the end, instead of at the beginning, as natural chauvinism would dictate, for

my own

reasons.

You’ll find out.)

For these, we

will first calculate the mass-ratio

and the

results turn

out as in Table 21.

As you Jupiter,

see,

the mass ratio

which

thousandth

as

is

by

far

is

really heavily in favor of the

the most massive planet,

massive as the Sun. In

satellites, planetoids,

fact, all

not quite one-

the planets together (plus

comets, and meteoric matter)

than 1/750 of the mass of the Sun.

is

Sun. Even

make up no more

ASIMOV ON ASTRONOMY V

Table 21 RATIO OF

MASS OF PLANET TO MASS OF SUN

PLANET

So

far,

Neptune

0.000052

Uranus

0.000044

Saturn

0.00028

Jupiter

0.00095

Mars

0.00000033

Earth

0.0000030

then, the tug of war

is all

on the

side of the Sun.

However, we must next get the distance planet heavily, for each satellite

And

than

it is

ratio

must be squared, making

to the Sun.

we can be But

is,

ratio,

and that favors the

of course, far closer to

its

primary

what’s more, this favorable (for the planet) it

even more favorable, so that

in the

end

reasonably sure that the Sun will lose out in the tug of war.

we’ll check,

Let’s take

anyway.

Neptune

first. It

has two

Triton and Nereid.

satellites,

average distance of each of these from the Sun

is,

The

of necessity, precisely

the same as the average distance of Neptune from the Sun, which 2,797,000,000 miles.

The

Neptune is, Nereid from

average distance of Triton from

however, only 220,000 miles, while the average distance of

Neptune If

we

is

is

3,460,000 miles.

divide the distance from the

for each satellite

and square the

Sun by the distance from Neptune

result

we

get 162,000,000 for Triton

and 655,000 for Nereid. We multiply each of these figures by the massratio of Neptune to the Sun, and that gives us the tug-of-war value, in Table 22: Table 22

TUG-OF-WAR

The

SATELLITE

RATIO

Triton

8,400

Nereid

34

conditions differ markedly for the two

tional influence of

Neptune on

its

nearer

satellites.

satellite,

Triton,

The is

gravita-

overwhelm-

!

JUST MOONING AROUND greater than the influence of the firmly in Neptune’s grip.

is

tracted

by

Neptune

Hie

Sun on the same

outer

considerably,

119

satellite,

but

not

satellite.

Triton

Nereid, however,

is

overwhelmingly,

more

at-

strongly than by the Sun. Furthermore, Nereid has a highly eccentric orbit, the most eccentric of any satellite in the system. It approaches to

within 800,000 miles of Neptune at one end of as far as 6 million miles at the other end.

a variety of reasons

orbit

and recedes to

When

tune, Nereid experiences a tug-of-war value as

For

its

most distant from Neplow as 1 1

(the eccentricih^ of Nereid’s orbit, for one

thing) astronomers generally suppose that Nereid is not a true satellite of Neptune, but a planetoid captured by Neptune on the occasion of a too-closc approach.

Neptune

s

weak hold on Nereid

certainly seems to support this. In

from the long astronomic view, the association between Neptune and Nereid may be a temporary one. Perhaps the disturbing effect of fact,

the solar pull will eventually snatch

it

out of Neptune’s grip. Triton, on

the other hand, will never leave Neptune’s

company

short of

some

ca-

tastrophe on a System-wide scale.

There’s no point in going through for all the satellites. sults.

I’ll

all

do the work on

Uranus, for instance, has

plane of Uranus’s equator and

five all

the details of the calculations

my own

known

and feed you the

satellites, all

revolving in the

considered true satellites by astrono-

mers. Reading outward from the planet, they are: Miranda, Ariel, briel, Titania, and Oberon.

The

re-

Um-

tug-of-war values for these satellites are given in Table 23:

Table 23

TUG-OF-WAR SATELLITE

RATIO

Miranda

24,600

Ariel

9,850

Umbriel

4750

Titania

1,750

Oberon

1,050

All are safely

and overwhelmingly

war values

with their status as true

fit

We pass on, *

so

it

The

in

Uranus’s grip, and the high tug-ofsatellites.

then, to Saturn, which has ten satellites: Janus,

satellite Janus was discovered four years after this article was wasn’t included. I am adding it now.

first

Mimas, published,

ASIMOV ON ASTRONOMY

120

Enceladus, Tethys, Dione, Rhea, Titan, Hyperion, lapetus, and Phoebe.

Of

these, the eight

innermost revolve

and are considered true clined orbit

The

and

is

in the

plane of Saturn’s equator

the ninth, has a highly in-

satellites. Plioebe,

considered a captured planetoid.

tug-of-war values for these satellites are given in Table 24:

Table 24

TUG-OF-WAR RATIO

SATELLITE Janus

23,000

Mimas

15,500

Enceladus

9,800

Tethys

00

Dione

4-150

Rhea

2,000

Titan

00

260

Hyperion

Note the low value

lapetus

45

Phoebe

3

for

first five:

Amaltheia,

lo,

them in two installments. Europa, Ganymede, and Callisto, all reand

I’ll

take

volve in the plane of Jupiter’s equator and satellites.

The

1/2

Phoebe.

Jupiter has twelve satellites

The.

0

all

are considered

true

tug-of-war values for these are given in Table 25:

Table 25

TUG-OF-WAR

and

all

SATELLITE

RATIO

Amaltheia

18,200

lo

3,260

Europa

1,260

Ganymede

490

Callisto

160

are clearly in Jupiter’s grip.

more satellites which have no official commonly known by Roman numerals (from

Jupiter, however, has seven

names, and which are

VI

to

XII) given

in the order of their discovery. In order of distance

from Jupiter, they are VI, X, VII, XII, XI, VIII, and IX, All are small

JUST MOONING AROUND and with

orbits that are eccentric

Jupiter

equator.

s

(Jupiter

is

Astronomers

121

and highly inclined

to the plane of

them captured planetoids. more massive than the other planets and is nearer the so it is not surprising that it would capture seven plane-

far

planetoid belt,

consider

toids.)

The

tug-of-war results for these seven certainly bear out the captured planetoid notion, as the values given in Table 26 show:

Table 26

TUG-OF-WAR SATELLITE

Jupiter

s

grip

to

VI

4-4

X

4-3

VII

4.2

XII

1-3

XI

1.2

VIII

1.03

IX

1.03

on these outer

Mars has two

satellites

feeble indeed.

is

Phobos and Deimos, each tiny and very close the plane of Mars’s equator, and are considered

satellites,

Mars. They rotate

true satellites.

RATIO

in

The

tug-of-war values are given in Table 27. So far I have listed ji satellites, of which 22 are considered true satellites and 9 are usually tabbed as (probably) captured planetoids.* I

would

like, for

ellite,

which happens

moment,

the

to

to leave out of consideration the

be our own

promise) and summarize the

31 in

Moon

get back to

(Fll

sat-

it,

Table 28

TUG-OF-WAR

NUMBER OF SATELLITES

SATELLITE

RATIO

PLANET

TRUE

CAPTURED

Phobos

195

Neptune

1

1

Deimos

32

5

0

Uranus Saturn

9

Jupiter

Mars I

am

including Janus in the appeared.

article first

I

Table 28:

Table 27

*

32nd

list,

remember, although

it

1

5

7

2

0

was unknowm when the

ASIMOV ON ASTRONOMY

122

And now

let's

analyze this

list

the true satellites the lowest tug-of-war value

On

among

the other hand,

highest tug-of-war value

Let us accept 30

is

member

for a true satellite

value will be found.

and

of a planet

We

then solving for

dp.

That

can expect to find a true Pluto, for

is

its

we can

distance from the Sun,

which

at

this tug-of-war

can use Equation 15 for the purpose, setting

known

equal to 30, putting in the

way

satellite

captured and probably tem-

from the planet’s center

calculate the distance

in this

and that any

figure

of the planet’s family.

Knowing the mass

fp/fs

and assume that the tug-of-war

in all likelihood, a

is,

32.

that of Nereid with an average of 34.

minimum

with a lower figure

that of Deimos,

is

the rttne satellites listed as captured, the

this state of affairs

a reasonable

porary

is

Among

in terms of tug-of-war values.

be the

will

satellite.

The

values for

maximum

rrip,

rris,

and

and

dg,

we

distance at which

only planet that can’t be handled

which the value

of

very uncertain, but

irip is

I

omit Pluto cheerfully.

We

can also

set a

minimum

distance at which

satellite; or, at least, a true satellite in

lated that

a true satellite

if

tance, tidal forces will break

ments already

exist at

it

its

expect a true

has been calcu-

It

primary than a certain

up into fragments. Conversely,

if

dis-

frag-

such a distance, they will not coalesce into a

single body. This limit of distance

named

for the astronomer E.

Roche

limit

is

the usual form.

closer to

is

we can

a distance

is

Roche,

called the ‘'Roche limit”

who worked

it

and

out in 1849.

is

The

from a planetary center equal to 2.44 times

the planet’s radius.”' So, sparing

you the actual calculations, here are the

results in

Table

29 for the four outer planets: Table 29

DISTANCE OF TRUE SATELLITE

(miles

PLANET

*

size

MAXIMUM

MINIMUM

(tug-of-war =: 30)

(roche limit)

Neptune

3.700.000

38.000

Uranus

2 . 200.000

39.000

Saturn

2,700,000

87.000

Jupiter

2,700,000

106,000

The Roche and

from the center of the primary)

limit only holds true exactly for satellites of

a few other qualifications but

we don't have

more than

a certain

to worry about that here.

JUST MOONING AROUND As you

each of these outer planets, with huge masses and far distant from the competing Sun, has ample room for large and comsee,

plicated satellite systems within these generous limits, satellites all fall

and the 22 true

within them.

—

Saturn does possess something within Roche’s limit its ring system. The outermost edge of the ring system stretches out to a distance of 85,000 miles from the planet’s center. Obviously the material in the rings could

have been collected into

a true satellite

had not been so

if it

near Saturn.

The

ring system

unique

is

as far as visible planets are concerned,

of course the only planets

tem. Even of these, tion with satellites

we can see are those of our own the only ones we can reasonably consider (Ell explain why in a moment) are the

but

Solar Sysin

connec-

four large

ones.

Of Its

these, Saturn has a ring system

innermost

planet

s

Amaltheia,

satellite,

center, with the

Roche

and Jupiter is

about 110,000 miles from the

limit at 106,000 miles.

miles inward and Jupiter would have rings.

suggestion therefore that once

systems

we

will discover

half the large planets

we

find will

would

I

A

few thousand

like to

make

the

reach outward to explore other stellar

(probably to our

we

just barely misses one.

initial

amazement) that about

be equipped with rings after the fash-

ion of Saturn.

Next we can

the inner planets are,

and much

do the same thing for the inner planets. Since one and all, much less massive than the outer ones

try to

competing Sun, we might guess that the range of distances open to true satellite formation would be more limited, and w'e would be right. Here are the actual figures in Table 30 as I have closer to the

calculated them:

Table 30

DISTANCE OF TRUE SATELLITE

(miles

from the center of the primary)

MAXIMUM planet

(tug-of-war= 30)

MINIMUM (

ROCHE limit)

Mars

1

5,000

5.150

Earth

29,000

9,600

Venus Mercury

19,000

9,200

00

3,800

N.

— ASIMOV ON ASTRONOMY

124

Thus, you

see,

million miles or tion

more

far

is

where ,each of the outer planets has a range of two

more within which

true satellites could form, the situa-

Mars and Venus have

restricted for the ifiner planets.

a permissible range of but io,oco miles. Earth does a little better, with

20,000 miles.

Mercury it

is

the most interesting case.

satellite against

can expect to form a natural

petition of the nearby

from

that,

if

my

Sun

reasoning

well within the

is is

The maximum

the overwhelming com-

Roche

limit. It follows

Mercury cannot have

correct, that

a true

a possible spattering of gravel

and that anything more than

satellite,

distance at which

is

not to be expected. In actual truth, no satellite has been located for

know, nobody has endeavored

as

I

it

as

knowledge of astronomic

me

that

I

have been anticipated

my

fact. If

detail

in this,

the news philosophically. At the very

screaming to the privacy of

as far

to present a reason for this or treat

anything other than an empirical

a greater

Mercim^ but,

any Gentle Reader, with will write to tell

than myself,

and by whom,

least,

I

will try to take

I

my

will confine

kicking and

study.

Venus, Earth, and Mars are better

off

than Mercury and do have a

beyond the Roche limit. It is not much room, however, and the chances of gathering enough material over so little

room

for true satellites

small a volume of space to

make anything but

a very tiny satellite

is

minute.

And,

as

it

happens, neither Venus nor Earth has any

satellite at all

(barring possible minute chunks of gravel) within the indicated limits,

and Mars has two small satellites, each scarcely deserve the name. It is

less

than 20 miles across, which

amazing, and very gratifying to me, to note

such delightful sense, and satellite

how

well

I

for;

one

all this

makes

can reason out the details of the

systems of the various planets.

thing remains unaccounted

how

It

is

trifling

such a shame that one small thing

I

have ignored so

far,

but

WHAT IN BLAZES

IS

OUR OWN MOON DOING WAY OUT

THERE? It’s

too far out to be a true satellite of the Earth,

beautiful chain of reasoning It’s

— which

is

too beautiful for

too big to have been captured by the Earth.

capture having been effected and the

if

Moon

The

me

to abandon.

chances of such a

then having taken up a

nearly circular orbit about the Earth are too small to eventuality credible.

we go by my

make such an

:

JUST MOONING AROUND T here are theories, of course, to the

much

my

closer to the Eartli (within

and then gradually moved away

lite)

have an objection to

that. If the

125

Moon

effect that the

permitted limits for a true

as a result of tidal action.

Moon

were a true

had

most

certainly be orbiting in the plane of Earth’s equator

circled Earth at a distance of, say, 20,000 miles,

the

Moon

is

captured one, what

is it?’^

This

and

if

satel-

Well,

would

it

and

neither a true satellite of the Earth nor a

may

surprise you,

but

have an answer;

I

what that answer is, let’s get back to my tug-of-war terminations. There is, after all, one satellite for which I have not it,

and that

is

and the average distance of the

Moon — Sun

Moon Moon

from the Earth from the Sun

Sun was given

The

is

ratio of the

earlier in the

is

cal-

2 yj,ooo miles,

93,000,000 miles.

Moon — Earth

distance to the

392. Squaring that gives us 154,000. to that of the

de-

our Moon. We’ll do that now.

Tlie average distance of the

Idle ratio of the

al-

it isn’t.

to explain

culated

I

that origi-

satellite

nally

But, then,

was once

distance

is

mass of the Earth

chapter and

is

0.0000030.

Multiplying this. figure by 154,000 gives us the tug-of-war value, presented in Table 31 Table 31

TUG-OF-WAR

The Moon, System

Sun

in other

in that its

attracts the

SATELLITE

RATIO

Moon

0.46

words,

is

unique among the

Moon

the

Moon,

then, as neither a true satellite of the

as a planet in its

the Sun in careful step with the Earth.

Earth-Moon system, the simplest way and

*

This

own

be

right,

sure,

from within the

of picturing the situation if

is

you were to draw a

to

pic-

is

about the Sun exactly to scale, everywhere concave toward the

always ''falling” toward the Sun. All the other was written

moving about

Moon

you would see that the Moon’s orbit It is

To

revolve about the Earth; but

ture of the orbits of the Earth

Sun.

The

twice as strongly as the Earth does.

Earth nor a captured one, but

Moon

the Solar

primary (us) loses the tug of war with the Sun.

We might look upon

have the

satellites of

satellites,

with-

was hoped that once the Moon was actually reached, a study of its surface might tell us how it originated and whether it was captured or not. So far, however, although the Moon has been visited several times, no answer has been obtained. The information we have received offers more puzzles than answers. article

in 1963. It

ASIMOV ON ASTRONOMY

126

out exception, “fair’ away from the Sun through part of their

orbits,

caught as they are by the superior pull of their primary— but not the

Moon.

And

consider this

— the Moon does not revolve about the Earth

plane of Earth’s equator, as would be expected of a true it

satellite.

in the

Rather

revolves about the Earth in a plane quite close to that of the ecliptic;

that

is,

to the plane in

Sun. This Is it

is

just

generally, rotate about the

which the planets,

what would be expected

possible then, that there

is

an intermediate point between the

situation of a massive planet far distant

about a single core, with numerous

of a planet!

from the Sun, which develops formed, and that of a small

satellites

planet near the Sun which develops about a single core with no satel-

Can

lites?

there be a boundary condition, so to speak, in which there

condensation about two major cores so that a double planet

Maybe Earth little

just hit the

too small, a

two halves

little

is

formed?

edge of the permissible mass and distance; a

too close. Perhaps

of the double planet

if

it

were better situated the

would have been more

both might have had atmospheres and oceans

of a size. Perhaps

and— life.

Perhaps in

other stellar systems with a double planet, a greater equality usual.

What a shame

if

is

we have missed

that

Or, maybe (who knows), what luck!

.

.

.

is

more

.

STEPPINGSTONES TO THE STARS

There’s something essentially unsatisfactory to me about the conquest of the Solar System which now seems to be at hand.* know too

We

much about what

and what we’ll find won’t be enough. After all, except for some possible lichenlike objects on Mars, the other worlds of the Solar System are all barren (barring a most unexwe’ll find,

pected miracle) Sure,

we

11

get

all sorts

of information

of reaching these barren w'orlds,

we

11

and knowledge. In the process

develop valuable

alloys, plastics,

We’ll work up useful techniques of miniaturization, automation, and computation. I wouldn’t minimize any of these advances. fuels.

there will be

superhumanly

no Martian

princesses,

intelligent energy beings,

no tentacled menaces, no

no dreadful monsters

to bring

back to zoos. In short, there won’t be any romance! For the proper results and rewards of space travel, we must reach the stars.

We must find

This

the Earth-type planets that possibly circle them, car-

appeared in print in ig6o and shows me with my usual optimism. The Moon has been “conquered," to be sure, but how far we will be able to continue space exploration in the present mood of disenchantment, I can't say. article first

ASIMOV ON ASTRONOMY

128

upon them

rying

their full

eomplement (we hope)

superman and monster. Only how do we get to the and Mars may be

stars?

The Moon may be on

just aeross the threshold,

and

of friend

but the

foe, of

our doorstep

way

stars are

to

helengone out of sight

The Moon

is

miles away at planets,

is

222,000 miles away at nearest.

its

Even

Mars

nearest and

its

Pluto, the

35,000,000

is

most distant of the known

never further than 4,650,000,000 miles from

us.

On

hand, the Alpha Centauri system, whieh ineludes the nearest

the other

stars to us,

25,000,000,000,000 miles away.

is

when we’ve

In other words,

labored our

the Solar System and stand on Pluto,

way to the

we have eovered

at best, less than ysooo oi the distanee that

is,

the nearest star It

would be

is

so nice

spell, a

whieh

a distanee

must be covered

if

even

to be reached. if

there were steppingstones to the stars;

were bodies between Pluto and the breathing

farthest edge of

stars that

would

if

there

at least give us a

place to stop and rest on the long trip to the nearest

stars.

And

having said that,

I

can smile cheerfully and say that there

reason to believe that such steppingstones do exist.

which may or may not

stars I

don’t I

mean

am

exist

I

don’t

is

good

mean dark

between us and Alpha Centauri; and

trans-Plutonian planets, which

may

or

referring, rather, to a shell of planetoids

may not

exist.

which surrounds the

Sun, far beyond Pluto’s orbit, with a dark halo; a shell of planetoids that

dwarfs the

known

and which,

Solar System

in all probability, actually

exists.

To

tell

the story of these planetoids,

I

shall, as

is

my

wont, begin at

the beginning. In this case, the beginning involves the comets.

From time immemorial, comets have been and with what seemed good reason. After all, the heavens are, for the most

considered portents of

dis-

aster,

part, a scene of quiet change-

lessness or, at most, of majestically periodic change. sets,

the

moon

runs through

positions exactly

among them All

is

in

its

The sun

is

like

and

phases, the “fixed” stars maintain their

from generation

to generation,

and the planets wander

complicated but predictable paths.

well. All

is

peaceful.

Then, hurrying into view, apparently from nowhere, comes It

rises

nothing

else in the heavens.

A fuzzy

patch of

light,

a comet.

the “coma,”

STEPPINGSTONES TO THE STARS

129

surrounds a bright starlike nucleus, and extending from the coma arched tail that can stretch halfway across the heavens. Having

is

an

come

from nou'here, the comet finally vanishes into nowhere. There seemed no way of predicting either its coming or going and all one could say was that it had disturbed the peace and serenity of the skies. This was in itself disturbing enough. Add to that the strangeness of its shape. terical

resembled a distraught woman, tearing across the sky in a hysfrenzy, her unbound hair streaming behind her in the wand. The It

word

very

comet

comes from the Greek koinetes meaning “long-

’

haired.”

Naturally, any sensible

man

and frightening apparition

could only suppose that such a sudden

by some god to w'arn humanity of disaster. And furthermore, since life and humanity are such that disaster strikes every year without fail, this theory seems to be borne out w'as sent

unmistakably. After a comet, disaster invariably follows. Within a year of the comet s appearance, there is sure to be a w^ar, plague, or

famine

somewhere, or some famous

man

dies, or

some

heretic appears or some-

thing.

The ceeded

would

last

in frightening

I,

come.

surely

death of

War

halfway spectacular comet showed up

many

in

1910,

and

it

suc-

people into believing the end of the world

(It also, as

any

fool can plainly see, foretold the

Mark Twain,

the sinking of the Titanic, the coming of and a whole slew of catastrophes.)

World

However, portent or not, what is the nature of a comet? Aristotle, and the ancient and medieval thinkers who followed him, believed the heavens were perfect and unchangeable. Since comets came and w-ent, having a beginning and an ending (which stars and planets did not) they were imperfect and changeable and, therefore, could not be part of the heavens. They were instead atmospheric phenomena; exhalations of

bad

air

and therefore part of our own corrupt and miserable Earth.

This notion

w'as

not destroyed until 1577.

Tycho Brahe, measured the year, plotting

tory in

its

position as

The Danish

astronomer,

comet that appeared that seen against the stars from his own observa-

parallax of a bright

Denmark and from another

proved too small to measure. This

is

observatory in Prague.

The

parallax

not surprising, considering the

rela-

tive shortness of the

base line (about 500 miles) and the fact that this w as before the days of the telescope. However, even so, if the comet had

been within 600,000 miles of the Earth,

its

parallax could have been

— ASIMOV ON ASTRONOMY

130

perceptible. Tycho’s conclusion, then, least three times as far

comet, at any

Even

They

Sun

as the

Moon

was.

That made that

heavens; and Aristotle was wrong.

rate, part of the

as part of the

troublesome.

from the Earth

was that the comet had to be at

heavens rather than of the Earth, comets remained

didn’t

into

fit

at the center of the Solar

any system.

When

System and Kepler made planetary orbits

into ellipses, the design of the planets began to

except for the comets.

They

where, and represented

Copernicus put the

fall

neatly into place

came from nowhere, vanished into an irritating lawlessness in the kingdom of still

no-

the

Sun.

COMETS The twentieth century has had rather poor luck with comets. In 1910 we had Halley’s comet returning and that was pretty spectacular but since then there has been scarcely a comet visible to the naked eye,

and

certainly nothing spectacular.

The nineteenth century saw

a

number

of giant

comets that stretched

halfway across the heavens at their closest approach and gleamed

like

foggy swords in the sky. These are long-period comets, with orbits that

may be many thousands or even rnillions of years before they return. They are fresh comets which have been in the neighborhood of the Sun only a few times {even, conceivably, for the first time) and have much in the way of volatile take

them

from the Sun that

so far out

it

matter to be heated into dust and vapor by the Sun and sent swirling into a long

tail

by the Solar wind.

Such comet spectaculars are absolutely unpredictable. There is no way of following comets very far outward into the outer Solar system and if the orbits are very elongated, then that part of it which we can observe

is

lion years.

just

about the same for comets which

Whichever

it is,

we

will

not be here to see that return.

And who knows how many comets which made a

hundred thousand years ago or

seen

it,

may be

shimmer

Of

in

will return every mil-

so,

when no

their last

appearance

civilized eye

could have

readying their plunge into the inner Solar system to

our skies

five years

course, every year a

usually small ones,

dim

anything but dim.

Some

from now.

number

We can only hope.

of comets are discovered.

They

are

which have been inhabitants of the planetary portion of the Solar system a long time and are either too far from the Sun at best, or have approached the Sun too many times to be one in

this

photo gyaph

ones, those

is

naked-eye specimens do appear, however.

an example.

The

photo: lick observatory

.

ASIMOV ON ASTRONOMY

132

Tlien

came Newton and

his

law of gravitation that so neatly explained

the planetary movements. Could

That would indeed be an In the year 1704

work out the

acid test.

Edmund

orbits of

also explain

it .

Newton, began to various comets over the regions for which observaHalley, a good friend of

tional records existed, in order to see fit

cometary movements?

if

their

motions could be made to

the requirements of gravitational mathematics, dlie records of twenty-

four different comets were studied. Tlie one with the best available data was the comet of 1682, which

photo; the bettmann archive

STEPPINGSTONES TO THE STARS Halley had himself observed.

Working out

its

orbit,

133

he noticed that

it

passed through the same regions of the sky as had the comet of 1607,

and the comet of 1531, seventy-six years before that. Checking back, he found records of another comet in 1456, seventyfive years further back still. Could it be that the same comet was coming back at intervals of seventy-five years before,

seventy-five years or so, after passing over an elliptical orbit so eccentric

that

end reached out way beyond the orbit of Saturn, then the

far

its

furthest planet

known?

EDMUND HALLEY London on November 8, 16^6. He was astronomy from his youth and when he was only twenty

Halley was born near ested in

inter-

years

old he went off for two years to the lonely island of St. Helena in the South Atlantic to map the stars of the souther?! hemisphere that were invisible

from the latitudes of Europe.

In 168^, Halley inherited a large

sum

money from his father, who had been found murdered, and he put that money to good use. For instance, when Isaac Newton was ready to publish his great book, Principia Mathematica, there was some scientific infighting with the cjuarrelsome Robert Hooke,

of

and the Royal

Society,

which had been

prepared to finance the publication, thought better of it and reneged. Halley then stepped in and paid all expenses. What’s more, he arranged for illustrations, read the galley proofs, and poured soothing oil on troubled waters in all directions.

One

of

Newton’s most devoted friends and admirers, Halley

tried to

apply the principles of gravitation to the comets, which hitherto had seemed to astronomers to come and go at their own sweet will and to

be amenable to no law. It struck Halley that a comet he observed in 1682 followed the same path as those of 14^6, 1531, and 1607. He decided that they were a single comet circling the Sun in a very elongated orbit and, after working out what that orbit might be, he predicted

So

its

it

return in 1J48.

did,

but Halley was not around to witness

1742. Nevertheless, the object has been since. Its last

known

appearance was in 1910 and

Halley was also the

first

to

show

it

it.

He had

as Halley’s

will

be back

died in

comet ever

in ig86.

that the fixed stars were not fixed

but that some of them had changed position since Greek times. In the last twenty years of his life he served as Astronomer Royal. He died in 1742.

ASIMOV ON ASTRONOMY

134 Halley

felt certain

dicted that the It

is

comet of 1682 would return once again

one of the frustrations of

was not

in 1758.

scientific history that

Halley

knew he

He would and he didn't. He

likely to live to see his prediction verified or exploded.

have had to

made

that just this was indeed so, and consequently pre-

live to

be one hundred and two

for that,

a valiant try, reaching the age of eighty-five,

but that wasn't good

enough.

On

it

Christmas night 1758 a comet was sighted and through early 1759, rode high in the sky. Tire comet had indeed returned and it has been

called Halley's

comet ever

since. (It

was Halley's comet that was

in the

sky in 1910.)

This created a sensation. Comets, or at least one comet, had been

re-

duced to a commonplace, law-abiding member of the Solar System.

many

Since then,

now, at

last,

there

And

others have been supplied with definite orbits. is

no

logical reason for considering

comets divinely

sent portents of disaster— which, however, will not prevent people pre-

paring for the end of the world at the next appearance of a large comet,

you may be

sure.

Granted that comets are ordinary members of the Solar System, subject to the

same laws

of

motion

as are the sedate planets,

what

are they?

Well, they aren't much.

Comets have frequently approached one planets and have

had

their orbits altered,

or another of the various

sometimes

sult of the gravitational attraction of the planet.

make

it

drastically, as a re-

(Such perturbations

The

rather difficult to pinpoint the time of a comet's return.)

planet, for

part, has never in

its

due to the comet's gravitational

any way showed any measurable

comet

attraction. Tlie

effect

of 1779 actually

passed through Jupiter's satellite system without affecting the satellites in

any way.

The

obvious conclusion

some comets very

is

that for

all

their gigantic volumes,

and

more voluminous than the Sun, comets have small masses. Tlie mass of even a large comet can be no larger than are actually

that of a middle-sized planetoid. If this is so,

the density of a comet must be extremely low, far lower

than the density of Earth's atmosphere. This fact that stars can

be seen through the

ble diminution in brightness. ley's

comet

in

tail

The Earth

of a

is

demonstrated by the

comet with no

passed through the

1910 and there was no discernible

percepti-

tail

of Hal-

effect. In fact, Halley's

STEPPINGSTONES TO THE STARS

135

comet passed between the Earth and the Sun and the whole thing appeared. Idle Sun shone through it as though it were a vacuum. Professor Fred

Whipple

of Harvard originated,

some

dis-

now

years ago, a

widely aecepted theory of the composition of comets that accounts for

Comets, he supposes, are made up

all this.

largely of 'dees,” that

is,

of

low-melting solids such as water, methane, carbon dioxide, ammonia,

When

and so on. the comet

is

far

from the Sun, these substances are indeed

a small, solid body.

As

it

approaches the Sun, however,

some of the ices evaporate and the dust and gas that form away from the Sun by the Solar wind.* Sure enough (as was first observed in 1531) a eomet’s away from the Sun.

It

the comet approaches the Sun, but

it

points generally

away from the Sun. Moreover, the

Not and

much atmosphere

as

is

are forced

tail

always

streams out behind the comet as precedes the comet as

it

moves

closer to the Sun, the larger the

tail.

formed, driven away by radiation pressure

you might think. The

lost, as

and

solid

ices

themselves are poor conductors of

heat and comets remain in the vicinity of the Sun only a comparatively

They

short space of time.

retreat with

Nevertheless,'at each return a

Whatever

passes into the

tail

most of

comet does

vanishes into

their substance intact.

some of its substance. space and never returns. A lose

few dozen passes at the Sun would probably

Even

comet that returns only at intervals expected to last more than several thousand a

suffice to finish a

comet.

of a century or so can’t

be

years at best. Therefore

we

ought, within historical times, to see comets shrivel and die.

And we

do. Halley’s

comet

at

return in 1910 was disappointingly

its

dim, when compared with previous descriptions.

more disappointing

at

its

And some comets have example

is

next scheduled appearance in 1986. actually died as

that of Biela’s comet,

Wilhelm von Biela. was observed on a number of its

astronomer,

split in

*

first

men

watched.

The

It is

known German

best

discovered in 1772 by the

had

dying.

about 6,6 years and returns. In 1846, it was found to have two, the halves traveling side by side. In 1852 the two parts had

separated further.

But

probably be even

It will

that’s

When

And

Biela’s

It

a period of

comet was never seen

not the end of the

stor)'.

again. It

had

died.

Traveling in the orbit of the

appeared I said it was “the radiation pressure of sunlight” that formed the comet’s tail. That’s what astronomers had thought but radiation pressure just wasn’t strong enough. The existence of the Solar wind (particles driven away from the Sun in every direction) was made clear in 1958, two years before this article

this article first

was written, but

ened out

in

time for

me

its

efficacy in

to include

it.

connection with comets’

tails

wasn’t straight-

ASIMOV ON ASTRONOMY

136

comet

are a group of meteorites.

We

know, beeause

in

1872 Biela’s

V

comet would have passed fairly close to the Earth if there had still been a Biela’s comet. There wasn’t, but that year we were treated with a meteor shower radiating out of the spot where the comet would have been located.

embedded

Apparently,

pebbles and pinpoints or ices are

in the iees of the less of

gone, the contents

teors that 611 space

Obviously,

if

fall

now may

metal and

apart.

The

comet

silicates.

When

of

the binding

small meteors and microme-

thus be the ghosts of comets long dead.

comets have such short lifetimes and are

new ones

ous as they are (several

number

are a vast

as

numer-

are discovered every year) even

though

still

the Solar System has been in existence for 6ve billion years, a continual

supply must be entering the system. But where are they eoming from,

then?

The

come from interstellar spaee. They may be wanderers among the stars. Some may oecasionally enter the gravitational held of the Sun, hash around it and go forever. Some enter, are eaptured by planets, and become periodic comets, doomed to a quick easiest

answer

that they

is

death.

There are interstellar

a couple of

arguments against

this possibility. First, to

migrants blundering into our Solar System at the rate they do

would require the

hlling of interstellar spaee with a

most unlikely num-

ber of eomets. Besides,

more would enter the system from the

toward which the Sun

is

is

not

so.

have

traveling than

Comets eome from

Secondly,

if

all

direetion

from the other. That, however,

directions equally.

comets entered the system randomly from outer spaee, a

number should come and go pin opened wide).

No comet

in distinetly hyperbolic orbits (like a hair-

with a distinctly hyperbolic orbit has ever

been observed.

more logical possibility is that the source of the comreservoir bound to the Sun. It was suggested some years ago

In view of this, a ets

is

a local

that this local reservoir exists in the form of a shell of ice planetoids, lo-

cated from one to two light-years from the Sun in every direction. It

is

easy to see

how

this shell

may have come

into existence. If the

Solar System began as a vast turbulent cloud of dust years in diameter, then as

it

and gas some lightswirled and contracted, the planets and pres-

ent-day Sun would be formed. At the outskirts of the original cloud,

however, the density would have been too low for planetary formation

S

TEPPING

TONE

S

S

TO THE STARS

137

and, instead, there would be numerous local concentrations. Since the temperature has remained near the absolute zero throughout billions of years in that far-flung region, the ices which composed much of the orig-

would be retained even by the tiny gravity of the planetoids.

inal cloud,

(Nearer the Sun, the higher temperature has caused even as the Earth to lose much of its supply of ices.) ^rhere

is

an estimate to the

as large a

body

effect that this shell of '‘cometary planet-

oids /1

contains 100,000,000,000 individuals with a mass, all told, up to 00 or even possibly that of the Earth. The average cometary

planetoid would then have a mass of 600,000,000 to 6,000,000,000 tons. If we assumed the density of such a planetoid to be equal to that of ice, the av^erage diameter would run, roughlv, close to a mile. You might think that a shell of a hundred billion planetoids

somehow

to

make

its

presence

known

to observers

ought

on Earth. However,

consider that the shell of space enclosing the Sun at a distance between' one and two light-years, has a volume of thirty cubic light-years. This is

immense!

the 100 billion cometary planetoids were evenly distributed through that volume, the average distance separating them If

would be

about 1V4 billion miles, which and Uranus.

is

nearly the distance between ourselves

Naturally, a v^olume of space containing a mile-wide hunk of ice every billion miles or so is not going to make any impression at all at a dis-

tance of a light-year or more. selves neither

The cometary

planetoids will reveal them-

by luminosity nor by blocking the light of the

stars.

Imagine a cometary planetoid somewhere in the middle of the shell, say 1V2 light-years from the Sun. The Sun, from that distance, would seem merely a star, though still the brightest star in the sky, with a magnitude of —2. Tlie planetoid would still be within the gravitational mfluence of the Sun (no other star would be as close) but that influence would be weak. A cometary planetoid, 1V2 light-years from the Sun, and traveling in

a circular orbit

about the Sun, would be whipped along under the feeble gravitational lash at a speed of only a little over miles a minute. This ^ may sound fast to the automobile driver, but the Earth

moves along

orbit at a rate of 1100 miles a

minute and even

far-off

its

Pluto never moves

at a rate of less than 150 miles a minute.

At

its

slow rate of movement,

30,000,000 years to complete a

it

re\

takes the average cometary planetoid

olution about the Sun. In

all

the exist-

ASIMOV ON ASTRONOMY

138

ence of the Solar System, those far-distant planetoids have not, on the

had time to revolve about the Sun 200 times. the cometary planetoids are circling quietly way out

average, yet

But

if

circle there forever?

do they not continue to

sends

only possible answer seems to involve the inter-

The

toward the Sun?

What

why them down there,

fering gravitational influence of the nearer stars. After

all,

the gravita-

Alpha Centauri on those cometary planetoids which happen to be directly between that star and the Sun, is 10 per cent that of the Sun and that is not negligible. (Remember, Alpha Centauri is scarcely farther from some of those planetoids than the Sun is.) A few

tional pull of

other stars exert gravitational attractions for those planetoids nearest

them

to

Now

an amount of over then,

circular orbit

its

orbital velocity,

becoming

must

ficiently, it

per cent that of the Sun.

these stellar attractions catch a particular planetoid in

if

such a way as to slow its

1

fall in

it

must

fall in

toward the Sun,

the orbital velocity

elliptical. If

toward the Sun so sharply

is

slowed

suf-

as to enter the Solar

would gather speed as it did so, whip around the Sun, and climb back to the point where the perturbation had taken place, then whip down again, climb back, whip down again, and so on. If it came close enough to the Sun, it would develop a gigantic tail and coma System proper.

It

of evaporating ices If

and would become

visible to

watchers on the Earth.

only the Sun and the comet existed, this new, highly elliptical orbit

would be permanent (barring additional stellar perturbation). A comet traveling in such an orbit would have a much shorter year than it did when it was in its shell, but its year would still be long by Earthly standards— about 10,000,000 years or As shots.

far as

man

is

Any comet

concerned, such ‘dong-period comets’" would be oneof this type appearing during historical times

man on

not have been viewed by exist.

Moreover there

is

course, once a

its

orbit affected. In

orbit will

become

it

he did not then

good chance that man may no

some

come

cases,

close

its

enough

slightly hyperbolic

and

it

no

some planet

may then

cases, its velocity will

often only recede

to

is

its

leave the Solar

be slowed and it

al-

to have

velocity will be speeded so that

longer gain the kinetic energy required to send shell. It will

for

enters the Solar System proper, there

will

System for good. In other

visit,

would

visit.

comet

ways the chance that

previous

its

a distressingly

longer exist to see the next

Of

so.

it

will

no

back to the cometary

further than the neighborhood of the

STEPPINGSTONES TO THE STARS planetary perturbation, so that

will, in effect,

it

139

have been captured by

the planet. All the outer planets

have "families” of comets, that of Jupiter, very naturally, being the largest. Perhaps the most remarkable of the Jupiter family

Encke’s comet, the orbit of which was worked out in 1818 by the German astronomer, Johann Franz Encke, after it had been disis

covered by the French astronomer, Jean Louis Pons. Encke s comet has the shortest period of any known comet It never recedes further from the Sun than about

— 3.^ years.

.^00,000,000 miles

which means that even as Jupiter

and

is.

It

at

approaches

its

most

distant,

fairly close to

it is

never as far from the Sun

Mercury’s orbit at

perihelion

its

perturbation by Mercury has been used to calculate the mass of that small planet. its

As you might expect, Encke’s comet never develops a

anything

else.

dim and unspectacular, and it has been near the Sun far too many times to be

tail. It

Most

of

its ices

consist largely of a fairly

is

are undoubtedly

compact

gone and

it

must now

silicate residue, thinly interlarded, per-

haps, with the remnant of the original ices. Naturally, the cometary shell is being depleted by these stellar perturbations. Any cometary planetoid slowed and sent down into the Solar System proper is condemned to death. In addition, other cometary planetoids are speeded by stellar perturbations and may be forced into a hyperbolic orbit that drives them away from the Sun altogether.

On

the other hand, no cometary planetoids are being added to the shell as far as we know, so that the number continually declines. Plowever, this need not be a source of worry. It has been estimated that perhaps three new comets are sent hurling into the Solar System proper each year. can suppose that three more are, on the average, speeded into hyperbolic loss in each year. At that rate, in the entire

We

five-billion-year history of the

Solar System,

planetoids have been lost or destroyed.

cent of the estimated

number

that

Despite the cometary death their usual

numbers

still

30,000,000,000 cometary

That amounts to only 30 per

remains.

rate, then,

our comets

will

be with us

in

for billions of years more.

these cometary planetoids, to get back to the remarks I the very beginning of the article, which may represent the It is

made

at

stepping-

stones to the stars. If

we could

ever reach Pluto,

it

might not be too great

a

hop

to reach

ASIMOV ON ASTRONOMY

140

cometary planetoids; one that had been slowed into a skimming approach toward the outskirts of the Solar System

one of the relatively

closer

v

proper. Certainly not as great an effort a planetoid as If a

would be required

to reach such

would be required

to reach

Alpha Centauri

base could be set up on such a mile-wide

hunk

in

one jump.

of ices, perhaps

we

could continue to press outward into space from planetoid to planetoid in

an island-hopping fashion, to the outermost fringes of the shell. Nor would the two-light-year mark necessarily end such island-hop-

ping

possibilities.

After

Centauri doesn’t have shouldn’t

there

is

no reason

to believe that

halo of cometary planetoids of

its

own.

Alpha

Why

have one? (Though perhaps a more complicated one, since

it

Alpha Centauri If it

a

all,

is

really three stars.)

has one, then Alpha Centauri and the Sun are close enough so

that the outermost fringes of the halo of one ought to be rather close to the outermost fringes of the halo of the other.

Perhaps, then, at

no point

will

we could

island-hop over the ice

an uninterrupted

trip of

more than

be required and perhaps we can reach the nearest

way

a

mountain climber

scales a

all

high peak

— by

a

the way. Perhaps

few

billion miles

star, at least, in

the

establishing a series of

intermediate bases on the way. I

cannot honestly say that

inviting,

but

if

this

makes

we’ve got to go, surely

a trip to the stars actually look

it is

easiest to

go a step at a time.

THE PLANET OF THE DOUBLE SUN

The

title

sounds

as

though

this

were going to be a rather old-fashioned

science-fiction story, doesn’t it?

Yet although the not be.

One

of the

The author

may sound

most glamorous

more than one sun

of

title

in

old-fashioned, the situation need

settings that can

be imagined

is

that

the sky.

of a story describing such a setting need not (and usually

does not) worry about the astronomic verities of the situation. The suns are usually described as looking like suns and both (or all) are made to

move independently

in the sky.

color by saying that

one sun was

passed zenith. erally)

He may make

by having one sun,

for

The author just rising,

will usually

throw

in local

while the other had just

matters more colorful (figuratively

example, red and the other blue.

and

lit-

Then he

can talk of double shadows and their various configurations and color combinations.* *

tmd

I

wrote such a story myself in 1954. h was called “Sucker Bait” and you will It in my book Fhe Martian Way and Other Stories (Doubleday, igi;!:).

At

rationalize the situation in that story, but in 1941, I wrote “Niehtf um' tall about a planet with six suns. It is collected in Nightfall and Other Stories (IJoubleoay, 1969). Despite the fact that the astronomic situation in “Niehtfall” is enormously unlikely, it remains my most popular short story.

ASIMOV ON ASTRONOMY

142

A

little

of this

is

enough to make us

sigh at our misfortune in having

only one sun in the sky; and a pretty colorless one at that. Oh, the missing glories!

What would

be

it

in the sky? Tliere

have more than one sun

like to

some are made up of two components and some of more than two. In some multiple stars, the components are near together; in other far apart. The com-

are, of course, a

may be

ponents

may be

wide

a

multiple

variet)^ of types of

stars;

one may be a red giant or one

similar or not similar;

white dwarf.

not make up any systems or look for something exotic or foreign. The fact of the matter is that we have an example in our back yard. The nearest star to us in space, a star so close we can almost reach

But

let's

out and touch

it,

a next-door neighbor

miles away, good old Alpha Centauri,

no more than 25,000,000,000,000 a multiple star.

is

^

Suppose we were on

would

To

it

be

Alpha Centauri system.

a planet in the

What

like?

begin with, what

Alpha Centauri

is

like?

Hemisphere.

It is

never visible in the sky north of about 30 degrees north latitude.

The

Alpha Centauri

is

a star in the Southern Celestial

chances are you've never seen ancient Greeks never saw

The

it;

I

know

I

never have. Moreover, the

it.

chief observatories of the medieval Arabs, in Cordova, Bagdad,

and Damascus,

v/ere all north of the 30-degree line.

Presumably

ordi-

nary Arabs in the Arabian and Sahara deserts must occasionally have seen a bright star very near the southern horizon, but

this,

apparently,

did not penetrate to the egghead level.

The

test of the

brightest star in the sky, bic.

Alpha Centauri, although the third has no name of its own, neither Greek nor Ara-

matter

is

that

(The name Alpha Centauri

Of

is

official

“astronomese.")

Europeans started adventuring down the coast of 1400s, the bright star must have been observed at once.

course, once

Africa in the late

Eventually, astronomers got around to making star of the Southern Celestial

was

Edmund

Hemisphere

invisible

maps

of those parts

from Europe. (The

first

Halley of Halley's comet fame who, in 1676, at the age of

twenty, traveled to

St.

Helena of future Napoleonic fame to map south-

ern stars.) Astronomers divided the southern heavens into constellations

scheme already begun in those which the ancients had been able to observe. to complete the

They named

the constellations

in

Latin,

parts of the heavens

naturally,

and included

THE PLANET OF THE DOUBLE SUN

M3

mythological creatures as a further match to what already existed in the sky (just as planets discovered in modern times received mythological

names matching the older ones). One

named

constellations was

of the

prominent southern

the Centaur. In Latin, this

the genitive (“of the Centaur")

is

Centaurus and

is

Centauri.

Centaurus contains two first magnitude stars. Tlie brighter was named Alpha Centauri and the other Beta Centauri. The words “alpha" and “beta" are not only the first two letters of the Creek alphabet but were also used by the Greeks to represent the numbers “one" and “two," a

habit never broken by scientists. lated, therefore

mean

“star

The names

number one

of the stars, freely trans-

of the Centaur"

and

“star

num-

ber two of the Centaur" respeetively.

The magnitude

of

Alpha Centauri

third brightest star of the sky.

(—0.86) and, of course,

Sirius

(

The

difference in

o.o6 which makes

only stars brighter are Canopus

brighter the star, in a logarithmie ratio.

magnitude of one unit means

of 2.512 times.

A

as said, the

it,

— 1.58).

(The lower the magnitude, the

A

is

a difference in brightness

difference in

magnitude of two units means a difference in brightness of 2.512X2.512 or about 6.31 times and so on). About 1650 telescopes became good enough to detect the fact that some stars, which looked like single points of light to the naked eye, were actually two closely spaced points of light. In 1685 Jesuit mission-

aries in Africa, taking

time out for astronomical observations, first noticed that Alpha Centauri is an example of such a double star. The

brighter

component

Alpha Centauri A, the other Alpha Centauri B. The magnitude of Alpha Centauri A by itself is 0.3 and that of Alpha

Centauri

B

is

Centauri

A

is

1.7.

is

The

1.4 difference in

3.6 times as bright as

magnitude means that Alpha Alpha Centauri B. To translate the

brightness into absolute terms; that

with our Sun,

it is

necessary to

know

is,

to

eompare

change

is

called stellar parallax

ereases.

A

star, resulting

and grows smaller

very distant star has virtually

no

is

it

revolved about

from Earth’s motion,

as the distance of a star in-

parallax at

treated as a motionless reference point against

nearby

shifts in the star’s

in Earth’s position as

the Sun. This tiny yearly motion of a

component

the distance of Alpha Centauri.

This distance could be measured by noting slight position, mirroring the

either

all,

so

it

ean be

which the parallax of

a

eould be measured. (Without some reference point, parallax meaningless.) star

However, astronomers had

for centuries

been trying to detect

stellar

ASIMOV ON ASTRONOMY

144

parallaxes without success, although they

Moon, then

parallax of the

of the

had succeeded

Sun and the

with the

first

planets. Apparently, even

the nearest stars had parallaxes so Mnall as to

make them

difficult to

measure.

Another trouble was that without knowing the parallaxes, one couldn’t tell which star was near and which far. How, then, know which star to

measure and which to use

as a motionless reference point?

Astronomers made the general assumption a bright star

is

closer to

Earth than

a

is

that, all things being equal,

dim

proper motion (a shift in position due to the space; a shift clic,

or back

which

and

is

continuous

— always

with high

star. Also, a star

star’s

in

forth, as parallactic shifts

own motion through

one direction and not

cy-

would be) was assumed

nearer the Earth than one with a low proper motion. These assumptions

would not

necessarily hold true in every case, for a bright star

more distant than a faint one, but be enough brighter, make up for that. Again, a near star might have a very motion, but one which was

in

our line of sight so that

might be

intrinsically, to

rapid apparent

wouldn’t show

it

up. Nevertheless, these assumptions at least give astronomers a lead.

By the 1830s the time was

ripe for a concerted attack

on the problem.

Three astronomers of three different nations tackled three different stars. Thomas Henderson (British) observed Alpha Centauri and Friedrich Wilhelm von Struve (German-born Russian) worked on Vega, the fourth brightest star in the sky. Both stars were not only bright but had pretty snappy proper motions. Friedrich plied his efforts to 61 Gygni. This was a

high proper motion. In each case, the

Wilhelm Bessel (German) apdim star but it had an unusually

star’s

position over at least a year

was compared with that of a dim and presumably very

far-off

neighbor

star.

Sure enough, each of the three stars being investigated shifted position slightly compared to its presumably distant neighbor. And so it hap-

pened

(as

it

often does in science) that after centuries of failure, there

were several almost simultaneous successes. Bessel got in

first,

in 1839,

measure the distance of a

and he

star. It

gets the credit of being the

turned out that 61 Gygni

years distant. Henderson, later in 1839, reported little

is

Vega

to

at

be a

about

27 light-years distant. No star has been found closer than those of the Alpha Gentauri

tem.

to

11 light-

Alpha Gentauri

over 4 light-years distant and Struve, in 1840, placed

first

sys-

the planet of the double sun Knowing

the distance of Alplia Centauri,

A

Alpha Centauri

our Sun. Since

as

perature,

it

is

it

(the brighter of the pair)

its

spectrum showed

it

to

is

is

M5

easy to calculate that

almost exactly as bright

be at the same surface tem-

our Sun’s tuin— same diameter, same mass, same bright-

same everything, apparently. As for Alpha Centauri B, if it were the same temperature as Alpha Centauri A, then it would be just as luminous per unit area. To be only ness,

luminous

Vs.n as

eters of the

two

as

companion,

its

it

must have

its

area.

The diam-

would be as the square roots of the respective areas and (assuming the two stars to be equally dense) the masses would be as the cube of the square roots of the respective areas. stars

would then turn out that Alpha Centauri A would have a diameter 1.9 times that of Alpha Centauri B and a mass about 7 times that of Alpha Centauri B. (Actually, Alpha Centauri It

B

Alpha Centauri A, so that the comparison but for the purposes of ments.)

this essay,

we

is

is

a triHe cooler

than

not exactly as I’ve given

it,

needn’t worr)^ about the refine-

The two stars rotate in elliptical orbits about a common center of gravity. The period of rotation is about 80 years. When the stars are closest,

they are about a billion miles apart.

When

they are furthest they

are 3.3 billion miles apart.

Now,

then, suppose

Centauri system here IS

we

in

try to duplicate (in

our

own

Solar System. Since Alpha Centauri

the twin of our Sun in every respect,

Centauri A, but

let’s

imagination) the Alpha

let’s

keep on referring to

it,

suppose our Sun

is

A

Alpha

for convenience’s sake, as

the Sun. Let’s imagine

about the Sun.

in orbit

making that

Alpha Centauri B (which we

it

will call

simply Sun B)

We

can avoid unnecessary complications by exactly one half the diameter of the Sun and equally dense so

one eighth the Sun’s mass. This may not be exactly the situation with respect to Alpha Centauri B, but it is a reasonably approxiit IS

mation.

Furthermore,

m

let’s

suppose Sun B

is

traveling in a nearly circular orbit,

the same plane as the planets generally, and at the average distance of

Alpha Centauri B from Alpha Centauri A (again a change in detail but not in essence). This would place it in orbit about 2,000,000,000 miles from the Sun. Tliis is almost as though we have taken the planet Uranus of our Solar System and replaced it with Alpha Centauri B.

ASIMOV ON ASTRONOMY

146 All this

a multiple-star system very closely

would make Earth part of

Now

resembling that of Alpha Centauri.

what would the heavens be

like?

some ways, our Solar System would be changed. Uranus, Neptune, and Pluto, as we know them, would be out. Their orbits would be tangled with Sun B. However, these planets were unknown in the preIn

telescopic era, so

tion

is

we can do without them

naked-eye observa-

as far as

concerned.

But even Saturn, the outermost of the planets known to the ancients would be nearer to the Sun than to Sun B in the position I placed the latter.

With

the Sun, on top of that, having a gravitational

times as intense as that of Sun B, still

closer planets with

effects

on the planetary

no

should hang on to Saturn and the

it

trouble. (There

orbits

but

field eight

Pm

might be interesting minor

not astronomer enough,

alas, to

be able to calculate them.)

Sun B would behave like a new and ver}' large “planet” of the Sun. The Sun and Sun B would revolve about a center of gravity which would be located in the asteroid belt. The motion of the Sun about this point once every eighty years would, however, not be detectable in pretelescopic days, because the Sun would carry all the planets, including Earth, with

it.

Neither the Sun’s distance nor Sun B’s distance from

Earth would be affected by that motion. (After the invention of the telescope, the Sun’s

tow— would become

noticeable through

displacement of the nearer

it

would net look

the other planets.

A

us in

reflection in the parallactic

stars.)

But what would Sun B look Well,

its

swing— with

like in

like a

our heavens?

Sun.

It

would be

a point of light like

diameter of 430,000 miles at a distance of 2,000,-

000,000 miles would subtend an angle of about 45 seconds of arc. Sun B would appear to the naked eye to be just about the apparent size of the smaller but closer Jupiter.

To

a naked-eye observer (such as the

would be one more point of

light

Greeks or Babylonians) Sun

moving slowly against the

stars.

B It

would be moving more slowly than the others, making a complete circuit of the sky in about 80 years, as compared with iqVz years for Saturn and 12

for Jupiter.

From

this,

the Greeks would

— rightly — conclude

that

Sun B was further from Earth than was any other planet. Of course, one thing would make Sun B very unusual and quite different from the other planets. It would be very bright. It would have

THE PLANET OF THE DOUBLE SUN an apparent magnitude of about -i8. bright as the Sun, to be sure, but it would the

full

Moon. With Sun B

in

It

M7

would be only

as

be 150 times as bright as the night sky. Earth would be well-illumistill

nated.

Another thing might be unusual about Sun B; not evitability, as

with

its

brilliance,

as a

matter of

in-

but as a matter of reasonable probabil-

ity at least.

As

“planet” of the Solar System, why should as the other planets have? (Of course, a

it

its satellites

about a Sun and would

really

be planets, but

not have

satellites

would be revolving

not worry about hav

let’s

ing a consistent terminology.)

To

be sure, since Sun B is much larger than the other planets, it could e expected to have a satellite much larger and more distant from itself

man It

is

true tor

any other planet.

might, for instance, have a

satellite

the size of Uranus.

Uranus would be much smaller compared pared to the Sun.

to

(Why noU

Sun B, than Jupiter

is

com-

the Sun can have Jupiter in tow, then it is perfectly reasonable to allow Sun B to have a planet the size of Uranus.) Uranus could be circling Sun B at a distance of 100,000,000 miles (Again, why not? Jupiter, which is considerably smaller than Sun B and considerably closer to the competing gravity of the Sun nevertheless rnanages to hold on to satellites at a distance of 15,000,000 miles from itse f If Jupiter can manage that. Sun B can manage 100,000,000.) ranus moved about Sun B in the plane of Earth’s orbit, it would move first to one side of Sun B, then back and to the other side then back and to the first side, and so on, indefinitely. If

Its

lon from

maximum

separa-

Sun B would be about

3 degrees of arc. This is about 6 times the apparent diameter of the Sun or the Moon and such a separation could be easily seen with the naked eye.

But would Uranus

itself

be

visible at that distance

from us? Well, right at the moment, without Sun B, Uranus is visible. It is 1,800,000 000 miles from the Sun (nearly as far as I have, in imagination, put Sun B) and it has a magnitude of 5.7

which makes

it

a vei}r faint star.

But d Uranus w^re rotating about Sun B, merely by the dim light of the distant Sun

it

would be

(the reflection

we

see

Ibe

Uranus

by, actually)

just visible as

up not of which is all lit

but also by the stronger rays of the

much

average magnitude of Uranus under these conditions would be

ASIMOV ON ASTRONOMY

148

wouldn’t be as bright

It

1.7.

brighter than the

North

B might make Uranus should

still

but

too,

lite,

be clearly let’s

the other planets, but

as

Star, for instance.

The

would be

glare of the nearby

Sun

harder to observe than the North Star, but

visible.

(Sun B might have more than one

not complicate the picture.

The Greeks would

it

One

it

satel-

satellite will do.)

thus be treated to the spectacle not only of an un-

and exceptionally brilliant point of light but also to another point of light (much dimmer) that oscillated back and forth as though

usually

caught

in the grip of the brighter point.

Both unique.

and

factors, brilliance I

have

a visible satellite,

a theory that this

would be completely

would have made an

interesting differ-

ence in Greek thinking, both on the mythological and the

scientific

level.

Mythology

first

(since

and that involves the

Greek mythology

is

older than

Greek science)

'‘synodic period” of a planet. This

the interval

is

and the Sun, in our sky. Jupiter Saturn and the Sun every 378 days.

a planet

between successive meetings of

and the Sun meet every 399 days; Sun B and the Sun would meet in Earth’s sky every 369 days. (This is to get just a measure of how frequently Earth in its revolution manages

Sun from the planet in question.) As the planet approaches the Sun it spends less and less time in the night sky and more and more time in the day sky. Eor ordinary planets it is this means it becomes less and less visible to the naked eye because out lost in the Sun’s glare during the day. Even the Moon looks washed

on the other

side of the

by day.

But Sun B would be bright as the full

Moon,

different. it

Gonsidering that

would be

by day. Allowing the use of smoked

up

it

is

150 times as

a clearly visible point of light even glasses,

it

could be followed right

to the Sun.

Now fire.

myth about how mankind learned the use of creation, man was naked, shivering, and miserable;

the Greeks had a

At the time

of

one of the weakest and most poorly endowed of the animal creation. The demigod Prometheus had piN on the new creature and stole fire from the Sun to give to mankind. With fire, man conquered night and winter and marauding beasts.

He

learned to smelt metals and developed

civilization.

But the anger of Zeus was kindled

at this interference.

Prometheus

was taken to the very ends of the world (which, to the Greeks, were the

THE PLANET OF THE DOUBLE SUN Caucasus Mountains) and there chained to a rock. there to tear at

liver ever}’ day,

liis

but

him

left

it

A

vulture was sent

at night in order that

might miraculously grow back and be ready

his liver

149

for the next day’s

torture.

Tliere now. Doesn’t of

all this fit in

perfeetly with the apparent behavior

Sun B? Every year Sun B commits the crime

be seen

in the

be seen to do

and than

it

daytime approaching the Sun, the only planet that can

this. It

can only be planning to

obviously succeeds. After the other planets,

all

of Prometheus. It ean

why it is so much brighter much brighter even than the

that

all, isn’t

why

it

so

is

from the Sun,

steal light

Moon? Moreover, sky,

it

illuminates the landscape into a

But the planet further

is

punished.

away than any other shape of

in the

satellite

tearing at again,

and

With in

it,

so

now have

Now

it

While the planet was busy was

satellite

all

Could

suggested the

Could

it

(because

it

was

swoops toward the bright planet, it

to recover, then swoops in

isn’t it just

name

about inevitable that

of you

it

if

Vulturius.

know), but

I

wouldn’t be surprised

people reading this might not think the parallel cidental.

visible

sober-minded and prosaic myself to think outlandish

far too

thoughts (as

it,

Sun B were would be named Prometheus? Or that the satellite would mind,

the Latin

I’m

even a vulture tearing at

an eternal rhythm.

in

all this in

our sky,

satellite

then moves away to allow

on

is

Once the planet was hurled though, and became visible in the night

The

appeared.

of day.

glare, of course).

to the edge of the universe, its

There

planet.

in the night

it is

out to the edge of the universe,

clearly visible satellite.

its

drowned out by the Bun’s sky,

dim kind

It is cast

from the Sun, no

stealing fire

when

brings this light to mankind, for

it

is

if

far too close to

some be

ae-

be that such a heavenly situation actually existed and

myth

in the first place?

be that the

human

race originated

on

a planet eircling

Centauri A? Could they have migrated to Earth about years ago, wiping out the primitive Neanderthals they

fifty

Alpha

thousand

found here and

men”? Could some disaster have destroyed their eulture and forced them to build up a new one? Is the Prometheus myth a dim memory of the distant past, when

established a race of “true

Alpha Centauri B

lit

up the

original of the Atlantis

No,

I

skies?

Was

the Alpha Centauri system the

myth?

don’t think so, but anyone

who wants

to use

it

in a scienee-

ASIMOV ON ASTRONOMY

150 fiction story

on

cult based

send

me

is

welcome

to

And anyone who wants

it.

to start a religious

notion probably can’t be stopped but please— don’t

this

any of the literature— and dbnt say you read

And what

would Sun B

effect

here

it

first.

“Prometheus”) have had on Greek

(or

science?

Well,

was

in the real world, there

balance.

The

time

a

when

matters

hung

in the

popular Greek theory of the universe, as developed by 300

PROMETHEUS Prometheus course,

all

one of

is

my

favorite myths,

As

who

with the suffering demigod,

even though he knows he cannot win

myth has

a matter of fact, the

to the situation that faces

—

man

and

my

sympathies

resists

are, of

ultimate power

for the sake of principle.

a peculiar significance with respect

today.

Zeus has

just defeated his father,

Kronos, and taken over the rule of the Universe. In the pride of his

power, he refuses to recognize the rights and needs of inferior creatures.

He

is

when Prometheus

not concerned with mankind, for instance, and

expresses concern

and does something about

it,

Prometheus

is

arbi-

and unjustly punished. But Prometheus knows something Zeus doesn’t know and that is the trump card in his hand. Zeus knows that there exists a goddess who is

trarily

fated to bear a child greater than his father, but he doesn’t

the goddess versatile

It

is.

inhibits his love-life

penchant

in that direction)

the result of a lighthearted

would

treat

Zeus

as

{and Zeus had an ample and

when new and younger god who

since he could never

amour would be a

Zeus had treated

know who tell

his father.

And Prometheus knew who the goddess was. capitulate. He learned that there was no such

In the end, Zeus had to thing as absolute power,

that without consideration for the rights of weaker ones his

ation was insecure. Prometheus was freed

be the sea Peleus,

match

And

nymph

and had

—

a son, Achilles

far

situ-

and revealed the goddess

Thetis was forced

Thetis.

own

marry a

to

to

mortal,

mightier than his father, but no

for the gods.

today.

Mankind

Zeus, possessing powers far mightier than

is

those the Greeks imagined to be at Zeus’s disposal. uses those powers as arbitrarily

garding the weaker species, fabric of earth,

Mankind’s

if

fate

and

as unjustly as

Mankind is

And

if

Mankind

Zeus did

in disre-

carelessly rends the ecological

also sealed.

Zeus learned, but

will

we?

THEPLANETOFTHEDOUBLESUN

1^1

put the Earth at the center and let everything in the heavens revolve about it. The weight of Aristotle’s philosophy was on the side of B.c.,

this theory.

About 280

B.c.

Aristarchus of

Samos suggested

that only the

Moon

re-

volved about the Earth, llie planets, including Earth itself, he said, revolved about the Sun, thus elaborating a heliocentric system. He even

had some good notions concerning the Moon and the Sun.

relative sizes

and distances of the

photo: the bettmann archive

ASIMOV ON ASTRONOMY For a while, the Aristarchean view seemed to have an outside chance despite the great prestige of Aristotle. However, about 150 b.c., Hipparchus of Nicaea worked out the mathematics of the geocentric system so thoroughly that the competiton ended. About 150 a.d. Claudius

Ptolemy put the

final

touches on the geocentric theory and no one

questioned that the Earth was the center of the universe for nearly

1400 years thereafter.

But had Prometheus and Vulturius been in the sky, the Greeks would have had an example of one heavenly body, anyway, that clearly did not revolve primarily about the Earth. Vulturius would have revolved about Prometheus. Aristarchus would undoubtedly have suggested Prometheus to be an-

other sun with a planet circling

it.

won

seems to me, certainly have

The argument by out.

analogy would,

it

Copernicus would have been

anticipated.

Furthermore, the motions of Vulturius about Prometheus would have given a clear indication of the workings of gravity. tion that gravity

immune

was confined

to

The

Aristotelian no-

Earth alone and that heavenly bodies

would not have stood up. Undoubtedly Newton too would have been anticipated by some two

were

thousand

to

years.

What would anyway?

it

have happened next?

Would

the Dark Ages

Would Greek

still

have intervened? Or would the

world have had a two thousand year head

now be

masters of space?

nuclear war fought in

So

that’s

how

it

Or would we

Roman

goes.

You

genius have decayed

start in science

and would we

possibly be the non-survivors of a

times? start off

checking on colored shadows in a

and end up wondering how different human history might have been (either for good or for evil) if only the Sun had had a

science-fiction story

companion

star in its lonely

voyage through eternity.

12

TWINKLE, TWINKLE, LITTLE STAR

It

came

as a great

shock to me,

in

childhood days, to learn that our sun

was something called a “yellow dwarf” and that sophisticated people scorned I

as a rather insignificant

it

member

of the

had made the very natural assumption,

little

things,

and eventhing

I

about the tiny

fairy tales

must be the

children of the sun and

sun being the father and the dim, retiring

When

I

found that

all

solar system.

it

all stars

many were

What’s more,

I

stars,

which

moon, the

moon

(I

gathered)

brightly shining

the mother.

not only upset the sanctity of the heavenly

also offended

Consequently,

learned that not a great

it

were

those minute points of light w^ere huge, glaring

suns greater than our owm, family for me, but

prior to that, that stars

had read confirmed the notion. Tliere

were innumerable little

Milky Way.

it

me

as a patriotic inhabitant of the

was with grim

relief

that

were greater than the sun after

all;

I

eventually

that, in fact,

smaller than the sun.

found some of those small

nating; and in order to talk about them,

I

stars to

will

be intensely

fasci-

begin Asimov-fashion at

the other end of the stick, and consider the earth and the sun.

.

ASIMOV ON ASTRONOMY

154

Tlie earth does not really revolve about the sun. Both earth and sun

(taken by themselves) revolve about'a rally,

the center of gravity

degree of closeness

is

common

more massive body and the the ratio of masses of the two

closer to the

proportional to

is

center of gravity. Natu-

bodies.

Thus, the sun

is

333,400 times as massive as the earth, and the center

of gravity should therefore be 333,400 times as close to the sun’s center

The

as to the earth’s center.

distance between earth and sun, center to

about 92,870,000 miles; and dividing that by 333,400 gives us the figure 280. Therefore, the center of gravity of the earth-sun system is center,

is

280 miles from the center of the sun. This means that as the earth moves around

annual revolution, the sun makes a small

this center of gravity in its

circle

280 miles

the same center, leaning always away from the earth. trifling

wobble

in radius

Of

about

course, this

quite imperceptible from an observation point outside

is

the solar system; say, from Alpha Centauri.

But what about the other planets? Each one the sun about a

common

more massive than the

earth

these factors working to sun’s center.

center of gravity.

(which, by the way,

I

revolves with

of the planets are both

and more distant from the sun, each of

move

To show you

Some

them

of

the center of gravity farther from the

the result,

have never seen

in

have worked out Table 32 any astronomy text) I

Table 32

DISTANCE (miles) OF CENTER OF GRAVITY OF SUN-PLANET SYSTEM

PLANET

FROM CENTER OF SUN

Mercury

6

Venus

80

Earth

280

Mars Jupiter

45 460,000

Saturn

250,000

Uranus

80,000

Neptune

140,000

Pluto

The

radius of the sun

ever)' case

but one

lies

1,500 (?)

is

432,200 miles, so the center of gravitv in

below the sun’s surface. The exception

is

Jupiter.

TWINKLE, TWINKLE, LITTLE STAR The

center of gravity of the Jupitcr-sun system

above the sun’s surface (always the sun and Jupiter were

If

server

from Alpha Centauri,

is

155

about 30,000 miles

in the direction of Jupiter, of course).

all

say,

that existed in the solar system, an ob-

though not able to see

Jupiter,

might

be able to observe that the sun was making a tiny

(in principle)

about something or other every twelve

circle

years. Tliis '"something or other”

could only be the center of gravity of a system consisting of the sun and

another body.

If

our observer had a rough idea of the mass of the sun,

how distant the other body must be to impose a twelverevolution. From that distance, as compared with the radius of the

he could year

tell

the sun was making, he could deduce the mass of the other body.

circle

In this way, the observer on Alpha Centauri could discover the presence

and work out

of Jupiter

ever actually seeing

mass and

its

its

distance from the sun without

it.

wobble on the sun imposed by Jupiter is still too small to detect from Alpha Centauri (assuming their instruments to be Actually, though, the

no better than ours).

What makes

Neptune (the other

planets can be ignored)

sun, too,

which complicate

But suppose that sive

than Jupiter.

much

its

worse

is

that Saturn, Uranus,

and

impose wobbles on the

motion.

circling the

sun were a body considerably more mas-

The sun would then make

a

much

larger circle

and a

simpler one, for the effect of other revolving bodies would be

swamped by

this super-Jupiter.

sun, but

possible that

is it

Yes, indeed,

In 1834 the

from

it

it

To

be

sure, this

might be so

is

not the case with the

for other stars?

possible.

it is

German astronomer

Friedrich

Wilhelm

a long series of careful observations, that Sirius

wavv

the sky in a

line.

Bessel concluded,

was moving across

This could best be explained by supposing that

the center of gravity of Sirius and another body was moving in a straight line

and that

some

period of Sirius,

and

for

it

was fifty

however, it

to

Sirius’s revolution

(in a

years) that produced the waviness.

is

about two and a half times

be pulled

as massive as the sun,

as far out of line as observation

the companion body had to be it

about the eenter of gravity

much more

showed

it

to be,

massive than Jupiter. In

faet,

turned out to be about one thousand times as massive as Jupiter, or

just

about

as

massive as our sun.

this thousand-fold-Jupiter letters

has

become

a

multiple star system.)

If

we

call Sirius itself "Sirius

companion would be

A,” then

"Sirius B.” (This use of

standard device for naming components of a

ASIMOV ON ASTRONOMY

156

Anything

and

as massive as the

he might, Bessel could see nothing

yet, try as

A

of Sirius

where

its

B ought

Sirius

B was

sion was that Sirius

had used up

sun ought to be a star rather than a planet

fuel.

a

tO‘be.

The seemingly

burned-out

neighborhood

in the

natural conclu-

blackened cinder that

star, a

For a generation, astronomers spoke of

Sirius’s

“dark companion.” In 1862, however, an American telescope-maker, Alvan

was testing

new

a

He

eighteen-inch lens he had made.

image

Sirius to test the sharpness of the

Graham

Clark,

turned

would produce, and,

it

on

it

to his

chagrin, found there was a flaw in his lens, for near Sirius was a sparkle of light that shouldn’t be there. Fortunately, before going back to his

grinding, he tried the lens on other stars,

Back It

to Sirius— and there

and the defect disappeared!

was that sparkle of light again.

couldn’t be a defect; Clark had to be seeing a

star.

In fact, he was

seeing Sirius’s “dark companion,” which wasn’t quite dark after it

was of the eighth magnitude. Allowing

was at

least

dim,

sun— there was

not dark, for

if

still

much

that

was only

it

of a

for

distance, however,

its

luminous

H20

dim glow amid

its

peratures, so that

lines

as

it

our

supposed ashes.

came

In the latter half of the nineteenth century, spectroscopy

own. Particular spectral

for

all,

into

its

could be produced only at certain tem-

from the spectrum of

a star

its

surface temperature

could be deduced. In 1915 the American astronomer Walter Sydney Adams managed to get the spectrum of Sirius B and was amazed to discover that

was not

it

a

dimly glowing cinder at

all,

but had a surface

rather hotter than that of the sun!

But as the

bright B was hotter than the sun, why was it only %20 The only way out seemed to be to assume that it was much

Sirius

if

sun?

smaller than the sun and had, therefore, a smaller radiating surface. In

account for both

fact, to

had to have was It

temperature and

its

low

total luminosity,

diameter of about 30,000 miles. Sirius B, although a

about the

just

size of the planet

it

was white-hot,

of that t\pe

But

came

to

too.

Consequently, Sirius

volume

about of

as

star,

B and

a star

all

might

other stars

be called “white dwarfs.”

Bessel’s observation of the

just

it

Uranus.

was more dwarfish than any astronomer had conceived

be and

still

a

its

mass of

massive as the sun.

To

Sirius

B was

squeeze

all

Uranus meant that the average density of

still

valid. It

was

that mass into the Sirius

B had

to be

38,000 kilograms per cubic centimeter, or about 580 tons per cubic inch.

Twenty

years earlier, this

consequence of Adams’ discovery would

have seemed so ridiculous that the entire chain of reasoning would have

— TWINKLE, TWINKLE, LITTLE STAR

157

been thrown out of court and the very concept of judging peratures by spectral lines would have

Adams’ time, however, the \^'orked out

and

it

was concentrated

density of Sirius

B— and,

nucleus at the very center of the atom.

star or for its density.

much

Van Maanen’s

larger than Mars. It

Star

is

it

(named is

still

dense as Sirius B.

A

its

discoverer) has

smaller than the earth and

one seventh

as massive as is

enough

to

our sun

make

it

cubic inch of average material from

Van Maanen’s Star would weigh 8,700 tons. And even Van Maanen’s Star isn’t the J.

times greater

for

(about 140 times as massive as Jupiter), and that fifteen times as

the

represents a record either for the smallness of a

a diameter of only 6,048 miles, so that

not very

If

pushed together, the

central nuclei

in fact, densities millions of

became conceivable. Sirius B by no means

atom had been

mass of the atoms

virtuallv all the

atom could be broken down and the

By

serious doubt.

internal structure of the

was known that

in a tiny

come under

tem-

stellar

1963 William Luyten of the University of Minnesota discovered a white dwarf

star

smallest. In

with a diameter of about 1,000 miles— only half that of the moon.*

Of

course, the white dwarfs can’t really give us

“little stars.”

They may be dwarfs

in

much

volume but they

satisfaction as

are sun-size in

mass, and giants in density and in intensity of gravitational

about

mass and temperature

really little stars, in

These are hard

making

to find.

We

a selection.

When we see

light-years in all directions,

even when they are

Judging by the cant dwarf, but

all

fields.

volume?

as well as in

look at the sky,

we

What

are automatically

the large, bright stars for hundreds of

but the dim

stars

we can

barely see at

all,

fairly close.

stars

we

we can

see,

our sun, sure enough,

is

a rather insignifi-

get a truer picture by confining ourselves to our

own immediate neighborhood. That is the only portion of space through which we can make a reasonably full census of stars, dim ones and all. Thus, within

five parsecs

a compilation prepared

(16V2 light-years) of ourselves, according to

by Peter

Van de Kamp

of

there are thirty-nine stellar systems, including our eight include

two

visible

Swarthmore College,

own

sun.

components and two include three

Of

these,

visible

com-

ponents, so that there are fifty-one individual stars altogether. "neutron stars” which are far smaller and far denser than any white dwarf can be. They are not mentioned in this article for the very good reason that the article first appeared in 1963, and neutron stars were not discovered until 1968. However, the omissicn of neutron stars does not invalidate any of the in*

There

are, of course,

formation in

this article.

3

ASIMOV ON ASTRONOMY

158

Of

these, exactly three^ stars are considerably brighter than our

and these

vve

can

call ''white giants/’ *

They

Table

are listed in

33.

%

Table 33

LUMINOSITY

DISTANCE STAR

(light-years)

(SUN=:

A

8.6

Altair

157

8.3

1.0

6.4

(see

Table 34) that are

can

call

Sirius

Procyon

There are then

A

1

dozen

a

nearly as bright as the sun.

stars

We

making any invidious judgments

as to

these "yellow

1 )

as

stars’

whether they are dwarfs

Table 34

DISTANCE STAR

(light-years)

Alpha Centauri

A

II

CO

1.01

4-3

—

Sun 70 Ophiuchi A

16.4

0.40

Tau

11.2

0.33

4-3

0.30

Ceti

1.00

Alpha Centauri B Omicron2 Eridani A

15.9

0

Epsilon Eridani

10.7

0.28

Epsilon Indi

Of

LUMINOSITY

1

1.2

0

0.1

70 Ophiuchi B

16.4

0.08

61 Cygni

A

11.1

0.07

61 Cygni

B

11.1

0.04

Groombridge 1618

14.1

6q

the remaining

stars, (see

Table 35)

all

of

which are

less

twenty-fifth as luminous as the sun, four are white dwarfs:

Table 35

DISTANCE STAR

(light-years)

B Omicron2 Eridani B Procyon B Van Maanen’s Star Sirius

LUMINOSITY

(SUN=

1 )

8.6

0.008

15.9

0.004

11.0

0.0004

13.2

0.00016

sun

:

TWINKLE,

T

I

NKLE

LITTLE STAR

,

1

not only considerably dimmer

Tliis leaves thirty-two stars that are

than the sun, but considerably cooler, too, and therefore distinctly red appearance.

much

To

be

59

in

sure, there are cool red stars that are nevertheless

brighter in total luminosity than our sun because they are so

gigantically voluminous. ation.)

(This

These tremendous cool

the reverse of the white-dwarf

is

stars are “red giants,”

of these in the sun’s vicinity— distant Betelgeuse

situ-

and there are none

and Antares are the

best-known examples.

The ver}'

nearest star to ourselves, the third

Alpha Centauri system. cause of is

An example of this is and dimmest member of

cool, red, small stars are “red dwarfs,”

only

its /{>

nearness,

the the

should be called Alpha Centauri C, but be-

It

more frequently

it is

called

3,000 as bright as our sun and, despite

Proxima Centauri.

It

nearness, can be

its

seen only with a good telescope.

To summarize,

then, there are, in our vicinity:

w'hite giants, twelve yellow stars, four

dwarfs. typical

If

we

giants, three

white dwarfs, and thirty-two red

consider the immediate neighborhood of the sun to be a

one (and we have no reason to think otherwise), then

half the stars in 'the heavens are red dwarfs

than the sun. Indeed, our sun

is

among

— luminosity “yellow dwarf” indeed! The

no red

w^ell

over

and considerably dimmer

the top 10 percent of the stars in

red-dwarf stars offer us something new.

When

I

discussed the

displacement of the sun by Jupiter at the beginning of the

article,

I

pointed out that the displacement would be larger, and therefore possible to observe from other stars,

An is

alternative

if

Jupiter were considerably larger.

would be to have the sun considerably

massive. It

not the absolute mass of either component, but the ratio of the masses

that counts. Thus, the Jupiter-sun ratio

detectable displacement. Sirius system, If a star

however,

is

The mass 1:2.5,

6 o.

is

1:1000, which leads to an in-

ratio of the

and that

is

two components of the

easily detectable.

were, say, half the mass of the sun, and

body eight times the mass 1

less

if it

were circled by a

of Jupiter, the mass ratio

would be about

The displacement would

of Sirius, but

it

not be as readily noticeable as in the case

would be detectable.

Exactly such a displacement was detected in 1943 at Sproul Observatory in Swarthmore College, in connection with 61 Cygni. From unevennesses in the motion of one of the major components, a third com-

ponent, 61 Cygni C, was deduced as existing; a body with a mass

^7^25

photo: yerkes observatory

TWINKLE, TWINKLE, LITTLE STAR

l6l

of our sun or only eight times that of Jupiter. In i960 similar displace-

ments were discovered

Lalande 21185 at Sproul Observatory. too, had a planet eight times the mass of Jupiter.

It,

And

for the star

same observatory announced

in 1963, the

the solar system. I’he star involved

This

star

was discovered

Emerson Barnard, and place

first

Barnard’s Star.

is

1916 by the American astronomer Edward

turned out to be an unusual star indeed. In the

the second nearest star to ourselves, being only 6.1 light-

it is

years distant.

it

in

a third planet outside

(The three

Alpha Centauri system, considered

stars of the

as a unit, are the nearest, at 4.3 light-years;

Lalande 21185 at

7.9 light-

BARNARD’S STAR In general, stars are “fixed” {that

move relative of them are

they don’t seem to

is,

one another) only because they are so far away. All moving, and very rapidly by Earthly standards dozens of miles per second but at even the distance of the nearest stars, such velocities to

—

—

result in

such tiny changes of position that they can only be seen after

many centuries. Of course, with a telescope even tiny changes can be made out in much shorter intervals of time but even so only for the very nearest stars

—

particularly

if

those stars happen to be

moving

at right angles to

our line of sight.

The most

rapidly

moving star is “Barnard’s star,” so-called because igi6 by the American astronomer E. E. Barnard.

it

was discovered

It

has the distinction of being the second closest star to our Solar

tem since star

in

only 6 light-years away.

The

only closer star

is

the three-

system of Alpha Centauri. Yet while Alpha Centauri

is

the third

it is

brightest star in the sky, Barnard’s star

The

out a telescope. of

sys-

reason for that

Alpha Centauri are

is

is

too

dim

to be

made out

that the two larger

stars rather like

with-

components

our Sun, and Barnard’s star

is

a small red dwarf.

Barnard’s star to

its

—

closeness

however, moving across our line of vision. Thanks has an apparent speed greater than that of any other

is,

it

J0.3 seconds of arc per year. In about 175 years, it would move the width of the Moon. That doesn’t sound much, but by starry stand-

ards

it is

enough

to give

it

The photographs show moved,

relative to the

twenty-two years.

the

name

of “Barnard’s

the distance

Runaway

Star.”

by which Barnard’s star has

more slowly moving

stars

about

it,

in a

mere

ASIMOV ON ASTRONOMY

162 years

third nearest.

is

system— at

Sirius

8.0

Next

and

is

and then the two

Wolf

the

stars of

8.6 light-years respectively.)

Barnard’s Star has the most raptd proper motion known, partly be-

cause

so close. It

it is

really, for in

the forty-seven years since

its

discovery

it

has only

much,

moved

a

over 8 minutes of arc (or about one quarter the apparent width of

little

the

10.3 seconds of arc a year. This isn’t

moves

moon)

across the sky.

For

a ‘"fixed” star,

however, that’s a tremen-

dously rapid movement; so rapid, in fact, that the star

Runaway

called “Barnard’s

is

sometimes

Star” or even “Barnard’s Arrow.”

dwarf with about one fifth the mass of the sun and less than 1/0500 the luminosity of the sun (though it is nine times as luminous as Proxima Centauri). Barnard’s Star

is

a red

Tlie planet displacing Barnard’s Star

is

Barnard’s Star

B and

the

it is

about 34 00 the times the mass of Jupiter. Put

smallest of the three invisible bodies yet discovered. It

is

mass of the sun and hence roughly 1.2 another way, it is about five hundred times the mass of the earth. possesses the over-all density of Jupiter,

about 100,000 miles

it

would make

a planetary

If it

body

in diameter.

All this has considerable significance.

Astronomers have about de-

cided from purely theoretical considerations that most stars have planets. Now we find that in our immediate neighborhood at least three stars

one planet apiece. Considering that we can only detect super-Jovian planets, this is a remarkable record. Our sun has one planet of Jovian size and eight sub-Jovians. It is reasonable to suppose that any

have at

least

other star with a Jovian planet has a family of sub-Jovians also. And indeed, there ought to be a number of stars with sub-Jovian planets only. In short, on the basis of these planetary discoveries,

it

would seem

quite likely that nearly every star has planets.

A

generation ago,

when

it

was believed that

through collisions or near-collisions of

Now we

family was excessively rare. true;

that

it is is

And

the truly lone

it

was

felt

systems arose

that a planetary

might conclude that the reverse

is

the one without companion stars or planets,

the really rare phenomenon.

seem to be from

yet the red dwarfs aren’t quite as little as they

their luminosity. less

star,

stars,

solar

Even the

smallest red dwarf, Proxima Centauri,

than one tenth the mass of the sun. In

uniform;

much more uniform than

nosities. Virtually all stars

stellar

fact, stellar

is

not

masses are quite

volumes, densities, or lumi-

range in mass from not

less

than one tenth of

the sun to not more than ten times the sun, a stretch of but two orders of magnitude.

There

is

good reason

for this.

As mass

increases, the pressure

and tem-

perature at the center of the body also increases and the amount of radiation produced varies as the fourth power of the temperature. Increase the temperature ten times, in other words, creases ten

and luminosity

in-

thousand times.

more than

Stars that are

ten times the mass of the sun are therefore

unstable, for the pressures associated with their vastly intense radiations

blow them apart

in short order.

On

the other hand, stars with

less

than

one tenth the mass of the sun do not have an internal temperature and pressure high enough to start a self-sustaining nuclear reaction. Tlie upper limit is fairly sharp. Too-massive stars, except in very rare cases, blow up and actually don’t exist. Too-light stars merely don’t shine and can’t be seen, so that the lower limit

may

light bodies

exist

even

if

own

Jupiter,

which

stars

solar system,

perhaps

is

an arbitrary one.

The

they can’t be seen.

Below the smallest luminous planets. In our

is

are,

indeed,

we have

the nonluminous

bodies up to the size of

the mass of the feebly glowing Proxima

A

body such as 6i Cygni C would have a mass one twelfth that of Proxima Centauri. Undoubtedly there must be bodies closing Centauri.

that remaining gap in mass. Jupiter, large as ter to

on

it is

for a planet, develops insufficient heat at

warmth

lend significant

Jupiter’s surface derives

for 6i

from

to

its

solar

its

Whatever warmth exists radiation.* The same may be true surface.

Cygni C.

How'ever, as

we

consider planets larger

still,

there

where the internal heat, while not great enough reactions,

enough

must come

to start

a point

runaway nuclear

keep the surface warm, perhaps enough to allow water to remain eternally in the liquid form.

We

is

might

great

call this a

in the infrared.

might, therefore,

to

super-planet but, after

*

director of Harvard College Observatory,

possible that such sub-stars are very

Actually,

radiating energy

a

Harlow Shapley, emeritus it

all, it is

warm

body would not glow visibly, but if our eyes infrared we might see them as very dim stars. They be more fairly called “sub-stars” than super-planets.

Such

were sensitive to

thinks

cen-

has recently been reported

common

in space,

and

that Jupiter radiates somewhat more heat than can be accounted for by solar tradiation. Maybe it has nuclear reactions going on in its compressed center and it is a very small and very un-warm star. it

ASIMOV ON ASTRONOMY

164

that they might even be the abode of

life.

To

be

sure, a sub-star

an earth-like density would have a diameter of about a surface gravity about eighteen times earth-normal. in the oceans, Is it

come

however, gravity

is

of

1

50,000 miles and

To

life

developing

no importance. might

possible that such a sub-star (with, perhaps, a load of life)

enough

rolling close

with

some

to the solar system,

day, to attract ex-

ploring parties?

We

can’t be certain

we can

it

won’t happen. In the case of luminous

detect invaders from afar, and

be coming

way

this

we can be

for millions of years.

sneak up on us unobserved; we’d never

—before

through

Then star

was

its

we

detected

its

sub-star,

know

might be right on top of us— say, within sun

A

certain that

it

its

reflected

mankind might go out

to see for themselves

childhood, ‘‘How

I

wonder what you

Only— it won’t be

twinkling.

are!”

light

what

set to rest that generations-long plaintive

and

It

miles of the

on the outer planets.

like

will

was approaching.

gravitational perturbations

at last

none

however, could

fifteen billion

presence through

stars,

and

a little

chant of

13

HEAVEN ON EARTH

The

nicest thing about writing these essays

ercise

it

me. Unceasingly,

gives

I

must keep

anything that wall spark something that

is

the constant mental ex-

my

will, in

eyes

my

and

ears

open

for

opinion, be of in-

terest to the reader.

For instance, a

letter arrived today, asking

about the duodecimal

tem, where one counts by twelves rather than by tens, and a

mental chain reaction that ended

gave

how

me it

My odd

a notion w'hich, as far as

I

in

this set

sys-

up

astronomy and, what’s more,

know,

is

original with

me. Here’s

happened. first

thought

corners.

dozen make

1

gross.

However,

number

which

has,

we

as far as

I

1

is

used in

dozen and 12

know, 12 has never been used

system, except by mathematicians in play.

on the other hand, been used

formal positional notation base just as

duodecimal system

For instance, we say that 12 objects make

as the base for a

A number

w^as that, after all, the

is

60.

The

as the base for a

ancient Babylonians used 10 as a

do, but frequently used 60 as an alternate base. In a

number based on 60, what we call the units column would contain any number from 1 to 59, while what w'e call the tens column would be the

ASIMOV ON ASTRONOMY

i66 “sixties”

column, and our hundreds column (ten times ten) would be

the “thirty-six hundred” column (sixty times sixty).

Thus, when we write a number, 123, what (iXio^)-l-( 2X10^) -[-(3X10**). 10 and 10^ equals

But

if

1,

the total

stands for

is

since lo- equals 100, 10^ equals

100-I-20-I-3 or, as aforesaid, 123.

is

the Babylonians wrote the equivalent of 123, using 60 as the

+ (2X60^)-!- (3X60^)-

would mean (1X60-) equals 3600, 60^ equals 60 and

base,

And

really

it

it

60*^

equals

1,

this

by our decimal notation. Using

i20-f-3, or 3-723

with the base 60

is

a “sexagesimal notation”

And

since bo-

works out to 3600-Ia positional notation

from the Latin word for

sixtieth.

As the word

“sixtieth”

suggests,

the sexagesimal notation can be

carried into fractions too.

Our own decimal where what you

see,

is

really

notation will allow us to use a figure such as 0.156,

meant

go up the scale

is

o-l-%o-f-%oo-f %ooo- The denominators,

in multiples of 10. In the

denominators would go up the scale represent o-t-%0

+ %600 + %1 6.000,

60X60X60, and so on. Those of you who know all about

sexagesimal scale, the

in multiples of

60 and 0.156 would

since 3600 equals

60X60, 216,000

equals

exponential notation will no doubt

be smugly aware that Yiq can be written io~h Vioo can be written io~2 and so on, while %o can be written 6o“h Hooo can be written

6o“2 and so on. Consequentlv,

number

a full

expressed in sexagesimal

notation would be something like this: (15) (45) (2). (17) (25) (59), or (25x60-2) 4(15X602) 4- (45x601) 4_ (2X60®) (17x60-1)

+

+

(59X60-2),

jf

yQu ^vant to amuse yourself by working out the

equivalent in ordinarv decimal notation, please do. As for me,

Lm

chickening out right now. All this

that

we

would be of purelv academic utilize sexagesimal

still

which date back

as

a

if it

weren’t for the fact

notation in at least two important ways,

to the Greeks.

The Greeks had lonians

interest,

a

base,

tendency to pick up the number 60 from the Baby-

where computations were complicated, since so

many numbers go evenly into 60 that fractions are avoided as often as possible (and who wouldn’t avoid fractions as often as possible?). One theory, for instance, is that the Greeks divided the radius of a circle into

60 equal parts so that

in dealing

with half a radius, or a third,

or a fourth, or a fifth, or a sixth, or a tenth (and so on) they could

ways express

it

as a

whole number of

sixtieths.

Then, since

al-

in ancient

HEAVEN ON EARTH days the value of

tt

(pi)

was often

167

rough and ready

set equal to a

since the length of the circumference of a circle

the radius, the length of that circumference

is

equal to twice

3,

tt

and

times

equal to 6 times the

is

Thus (perhaps) began the custom

radius or to 360 sixtieths of a radius.

of dividing a circle into 360 equal parts.

Another possible reason completes

day

it

for

doing so

with the fact that the sun

rests

circuit of the stars in a little over 365 days, so that in

its

moves about

^6 5

of the

way around the

sky.

each

Well, the ancients

weren’t going to quibble about a few days here and there and 360

is

much easier to work with that they divided the circuit of the sky into that many divisions and considered the sun as traveling through one of so

those parts (well, just about) each day.

A

360th of a circle

“step down.” stairway,

is

the sun

If

takes one step

it

Each degree,

if

we

from Latin words meaning

called a “degree” is

viewed as traveling down a long circular

down

stick

(well, just

about) each day.

with the sexagesimal system, can be divided

and each of those smaller

into 60 smaller parts

The

parts into 60

still

smaller

parts

and

(first

small part) and the second was called pars minuta secunda (sec-

so on.

ond small

part),

first

was called

division

Latin pars minuta prima

in

which have been shortened

in

English to minutes and

seconds respectively.

We symbolize a single stroke,

the degree by a

little circle

and the second by

double stroke, so that when we say

a

that the latitude of a particular spot ing that

minute by

(naturally), the

on earth

distance from the equator

is

39° 17' 42",

we

oi 39 degrees plus plus '^%6 00 ot 3 degree, and isn’t that the sexagesimal system? its

The second

is

place where sexagesimals are

used

still

is

in

are say-

3.

degree

measuring

time (which was originally based on the movements of heavenly bodies).

Thus we

divide the hour into minutes and seconds and

hour, 44 minutes, and 20 seconds, we are speakhour plus of an hour plus ^%600 of an hour.

speak of a duration of ing of a duration of

You can

1

when we

1

carry the system further than the second and, in the

Ages, Arabic astronomers often did. There divided one sexagesimal

fraction

into

is

a

Middle

record of one

who

another and carried out the

quotient to ten sexagesimal places, wLich

is

the equivalent of 17 dec-

imal places.

Now

let’s

take sexagesimal fractions for granted, and

let’s

consider

next the value of breaking up circumferences of circles into a fixed

num-

.

.

ASIMOV ON ASTRONOMY

i68

ber of pieces. And, in particular, consider the circle of the ecliptic along

which the sun, moon, and planets trace their path in the sky. After all, how does one go about measuring a distance along the sky?

One

can’t very well reach

essentially,

is

to

up with

draw imaginary

a tape measure. Instead the system,

lines

from the two ends of the distance

traversed along the ecliptic (or along to the center of the circle,

to measure the angle

The

do

shall try to

come

draw one

to

my

with

so,

as

where we can imagine our eye to be, and

made by

value of this system

I

circular arc, actually)

any other

is

those two lines.

hard to explain without a diagram, but

usual dauntless bravery

go along,

just in case

I

(

though you

I

re wel-

turn out to be hopelessly

confusing)

Suppose you have a

circle

and another diameter of 230 feet, and

with a diameter of 115

drawn about the same center with a another drawn about the same center with

circle still

a

feet,

diameter of 345

feet.

and would look like a target.) The circumference of the innermost circle would be about 360

(These are “concentric

circles”

and that of the outermost 1,080

that of the middle one 720 feet

Now of arc

mark

off

foot long, and from the two ends of the arc draw lines to the

1

ot the circumference

the center

called

quently

1

across the

and

different, 1

1

degree, the angle formed at

degree also (particularly since 360 such arcs

1

-degree angle

two outer

is

now extended outwards

circles,

so that the arms cut

they will subtend a 2-foot arc of the middle

The arms diverge just enough The lengths of the arc will be

a 3-foot circle of the outer one.

match the expanding circumference.

to

is

the entire space about the center)

fill

the

circle

%60 may be

the entire circumference and 360 such central angles will conse-

fill

If

feet.

the innermost circle’s circumference, a length

Ysqo

center. Since

will

but the fraction of the

circle

subtended

will

be the same.

-degree angle with vertex at the center of a circle will subtend a

gree arc of the circumference of any circle, regardless of

whether (if

it

is

we assume

a Euclidean geometry,

Suppose your eye was

circle,

it.

The two marks

that

is

its

1

A

-de-

diameter,

the circle bounding a proton or bounding the Universe

ogously true for an angle of any

upon

feet,

by

I

quickly add).

The same

anal-

size.

at the center of a circle that

are separated

by

or 60 degrees of arc.

from the two marks to your

is

% If

had two marks

the circumference of the

you imagine

eye, the lines will

a line

drawn

form an angle of 60 de-

HEAVEN ON EARTH you look

grees. If

first

one mark, then

at

169

at the other

you

will

was

a mile

have to

swivel your eyes through an angle of 6o degrees.

And

wouldn’t matter, you

it

whether the

see,

}'Our eye or a trillion miles. If the

circle

two marks were Ve of the circumfer-

ence apart, they would be 6o degrees apart, regardless of distance. nice to use such a measure, then,

away the

ho\^' far

circle

when you haven’t

How

the faintest idea of

is.

most

So, since through

from

of man’s history astronomers

had no notion

of the distance of the heavenly objects in the sky, angular measure was just the thing.

And

you think

if

it isn’t,

making use

try

of linear measure.

about

reply, judiciously, ''Oh,

But

as

distance,

full

a foot.”

soon as he makes use of linear measure, he

whether he knows

as large as the full

that anyone

who

moon,

or not.

it

it

judges the

aver-

moon in appearmeasure. He is liable to

age person, asked to estimate the diameter of the ance, almost instinctively makes use of linear

The

is

setting a specific

For an object a foot across

would have

to look

be 36 yards away. I doubt to be a foot wide will also judge it

moon

to

no more than 36 yards distant. we stick to angular measure and say that the average width of the

to be If

full

moon

and are

But

is

(minutes),

31'

making no judgments

as to distance

we’re going to insist on using angular measure, with w'hich the

if

way

of

this,

and

mon

circle

making

which

One

are

safe.

general population

at

we

it

is

unacquainted,

clear to everyone.

moon’s

to picture the

it

with wTich

we

must be held

such

circle

is

are

becomes necessary

all

to find

The most common way

size, for instance,

is

to take

some

of doing

some com-

acquainted and calculate the distance

to look as large as the

moon.

that of the twenty-five-cent piece.

about 0.96 inches and we won’t be diameter.

it

far off

if

we

consider

Its it

diameter just

is

an inch

held 9 feet from the eye, it will subtend an arc of 31 minutes. TTat means it will look just as large as the full moon in

does, and,

moon,

it

Now

if

If

it

a quarter

held at that distance between your eye and the

is

will just cover

if

is

full

it.

you’ve never thought of

this,

you

will

undoubtedly be

sur-

prised that a quarter at 9 feet (wdiich you must imagine will look quite small) can overlap the full moon (wdiich you probably think of as quite large).

To which

I

can only say: Try the experiment!

ASIMOV ON ASTRONOMY

170

Well, after

this sort of

thing will do for the sun and the

moon

but these,

of all the heavenly bodies, the largest in appearance. In

all, are,

the only ones (barring an occasional comet) that

fact, they’re

measured

visible disc. All other objects are

show

a

minute or

in fractions of a

even fractions of a second.

enough

easy

It is

to continue the principle of

comparison by saying

that a particular planet or star has the apparent diameter of a quarter

held at a distance of a mile or ten miles or a hundred miles and this

what

in fact,

quarter at

generally done.

is

But

one unvisionable measure

just substituting

There must be some better way of doing

And

at this point in

my

thoughts,

I

had

Suppose that the earth were exactly the

And

low, smooth, transparent sphere.

that?

You its

can’t see a

size.

You’re

for another. it.

my original size

(I

hope)

idea.

but were a huge hol-

it is

suppose you were viewing the

not from earth’s surface, but with your eye precisely at earth’s cen-

skies ter.

is

and you can’t picture

at such distances,

all,

what use

of

is,

You would then

see

all

the heavenly objects projected onto the

sphere of the earth. In effect,

would be

it

as

though you were using the entire globe of

the earth as a background on which to paint a replica of the celestial sphere.

The

value of this

which we can

is

that the terrestrial globe

easily picture angular

the one sphere

is

upon

measurement, since we have

all

learned about latitude and longitude which are angular measurements.

On

equal to 69 miles (with minor variacan ignore, because of the fact that the earth is not a

the earth’s surface,

tions,

which we

1

degree

perfect sphere). Consequently,

degree,

is

equal to

You

is

minute, which

1

equal to 1.15 miles or 6,060 feet, and

%o

oi

3.

minute,

see, then, that

heavenly body,

drawn on the

The moon,

if

we know

is

equal to

is

^

second, which

1

is

equal to 101 feet.

we know exactly

the apparent angular diameter of a

what

its

diameter would be

if

it

were

earth’s surface to scale. for instance,

with an average diameter of 31 minutes by

angular measure, would be drawn with a diameter of 36 miles, to scale

%o

on the

earth’s surface. It

would neatly cover

all

if

painted

of Greater

New

York or the space between Boston and Worcester.

Your as

it

first

seems.

impulse

may be

Remember, you

a

'AVHAT!” but

this

is

not

are really viewing this scale

really as large

model from the

HEAVEN ON EARTH

171

center of the earth, four thousand miles from the surface, and just ask

how

yourself

New

large Greater

Or

of 4,000 miles.

York would seem, seen from

look at a globe of the earth,

a distance

you have one and

pic-

ture a circle with a diameter stretching from Boston to W^orcester

and

you

will see that

it

is

small indeed compared to the whole surface of

moon

the earth, just as the

itself

surface of the sky. (Actually,

our

But

moon to fill painted moon to fill

at least this

and

posing,

it

is

small indeed compared to the whole

would take the area of 490,000 bodies the entire sky, and 490,000 bodies the

it

the size of the size of

all

of the earth’s surface.)

shows the magnifying

comes

if

moon

I

am

pro-

handy where bodies smaller

in ap-

are concerned just at the point

where

in particularly

pearance than the sun or the

effect of the device

the quarter-at-a-distance-of-so-manv-miles notion breaks down.

For instance,

in

Table

36,

I

present the

maximum

angular diameters

of the various planets as seen at the time of their closest approach to earth, together with their linear diameter to scale

if

drawn on

earth’s

surface. I

omit Pluto because

ever,

if

we assume

furthest point in

its

angular diameter

is

that planet to be about the size of

its

orbit,

it

will still

How-

not well known.

Mars then

at

its

have an angular diameter of 0.2

seconds and can be presented as a circle 20 feet in diameter.

Table 36

PLANETS TO SCALE

ANGULAR DIAMETER

LINEAR DIAMETER

PLANET

(seconds)

(feet)

Mercury

12.7

1,280

Venus Mars

64,5

6,510

25.1

2,540

Jupiter

50.0

5.050

Saturn

20.6

2,080

Uranus

4.2

Neptune

2.4

425 240

Each planet could have ience.

its satellites

For instance, the four large

drawn to

scale with great conven-

satellites of Jupiter

would be

ranging from 110 to 185 feet in diameter, set at distances of miles from Jupiter.

The

entire Jovian system to the orbit of

circles

3 to

its

14

outer-

ASIMOV ON ASTRONOMY

172

most

(Jupiter IX, a circle about

satellite

5

inches in diameter) would

cover a circle about 350 miles in diameter. «

The

»

such a setup, however, would be the

real interest in

the planets, do not have a visible disc to the eyes. Unlike the

stars, like

planets, however, they

The

telescope.

planets

do not have

a visible disc even to the largest

but Pluto) can be blown up to discs even

(all

by moderate-sized telescopes; not so the

By

The

stars.

stars.

methods the apparent angular diameter

indirect

of

some

stars

has

any

star

largest angular diameter of

been determined. For instance, the

0.047 seconds. Even the huge 200-inch telescope cannot magnify that diameter more than a thousandis

probablv that of Betelgeuse, which

is

and under such magnification the

fold,

minute of arc

in

appearance and

200-incher than Jupiter

is

largest star

to the unaided eye.

are far smaller in appearance than

no more

therefore

is

is

is

And

still

less

than

1

of a disc to the

most

of course,

stars

giant Betelgeuse. (Even stars that

away

are in actualitv larger than Betelgeuse are so far

to

as

appear

smaller.)

my

But on

earth scale, Betelgeuse with

an apparent diameter of

0.047 seconds of arc would be represented by a circle about 4.7 feet in

(Compare

diameter.

However,

it’s

that with the 20 feet of even distantest Pluto.)

no use trying

to get actual figures

because these have been measured for very few the assumption that

sun has. (This

is

all

not

on angular diameters

stars.

Instead,

let’s

make

the stars have the same intrinsic brightness the

so, of course,

so the assumption won’t radically

but the sun

is

an average

star,

and

change the appearance of the uni-

verse.)

Now

then, area for area, the sun (or any star) remains at constant

brightness to the eye regardless of distance. to twice

present distance,

its

four times but so would of

its

it,

that’s

area

I’he

would be

its

it

the sun were

moved out

apparent brightness would decrease by

apparent surface area.

just as bright as

it

What we

ever was; there

could see

would be

less of

all.

same

is

true the other way, too. Mercury, at

to the sun, sees a sun that

but

its

If

sees

is

Well, then,

if all

is

closest

approach

no brighter per square second than ours

one with ten times

that Mercury’s sun

its

as

many

is,

square seconds as ours has, so

ten times as bright as ours in total.

the stars were as luminous as the sun, then the ap-

parent area would be directly proportional to the apparent brightness.

HEAVEN ON EARTH

W

e

know

the magnitude of the sun

of the various stars, ness,

and that

— 26.72)

(

173

as well as the

gives us our scale of comparative bright-

from which we can work out

a scale of comparative areas and,

therefore, comparative diameters. Furthermore, since

gular measure of the sun,

we can

we know

vert to linear diameters (to scale)

paragraph already).

I’ll

details

the an-

use the comparative diameters to cal-

culate the comparative angular measures which, of course,

But never mind the

magnitudes

on the

we can

earth.

(you’ve probably skipped the previous

give you the results in Table 36.

(The fact that Betelgeuse has an apparent diameter of 0.047 no brighter than Altair is due to the fact that Betelgeuse, a red

is

has a lower temperature than the sun and

that

is

Remember

consequence.

in

that Table 37 stars are as luminous as the sun.)

all

con-

much dimmer

giant,

per unit area

based on the assumption

is

So you see what happens once we leave the Solar System. Within that system, we have bodies that must be drawn to scale in yards and miles. Outside the system, in

mere

we

deal with bodies which, to scale, range

inches.

Table 37 STARS TO SCALE

ANGULAR DIAMETER

LINEAR DIAMETER

(seconds)

(inches)

MAGNITUDE OF STAR

—

1

(e.g. Sirius)

0

(e.g.

1

(e.g. Altair)

Rigel)

0.014

17.0

0.0086

10.5

0 b0

6.7

\.n

2 (e.g. Polaris)

0.0035

4.25

3

0.0022

2.67

4

0.0014

1.70

5

0.00086

1.05

6

0.00055

0.67

If

you imagine such small patches of

earth’s center,

I

think you will get a

are in appearance

T he

total

which two

earth’s surface, as seen

new

vision as to

and why telescopes cannot make

number thirds are

dim

stars

in

small the stars

visible discs of

of stars visible to the naked eye

them.

about 6,000, of of 5th and 6th magnitude. might

then picture the earth as spotted with 6,000

about an inch

how

from the

is

We

stars,

most of them being

diameter. There would be only a very occasional

.

ASIMOV ON ASTRONOMY

174

larger one; only 20, all told, that

would be

as

much

diam-

as 6 inches in

eter.

Tlie average distance between two stars on earth’s surface would be

180 miles. There would be one

and one hundred

more

stars,

or, at

or

less,

New

most, two stars in

York

State,

within the territory of the United

States (including Alaska)

Tlie sky, you see,

Of

quite uncrowded, regardless of

is

course, these are only the visible stars.

iads of stars too faint to

appearance.

its

Through

a telescope, myr-

be seen by the naked eye can be made out and

the 200-inch telescope can photograph stars as

dim

22nd magni-

as the

tude.

A star

of

magnitude

22,

drawn on the earth

would be

to scale,

a

mere

0.0004 inches in diameter, or about the size of a bacterium. (Seeing a shining bacterium on earth’s surface from a vantage point at earth’s

down,

center, 4,000 miles

is

modern telescope.) The number of individual be roughly

tw^o billion.

stars visible

(There

nucleus which billion

is

dowm

to this

all

them

of

power

of the

magnitude would

are, of course, at least a

our Galaxy, but almost

stars in

two

a dramatic indication of the

hundred

billion

are located in the Galactic

completely hidden from our sight by dust clouds.

we do

The

own neighborhood

see are just the scattering in our

of the spiral arms.)

Drawn to scale on the earth, this means that among the 6,000 circles we have already drawn (mostly an inch in diameter) we must place a pow^dering of two billion more dots, a small proportion of which are still

large

The would

enough

to see, but

still

be,

on the

earth-scale, 1,700 feet.

a person looks at a

to a large telescope,

beyond

all

tween them all

stars of

still

photograph showing the myriad

the vast numbers of

comparatively huge. In

the starlight that reaches us

magnitude

1.

Tliis

From

it

stars,

fact,

it

is

in the past. stars visible

possible to see

galaxies.

the clear space be-

has been estimated

equivalent to the light of 1,100

is

means that

were massed together, they would

would be

have asked myself

for one,

powder and observe the outer

see, despite is

I,

he can’t help wondering how

that talcum

Well, you

that

in size.

average distance betw^een stars even after this mighty powdering

This answers a question

Once

most of which are microscopic

if all

fill

a

the stars that can be seen

circle

(on earth-seale)

that

18.5 feet in diameter.

this

we can conclude

that

all

the stars combined do not cover

HEAVEN ON EARTH up

as

much

moon,

all

of the sky as the planet Pluto.

by

itself,

As a matter of

much

obscures nearly 300 times as

other nighttime heavenly bodies, planets,

all

175 fact,

of the sky as

the

do

satellites, planetoids, stars,

put together. Tliere

would be no trouble whatever

our Galaxy obstacle,

if

it

in

viewing the spaces outside

weren’t for the dust clouds. These are really the only

and they

can’t be

removed even

if

we

set

up

a telescope in

space.

What

a pity the universe couldn’t really

face temporarily— just long

maids with seven mops with

be projected on earth’s

sur-

enough to send out the Walrus’s seven strict orders to give

the universe a thorough

dusting.

How

happy astronomers would then be!*

have always thought, deep in my heart, that this notion of mapping the heavens on the globe of the earth was the most brilliant I have ever had. In the dozen years since this article first appeared in 1961, I never even had a single letter saying, “Gee, Isaac, that was brilliant.” Oh, well *

I

.

.

.

THE FLICKERING YARDSTICK

Every once

in a while,

astronomical opinion concerning the size of the

Universe changes suddenly

— invariably for

the larger. Tlie last time this

happened, the responsibility could be placed directly at the door of

a

wartime blackout.

As

late as the turn of the century,

astronomers had only the foggiest

The best Dutch astronomer named Jacobus

notion of the size of the Universe, as a matter of of the time

was made by

a

fact.

estimate Cornelis

Kapteyn. Beginning in 1906, he supervised a survey of the Milky Way. He would photograph small sections of it and count the stars of the various magnitudes.

Assuming them

to

be average-sized

culated the distance they would have to be in order to

he

stars,

show up

as

cal-

dimly

as they did.

He ended up

with the concept of the Galaxy as a lens-shaped object

(something which had been more or

less

generally accepted since the

days of William Herschel, a century earlier). Tlie Milky

the cloudy haze formed by the millions of distant stars

Way

we

see

is

simply

when we

THE FLICKERING YARDSTICK

177

look through the Galactic lens the long way. Kapteyn estimated that the Galaxy was 23,000 light-years in extreme diameter and 6,000 lightyears thick.

And

anyone, could

as far as he, or

tell at

the time, nothing

existed outside the Galaxy.

He

also decided that the Solar

Galaxy, by the following line of reasoning.

ter of the

Way cut

System was located quite near the cenFirst,

the heavens into approximately equal halves, so

the median plane of the lens.

Way

median plane, the Milky

If

we were much above

the Milky

we must be on or below the

would be crowded into one half of the

sky.

Way

Secondly, the Milky If

we were toward one end

was about equally bright

all

the

way around.

or the other of the lens, the Milky

Way

in

the direction of the farther end would be thicker and brighter than the section in the direction of the nearer end.

In short, the

Sun

is

in the center of the

Galaxy, more or

less,

because

the heavens, as seen from Earth, are symmetrical, and there you are.

But there was one characteristic of the heavens which showed a disturbing asymmetry. Present in the sky are a number of '‘globular clus-

These are

ters.” less

collections of stars packed rather tightly into a

spherical shape.

hundred thousand

them

exist in

Each globular

stars

to a

cluster contains anyw'here

more

or

from

a

few million and about two hundred of

our Galaxy.

Well, there's no reason why such clusters shouldn't be evenly distributed in the Galaxy, and if we were at the center, they should be spread evenly

seem

to be

all

over the

sky— but

crowded together

in

they're not.

A

large part of

one small part of the

them

sky, that part

covered by the constellations of Sagittarius and Scorpio. It's

the sort of odd fact that bothers astronomers and often proves

the gateway to important Tlie

way

new

views of the Universe.

to a solution of the problem,

and to the new view of the

Universe, lay through a consideration of a certain kind of variable a star that flickers, if

is

you

which

is

star;

constantly varying in brightness; a star which

like.

There are a number of different kinds of variable stars, differentiated from one another by the exact pattern of light variation. Some stars flicker for

outside reasons; usually because they are eclipsed by a

companion which the constellation

dim

way of our line of sight. The star Algol in Perseus has a dim companion which gets in our way gets in the

every 69 hours. During that time of eclipse, Algol loses two thirds of

its

.

ASIMOV ON ASTRONOMY

178 light (it

is

not a total eclipse)

in a

matter of a couple of hours and

re-

V

gains

as quickly.

it

More

interesting are stars

changes

which

really vary in brightness

because of

Some explode with

greater or

their internal constitution.

in

some vary all over the lot in irregular fashion for mysterious and some vary in very regular fashion for equally mysterious

lesser force;

reasons;

reasons.

One is

and more noticeable examples of the

of the brighter

a star called Delta Cephei, in the constellation

Cepheus.

last

group

brightens,

It

dims, brightens, dims with a period of 5.37 days. From its dimmest point it brightens steadily for about two days, reaching a peak bright-

dim point. It then spends about three and a third days fading off to its dim point again. The brightening is distinctly more rapid than the dimming. From its spectrum, it would seem that Delta Cephei is a pulsating star. That is, it expands and contracts. If it remained the same temperature during this pulsation, it would be easy to understand that it was brightest at peak size and dimmest at least size. However, it also changes temperature and is hottest at peak brightness and coolest at the dim point. The trouble is that the peak temperature and peak brightness that

is

just

double

its

brightness at

maximum size, but when it is expanding and halfway toward peak size. The lowest temperature and dimmest point comes when it is contracting and halfway toward minimum size. This means ness come, not at

that Delta Cephei ends up being about the brightness

and

in the

same

trough of dimness. In the

size at the

first

peak of

though,

case,

it is

in the process of expansion; in the latter, in the process of contraction.

THE MILKY WAY The Milky

Way

is

dark Moonless night

ous

light,

a bright

dweller never sees

Guesses as to

it

its

a it

band

of soft light that encircles the sky.

looks like a faintly luminous cloud.

Moon,

city glare,

(or any but a

washes

it

out.

Any

nature abounded in ancient times.

city-

stars)

could be a

It

Heaven and Earth

a

extrane-

A modern

few hundred of the brighter

stairway of the gods, stretching between

On

(

though the

rainbow seemed a better candidate for that role). The prettiest notion I know of was that it represented an overflow of milk from Hera's breast (Hera being the wife of Zeus, the chief of the

Greek gods).

ism

XV v’

photo:

Of

mount wilson and palomar observatories

some unromantic astronomers guessed it might consist of crowds of stars so numerous and so dim as to appear to melt together in a kind of dimly luminous fog. And in 1610, when Galileo turned his telescope upon the Milky Way, that turned out to be exactly the case. It was composed of crowds of stars and looked, indeed, like lumicourse,

nous grains of talcum powder. The Milky Way seems to be more or tions as

though we were located

be the case

if

less

evenly bright in

all direc-

That would

in a ring of distant stars.

the stars were gathered together in a lens-shape with

ourselves roughly at the center. If

we looked out through

the long axis

we would see many millions of distant stars that would coalese into a Milky Way. At right angles to the long axis we would see relatively few stars and there would be no distant lummous haze. of the lens,

Yet the Milky be at

its

Way

is

not precisely even in brightness.

brightest in the constellation of Sagittarius

graph shows some of the clouds of

Way. The

and

It

this

stars in that portion of the

dots of light are too closely spaced to he counted

one of them

is

or less like ours.

a

Sun more

seems

or less like ours

to

photo-

Milky

and every

and may have planets more

photo:

mount wilson and PALOMAR

0BSERVAT(

STAR CLUSTER Stars are not spread evenly through the Galaxy.

They tend

to exist

Some are loose clusters containing a few dozen or a few hundred stars. Some are much larger clusters. When a cluster contains stars by the many thousands, the mutual gravitational force draws them in clusters.

close into a

more

or less spherical ball, at the center of

which

stars are

separated by only a few hundred billion miles. That

still

leaves plenty

room between indeed compared

but

it is

of

between

Such

They

stars

is

stars,

to

with

our

in the

little

danger of

collisions,

own neighborhood where

the average distance

dozens of thousands of billions of miles.

‘'globular clusters’ are not visible to the

are all quite far

crowded

away and even

best, like small fuzzy patches.

In

unaided eye as

clusters.

in small telescopes they look, at

fact, the largest of

them, the Great

Hercules Gluster [not the one shown in the photograph but very it

in

appearance), was

object. It

was thirteenth

first

like

recorded as nothing more than a fuzzy

in a list

Charles Messier in 1781, so that

it

compiled by the French astronomer

was called

Mi 3.

TIIEFLICKERINGYARDSTICK

l8l

\Vliy the regular but non-synehronous pulsation in size and tempera-

That part is still a mystery. Tliere is enough that is charaeteristie

ture?

make astronomers fashion, that

all

realize

when they found

must belong

Cepheids vary among themselves

way through the

the

range.

in all this to

other stars behaving in like

in

honor of the

first

stars,

of the group.

in the length of their period.

some

periods are as short as one day,

Cephei

group of structurally similar

to a

which they called “Ccpheid variables”

all

of Delta

Some

long as 45 days, with examples Cepheids closest to us have periods

The

as

of about a week.

The

brightest

and

closest

Cepheid

is

none other than the North

Star.

has a period of 4 days, but during that time its flicker causes it to vary in brightness by only about 10 per cent, so it’s not surprising no nonIt

astronomer notices tention to the

it,

and that astronomers themselves paid more

somewhat dimmer but more

drastically

at-

changeable Delta

Cephei. Tliere are a

number

of stars with Cepheid-like variation curves that

are to be found in the globular clusters. Their

ordinary Cepheids nearer us,

The

longest period

an hour and

among

a half are

is

these

main

distinction

from the

that they are extremely short-period. is

about a day, and periods

known. These were

first

as short as

called cluster

Cepheids

while ordinary Cepheids were called classical Cepheids. However, cluster

Cepheids turned out to be

a

misnomer, because such

found with increasing frequency outside

The

cluster

stars

were

clusters, also.

Cepheids are now usually called by the name of the best-

studied example (just as the Cepheids themselves are).

The

best-studied

About twenty years later, William Herschel studied Mi 3 with a better telescope and was the first to record that it consisted of a large

The Great Hercules Cluster, the largest we can see, certainly contains 100,000 stars and may contain as many as a million. The globular star cluster shown in this photograph is in the constel-

mass of

lation list,

stars.

Canes Venatici (“T/ic Hunting Dogs”).

so that

We

it

can be called

It

too was on Messier's

M3.

can make out globular clusters in the neighboring Andromeda

Galaxy and very

likely there are similar objects in all galaxies.

perhaps two hundred in our

own Galaxy and

in the

There are

Andromeda and, we

can suppose, correspondingly fewer in smaller galaxies.

ASIMOV ON ASTRONOMY

182

MAGELLANIC CLOUDS *

The Magellanic Clouds luminous patches; rather

And

that's for a very

and

structure

«

like

portions of the Milky

good reason,

studied in detail. Then,

Cape

He

of

two small

like

Way

for they resemble the

far in the

not visible to Europeans and

the

eye,

broken loose.

Milky

Way

in

distance.

Because they are located

of Uranus)

naked

appear, to the

it

southern half of the sky they are

was not until 1834 that they were was John Herschel (the son of the discoverer it

who investigated them from an Good Hope.

found the Magellanic Clouds

observatory he set up at

dim stars, just as the Milky Way was. They were farther than the Milky Way, however. The Milky Way is made up of the more distant stars

own

of our

to be large collections of very

Galaxy, while the Magellanic Clouds are collections outside

our Galaxy.

The Milky which

stars,

Way is

Galaxy

is

a huge collection of well over 100 billion

100,000 light-years across. The Magellanic Clouds are

160,000 light-years away and contain a total of about 6 billion Nevertheless,

(shown

in the

despite

the

photograph

smaller

in a

contain some kinds of objects

—

size

of

the

stars.

Magellanic

Clouds

somewhat overexposed condition) they larger and more spectacular than any

found in the Galaxy at least in those parts of the Galaxy we can see. For instance, the most luminous known star S Doradus is in the Large Magellanic Cloud. It is about 600,000 times as luminous as the Sun. 1 he Large Magellanic

Cloud

also

contains the Tarantula Nebula,

a bright cloud of dust like the familiar Orion Nebula in our Galaxy,

but 5,000 times

No They

example

larger.

objects outside our Galaxy are as close as the Magellanic Clouds. are

is

what we might consider

a star called

RR

"‘satellite

galaxies" of ours.

Lyrae, so the cluster Cepheids are called

RR

Lvrae variables.

Now

none

of this

seemed to have any connection with the

size of the

when Miss Henrietta Leavitt, studying the Small Magellanic Cloud, came across a couple of dozen Cepheids in them. Universe until 1912

(The Large and the Small Magellanic Clouds that look like detached remnants of the Milky

are

two foggv patches

Way. They

are visible in

photo: yerkes observatory

the Southern Hemisphere and were

first

sighted by Europeans during

the round-the-world voyage of Ferdinand Magellan and his crew back in

1520— whence the name.)

The Magellanic Clouds scope and

it

is

can be broken up into

stars

by

a

good

tele-

only because they are a long way distant from us that

these stars fade into an undifferentiated foggy patch. Because the clouds are so far distant

from

us, all

the stars in one cloud or the other

considered about the same distance from us.

Whether

may be

a particular star

ASIMOV ON ASTRONOMY

184 is

at the near edge of the cloud or the far edge

(This

is

like

an individual

little

difference.

making the equally true statement that every man

Washington

state of

makes

in

is

roughly the same distance,

i.e.

in the

3,000 miles, from

Boston.)

This also means that when one appears twice as bright as another

Small Magellanic Cloud

star in the star, it is

twice as bright. There

is

no

distance difference to confuse the issue.

Well, when Miss Leavitt recorded the brightness and the period of variation of the Cepheids in the Small Magellanic Cloud, she

smooth

relationship.

She prepared

a

The

found

a

brighter the Cepheid, the longer the period.

graph correlating the two and

this

is

called the ''period-

luminosity curve.”

Such a curve could not have been discovered from the Cepheids near us, just

because of the confusing distance difference. For instance. Delta

Cephei

is

period.

more luminous than the North Star and therefore has But the North Star is considerably closer to us than

Cephei, so that the North Star seems brighter to longer period seems to go with the

dimmer

the

period-luminosity

was

curve

promptly made the assumption that

it

Delta

For that reason, the

Of

course,

if

straighten the matter out, but at the time the distances were not

Once

is

we knew Delta Cephei, we could

star.

the actual distances of the North Star and of

us.

a longer

established,

held for

all

known.

astronomers

Cepheids, and were

then able to make a scale model of the Universe. That

is, if

astronomers

spotted two Cepheids with equal periods, they could assume they were also equal in actual luminosity. If

bright as Cepheid B,

it

distant from us as was

Cepheid

A

seemed onlv one fourth

as

A

as

could only be because Cepheid

Cepheid B. (Brightness

was twice

varies inversely as the

square of the distance.) Cepheids of different period could be placed, relatively to us,

with only slightly more trouble.

With all the Cepheids in relative place, astronomers would need know the actual distance in light-years to any one of them, in order know the actual distance to all the rest. There was only one trouble

here. TTie sure

the distance of a star was to measure

its

than

parallax

100 light-years,

measure. is

And

however,

the

method

parallax.

to to

of determining

At distances of more

becomes too small

to

unfortunately even the nearest Cepheid, the North Star,

several times that distance

from

us.

Astronomers were forced into long-drawn-out, complex, analyses of medium-distant

statistical

(not globular) star clusters. In this way.

THE FLICKERING YARDSTICK

185

they determined the actual distance of some of those clusters, including the Cepheids they contained.

Ihe

model

scale

Cepheid

variables

of

the Universe thus

had become

became

a

real

flickering yardsticks in the

map. llie

hands of the

astronomers.

In 1918

Harlow Shapley

started calculating the distance of the various

RR

globular clusters from the

Lyrae variables they contained, using

Miss Leavitt’s period-luminosity curve. His little

too large and were corrected

figures turned out to

downward during

be a

the next decade,

but the new picture of the Galaxy which grew out of his measurements has survived.

The

globular clusters are distributed spherically above and below the

median plane of the Galaxy. The center of ters

is

this

sphere of globular clus-

plane of the Galactic lens, but at a spot some tens of thou-

in the

sands of light-years from us

in

the direction of the constellation Sagittar-

ius.

That explained wTy most

of the globular clusters were to be found in

that direction. It

seemed

to Shapley a natural

assumption that the globular clusters

centered about the center of the Galaxy and later evidence from other directions bore him out. So here we are, not in the center of the Galaxv at

all,

We

but w ell out to one

side.

median plane of the Galaxy, for the Milky Way does split the heavens in half. But how account for the fact that the Milky W^ay is equally bright throughout if we are not, in fact, centered? are

in the

still

Tlie answer

is

that the

median plane on the

(where we happen to be) lie

is

outskirts of the

loaded with dust clouds. These happen to

between ourselves and the Galactic center, obscuring

The

result

is

that, wdth or

it

completely.

without optical telescopes, we can only

see our portion of the Galactic outskirts.

part of the Galaxy which

we can

We

are in the center of that

see optically,

and that part

too far off in size from Kapteyn’s estimate. Kapteyn’s error

was at the time excusable) was the Galaxy there

all

in

assuming that wdiat

at the center.

is

The

is

not

(which

w'e could see w'as

w^as. It w^asn’t.

Tlie final model of the Galaxy, now' thought to be correct, a lens-shape that

Galaxy

is

that of

ioo,coo light-years across and 20,000 light-years thick thickness

the position of the Sun

falls off as

the edge

is

approached and

in

(30,000 light-years from the center and two

THE ANDROMEDA GALAXY There like a

is

a small object in the constellation of

cloudy patch of

light.

In small telescopes

Andromeda

it

that looks

looks like an elliptical

was called the ''Andromeda Nebula.” Until about half a century ago, it was taken to be an object within our galaxy. In fact, until about half a century ago, it was supposed that everything was bit of fog

and

it

within our galaxy; that our galaxy, various clouds, both luminous objects,

made up

and

made up

lOO

billion stars plus

dark, plus unseen planets

and other

the entire Universe. Since our galaxy was 100,000

light years across, that

didnt seem

to constrict matters too

However, the Andromeda Nebula was a

when analyzed by

off

istic

of a luminous cloud of dust like starlight

and

nosity could a star be seen.

yet

and

gas.

much.

bit puzzling.

a spectroscope did not

gave

markably

of

seem

to

Rather the

The

light

it

be characterlight

was

re-

nowhere within that oval patch of lumi-

— THE FLICKERING YARDSTICK thirds of the

way toward the extreme edge

l8y

of the Galaxy)

only

is

3,000 light-years thick.

Even before the Galactic measurements had been finally determined, the Gepheids in the Magellanic Glouds had been used to determine their distance. Those turned out to be rather more than 100,000 lightyears distant. (Our best modern figures are 130,000 light-years for the Large and 170,000 light-years for the Small Magellanic Gloud.) They enough

are close

to the

body

of our Galaxy

and small enough

in

com-

parison, to be fairly considered “satellite Galaxies” of ours.

From

the rate at which our Sun and neighboring stars are traveling

in their 200,000,000-year circuit of the Galactic center,

it

is

possible to

mass of the Galactic center (which contains most of the the Galaxy) and it turns out to be something like 90,000,000,000

calculate the stars in

times that of our Sun. mass, then

we can

100,000,000,000

If

fairly

stars.

we

consider our Sun to be an average star in

estimate the Galaxy as a whole to contain

In comparison, the two Magellanic Clouds to-

gether contain a total of about 6,000,000,000.

The

question in the 1920s was whether anything existed in the Uni-

verse outside our

Galaxy and

its

Suspicion rested on certain

satellites.

dim, foggy structures, of which a cloudy patch

Andromeda was

moon

in

the constellation

the most spectacular. (It was about half the size of the

naked eye and was called the Andromeda nebula “nebula” coming from a Greek word for cloud.) full

Could

to the

Andromeda Nebula was like our Milky Way, a luminous fog made up of a vast number of distant stars. If so, the stars must be dimmer and more distant than those of the Milky Way, for it

be that the

telescopes capable of easily revealing the Milky

separate stars could do no such thing for the

could

mean

What

its

settled the matter

couldnt he seen

in the

was the

fact

to

be composed of

Andromeda Nebula. That

Andromeda Nebula was far turn, put a new look on the size

that the

that would, in

Way

outside our galaxy of the Universe.

that although ordinary stars

Andromeda Nebula,

occasional novae, exploding

more than usual brightness, could be. In fact, a nova, a supernova, S Andromedae, appeared there in 188^. stars of far

and

particular

The Andromeda Nebula is, in short, a galaxy even larger than our own and it is something like 2.3 million light-years away. It is the farthest object we can make out with the unaided eye.

ASIMOV ON ASTRONOMY

i88

There were some nebulae which were knowm to be

parts

of our

Galaxy because they contained hot (and not too distant) stars that were the cause of their luminosity. The Orion nebula is an example.

The Andromeda

nebula, however, contained no such stars that anyone

could see and seemed to be self-luminous. Could of haze that could

be broken up into many

far, far

the proper magnification), as could the Milky

be, then, a patch

it

Way

distant stars (w’ith

and the Magellanic

Clouds? Since the same telescopes that resolved the Milky

Way

and the

Magellanic Clouds did not manage to do the same for the Andromeda

be that the Andromeda nebula was far more distant? Tlie answer came in 1924, when Edwin Powell Hubble turned the new 100-inch telescope at Mount Wilson on the Andromeda nebula nebula, could

it

and took photographs that showed the outskirts of the nebula resolved into stars. Furthermore, he found Cepheids among the new'ly revealed and used them

stars

to

determine the distance.

The Andromeda nebula

turned out to be 750,000 light-years distant and this is the value found in all the astronomy books published over the next thirty years.

Allowing for the distance, the Andromeda nebula

w^as obviously

an

Andromeda galaxy. Hubble established the fact that many other nebulae of the Andromeda type w^ere galaxies, even more distant than the Andromeda galaxy (which is a near neighbor of ours, in fact). The size of the Universe sprang in-

object of galactic size, so

stantly

one

from

in the

a

diameter

it is

now^ called the

in the

hundreds of thousands of

light-years to

hundreds of millions.

How'ever, there w^ere a few disturbing facts which lingered. For one

seemed to be considerably smaller than our should our owm Galaxy be the one outsize member of a large

thing, the other galaxies

own.

Why

all

group?

For another, the Andromeda galaxy had

a halo of globular clusters

own Galaxy did. Tlieir clusters, and dimmer than ours. Why?

how’ever, were considerably

just as

smaller

our

Thirdly, allowing for the distance of the galaxies and the speed at

wTich the Universe was know n tw’o billion years

some gists

ago that

all

central starting point.

swore up and

dowm

to

be expanding

it

could only have been

the galaxies w'ere squashed together at

The

trouble with that w’as that the geolo-

that the earth itself was considerably older

than two billion years. How' could the earth be older than the Universe? Tlie beginning of an answer came in 1942, when Walter Baade took

THE FLICKERING YARDSTICK

189

another look at the Andromeda galaxy with the 100-inch telescope. Until then only the outer fringes of the galaxy had been broken up into the central portions had remained a featureless fog. But

stars;

now Baade

had an unusual break. It was wartime and Los Angeles was blacked out. That removed the dim background of distant city light and improved seeing.

For the

portions of the

Andromeda

galaxy.

Baade could study the very brightest

the interior.

stars of It

time, photographs were taken that resolved the inner

first

turned out that there were striking differences between the bright-

est stars of the inner regions stars

in

and those

The

of the outskirts.

brightest

the interior were reddish while those of the outskirts were

That alone accounted for the greater ease with which photographic plates picked up the outer stars, since blue more quickly affects

bluish.

the plates than red does (unless special plates are used). Add to this, the fact that the brightest (bluish) stars on the outskirts were up to a

hundred times

To

Baade,

as bright as the brightest (reddish) stars in the interior.

it

seemed there were two

sets of stars in

galaxy with different structures and history. outskirts Population

He

the

Andromeda

called the stars of the

and those of the interior Population II. Population II is the dominant star-group of the Universe, making up perhaps 98 per cent of the total. They are, by and large, old, moderate-sized stars fairly uniform in characteristics and moving about in I,

dust-free surroundings.

Population

stars are

I

found only

those galaxies that have spiral arms.

in the

On

dust-choked spiral arms of

the whole, they are far more

and structure than the Population II stars, including very young, hot, and luminous stars. (Perhaps Population I stars sweep up the dust through which they pass gradually growing more massive,

scattered in age

hotter,

and brighter

— and

shortening their

lives, as

humans

do, by over-

eating.)

Our own Sun, by the way, happens

to

be occupying a

that the familiar stars of our sky belong to Population fortunately,

is

an old, quiet, settled

star

I.

spiral

arm

so

Our own Sun,

not typical of that turbulent

group.

Once

the 200-inch telescope was set up on

Mount

Palomar, Baade

continued his investigations of the two populations. ITere were Cepheid variables in both populations and this brought up an interesting point. I

he Cepheids of ^he Magellanic Clouds (which have no

spiral

arms)

ASIMOV ON ASTRONOMY

190

belong to Population

11

.

So do the

clusters for

Lyrae variables in the globular

of the moderately distant non-globular

So do the Cepheids

clusters.

RR

which the actual distances were

In other words

all

first

worked out

statistically.

the work done on the size of the Galaxy and the dis-

tance of the Magellanic Clouds, as well as on the original establishment of the

Cepheid

done on Population

yardstick, were

Cepheids. So

II

far,

so good.

But what about the distance of the outer

made out

that could be

The only stars Andromeda by

galaxies?

in the outer galaxies

such as

Hubble and his successors were the extra-large giants of the spiral arms. Those extra-large giants were Population I, and the Cepheids among

them were Population from Population Cepheids would

Cepheids. Since Population

could

II, fit

I

into a

one be certain

that

I

is

the

so different

Population

I

period-luminosity curve which had been

worked out from Population II Cepheids only? Baade began a painstaking comparison of the Population in the globular clusters with the Population I Cepheids

II

in

Cepheids our

own

1952 announced that the latter did not fit the Leavitt period-luminosity curve. For any particular period, a Population

neighborhood and

I

in

Cepheid was between four and

II

Cepheid would

the Population

Well, then,

I if

be.

A new

five

times as luminous as a Population

period-luminosity curve was drawn for

Cepheids. the Population

I

Cepheids of the Andromeda

spiral

arms were each considerably more than four times brighter than had been thought, then to be as bright as they seemed (the apparent

same of course) they had to be considerably more away as had been thought. The flickering yardstick of

brightness stayed the

than twice as

far

the Cepheids, which the astronomers had been using to measure the distance of the outer galaxies suddenly turned out to be roughly three

times as long as they had thought. All the nearer galaxies,

were suddenly pushed a galaxies

the

distance out into space.

distances of the nearer galaxies,

Universe had again increased in

w'as penetrating,

but a

triple

The

further

whose distances had been estimated by procedures based on

“known”

The

which had been measured by that yardstick

full

two

not somewhat

less

size,

all

retreated likewise.

and the 200-inch telescope

than a billion light-years into space,

billion light-years.

This solved the puzzles of the galaxies. three times the distance that

If all

the galaxies were about

had been thought, they must be

larger

THE FLICKERING YARDSTICK (in actuality)

than had been

tlioiiglit.

191

^^htll all the galaxies

suddenly

grown up, our own Galaxy is reduced to run-of-the-mill size and is no longer the one outsize member of the family. In fact, the Andromeda galaxy

at least twice the size of ours in terms of

is

numbers

of stars

contained.

Secondly, the globular clusters around Andromeda, being actually

much

away than had been thought, must be more luminous in than had been thought. Once the greater distance had been

further

actual fact

allowed

for,

the globular clusters of

comparable to our own

Andromeda worked out

be quite

in brightness.

Finally, with the galaxies

much

further spread apart, but with their

actual speeds of recession unaffected by the change (the of speed of recession does not

being tested)

to

measurement

depend on the distance of the object

would take a much longer period for the Universe to have reached its present state from an original compressed hunk of matter. This

it

meant the age

or even six billion years.

no longer had

Which was

of the Universe

With

minimum, five were content. They

to be, at

this figure, geologists

to consider the Earth to a great relief.

had

be older than the Universe.

THE SIGHT OF HOME

Now man will

is

struggling toward the

be bouncing among the

Moon*

far stars.

may come when some homesick

but someday, we hope, he

Can we imagine

astronaut will

lift

that the time

his eyes to the strange

skies of planets of distant suns in order to locate the tiny

speck that

is

“Sol”?— home, sweet home, across the frigid vastness of space. A touching picture, but what occurs to me is: How far away can said astronaut be and still make out the sight of home? For that matter,

we can make any

stellar

it

system be and

still

How

away can the inhabitant of make out the sight of the star in whose

general and ask:

far

planetary system he was born?

how bright the particular star is. I say is From where we sit here on Earth’s surface, we see stars

This, of course, depends on

and not seems. of

all

That brightness is partly due luminosity, but is also partly due to the distance

gradations of brightness.

star’s particular

pens to be from *

And

1969.

us.

A

star

nine years after this

not particularly bright,

first

appeared, the

Moon

as stars go,

to the it

hap-

might

was actually reached

—

in

THE SIGHT OF HOME seem

much

star

specimen to ns because

a brilliant

much more

brighter, but also

it

is

193

relatively close; while a

distant,

might seem

trivial in

comparison.

Consider the two

stars

Alpha Centauri and Capella,

Both are about equally bright 0.1

and

in

appearance, with magnitudes of about

(Remember

0.2 respectively.

for instance.

that the lower the magnitude, the

brighter the star, and that each unit decrease of magnitude

is

equal to

a multiplication of 2.52 in brightness.)

How ever,

the closest of

is

all

To

explain.

us.

Alpha

only 1.3 parsecs from us. (I am distances in parsecs in this chapter for a reason I wall shortly

Centauri giving

the two stars are not the same distance from

and

all stars

guide you, a parsec

is

equal to 3.26 light-vears, or to

is

150,000,000,000 miles.) Capella, on the other hand,

is

19,-

iq parsecs from

over 10 times the distance of Alpha Centauri.

us, or

Since the intensity of light decreases as the square of the distance,

the light of Capella has had a chance to decrease by

more than has the

10X10

or 100 times

Alpha Centauri. Since Capella ends by apAlpha Centauri, it must in reality be 100 times as

light of

pearing as bright as bright.

know^ a

If w'e

what from

star’s distance, w’e

brightness w'ould be

its

us.

The

connection

if it

can correct for

w'ere located at

it.

We

can calculate

some standard distance

distance actually used by astronomers as standard in this

10 parsecs (which

is

is

W'hy

I

am

giving

distances as

all

parsecs in this chapter).

Thus the apparent magnitude see

it)

Alpha Centauri

of

0.1

is

and of Capella

magnitude, however (the brightness actly 10 parsecs aw'ay)

The Sun, by

the way,

absolute magnitude

It is

is

is

4.8 for is

4.86.

just

it

TTie absolute

0.2.

w'ould appear

if

a star

Alpha Centauri and —0.6 about

as bright as

Both are average,

means

of

Equation

M=m-|-5— 5 w'here

as

is

were

M

tude and

is

D

Alpha Centauri.

run-of-the-mill stars.

5,

and

log

D

(Equation 16) star,

m

is

the apparent magni-

the distance in parsecs. At the standard distance of 10

D M=m.

parsecs the value of

M=m + 5—

Its

16.

the absolute magnitude of a is

ex-

for Capella.

possible to relate absolute magnitude, apparent magnitude,

distance by

we

(the actual brightness of a star as

or

is

10 and log 10 equals

The

1.

Tlie equation becomes

equation at least checks by telling us that

ASIMOV ON ASTRONOMY

194

at the standard distance of lo parsecs the

apparent magnitude

is

equal

Our

astro-

to the absolute magnitude.

But naut

use the equation for something

let’s

is

on

the local gentry. it

another

a planet of

He

wants

star

more

and he wants

significant.

do so with pride so he would

to

Sun to

to point out the

like to

have

a first-magnitude star.

The

equation will

might be

That

we

possible.

how

us

tell

far

away we can be

in order that this

absolute magnitude of the Sun

The

(M)

is

We want the apparent magnitude to be m. We now calculate for D which turns out

can’t be changed.

substitute that for

be equal to

4.86.

so

1,

to

1.7 parsecs.

Only Alpha Centauri is within 1.7 parsecs of the Sun. This means from a planet in the Alpha Centauri system only can the Sun be seen as a first-magnitude star, and from no other planetary system in the Univery close to us (less than 3 parsecs away, to be incomparably the brightest star in the sky though

verse. Sirius, for instance,

close

enough

is

only Ve as bright as Capella in actuality) and yet even from the Sirian svstem, the Sun would be seen as only a second-magnitude

star.

Well, then, his pride chastened, but homesick nevertheless, our astronaut might abandon first-magnitude pretensions and be willing to

any glimpse, however

settle for

faint, of

Since a star of apparent magnitude

by

comes out

as equal to

naked-eye

visibility at a

Of that

course,

it

is

6.15

can

1

and calculate

20 parsecs. Tlie Sun

a is

m

make

let’s

Now

it

limit of

distance of 20 parsecs.

visible for this distance in all directions

can be seen by naked eye anywhere

in a

like that)

(assuming so that

sphere of which the Sun

center and which has a radius of 20 parsecs. is

be made out

new value for D. down to the very

not obscured by dust clouds or anything

it is

sphere

just barely

under ideal seeing condition,

a pair of excellent eyes

equal to 6.5 instead of to

home.

The volume

is

it

the

of such a

about 32,000 cubic parsecs.

This sounds

like a lot

but

of stars (or multiple stars)

in the is

neighborhood of our Sun, the density

about 4V2 per 100 cubic parsecs. Within

the visibility sphere of the Sun there are therefore about 1450 stars or multiple-star systems. Since the Galaxy contains about a stars,

the

by naked

number

of stellar systems

eye, represents

hundred

from which we can be seen

an insignificant percentage of those

billion at in

all,

the

Galaxy.

Or put

it

another way.

The Galaxy

is

about 30,000 parsecs across the

THE SIGHT OF HOME width of

full

%oo

its

lens-shape.

if

we

The

range of

195 the Sun

visibility of

only

is

of this.

Obviously,

we can

Galaxy,

going to go

are

just take

it

flitting

for granted that

here and there in

when we

homesick eyes to the alien heavens, a sight of home

lift

our

tear-filled,

what we

is

the

will

not

get.

Of

course, let’s not be too sorry for ourselves.

There are

luminous than the Sun and therefore far less extensively 1 he least luminous star known is one which is listed

“Companion

of

BD4-4°4048” which,

magnitude of though

it is

19.2. It

is

visible.

Joe.

Now

books as

in the

for obvious reasons

(for purposes of this chapter only)

call

stars far less

we

suggest

I

Joe has an absolute

only two millionths as bright as the Sun and

only about 6 parsecs from

us,

it

is

al-

barely visible in a good

large telescope.

Using the equation,

it

turns out that at a distance of 0.03 parsecs,

just barely visible to the

Joe

is

put

in the place of the

Sun,

it

naked

eye.

This means that

Joe were

if

would disappear from naked-eye

sight at

a distance just six times as great as that of the planet Pluto. It is

unlikely that anywhere in the Galaxy there exist two stars this

close together, unless, of course, they

tem. (And Joe star

is

it is

the “Companion.”)

follows that the existence of a star like Joe

secret to

would be

a planet that actually revolves about Joe or about

own

living

companion.

its

Joe could ever get a naked-eye sight of

planet outside his

On

complete

a

any race of beings not possessing telescopes and not

man from

sys-

part of a multiple-star system, one which includes the

BD-|-4°4048, of which

It

form part of a multiple-star

on

No

home from any

multiple system; from any planet at

all.

the other hand, consider stars brighter than the Sun. Sirius, with

an absolute magnitude of 1.36 can be made out at parsecs, while Capella with an absolute

a distance of

magnitude of

— 0.6

100

could be

seen as far off as 260 parsecs. Sirius could be seen through a volume of

space 600 times and Capella through a volume over 2,000 times as great as that

Nor

through which the Sun can be seen. is

Capella the most luminous star by any means.

visible to the

naked

eye, Rigel

—

absolute magnitude of

nous

as the

Sun and

bright Capella.

3.8,

rather

is

Of

all

about the most luminous.

which makes

it

the stars

It

has an

over 20,000 times as lumi-

more than 100 times

as

luminous

as

even

ASIMOV ON ASTRONOMY

196

Rigel can be seen by the naked eye for a distance of 2,900 parsecs in

any direction, which means over Tliis

a range of Vs the

width of the Galaxy.

rather respectable.

is

means that over a large section of the Galaxy we might at least count on identifying our Sun by its spectacular neighbor. We could say to the local Rotarians, ‘‘Oh, well you can’t see our Sun from here, but It

pretty close to Rigel, that star over there, the one you call Bjfxlpt.”

it’s

and day-out luminosity is not held by any member of our own Galaxy. There is a star called S Doradus in the Large Magellanie Gloud (which is a kind of satellite galaxy of our own, about 50,000 parsecs away) and S Doradus has an absolute magnitude

But the record

— 9.

of

for steady day-in

can be seen by naked eye for

It

made out all length of our own

own

could be

across

full

large one,

Of

course,

no normal

star

explodes. Exploding stars

its

if it

a distance of 12,500 parsecs. It

small galaxy and across nearly the

were

in

our Galaxy.

can compete in brightness with a star that into two classes. First there are ordinary

fall

novae, which every million years or so blow off a per cent or so of their

mass and grow several thousand times brighter (temporarily) when they

do

so.

In between blowoffs they lead fairly normal lives as ordinary stars.

—

Such novae may reach absolute magnitudes of 9, which makes them only as bright as S Doradus is all the time, but then S Doradus is a

most unusual

star.

Gertainly the novae are a million times as luminous

as are average stars like

our Sun.

But then there are supernovae. These are smash

one big explosion, releasing

in

Sun does left

is

in sixty years.

Most

as

If

we imagined

— 17,

of

it

a

as

energy in a second as the

mass

is

The upper

magnitude reaches anywhere between

go completely to

much

of their

converted to a white dwarf.*

supernova can be 1,500 times

stars that

— 14

luminous

as

blown

off

and what

limit of their absolute

and

— 17

so that a large

even S Doradus.

good supernova reaching an absolute magnitude

could be seen by naked eye, at peak brightness, for a distance

up any-

of 500,000 parsecs. In other words, such a supernova flaring

where

in

is

our Galaxy could be seen by naked eye anywhere else in our

Galaxy (except where obscured by

interstellar dust). It

could even be

seen in our satellite galaxies, the Large and Small Magellanie Glouds.

However, the distance between our Galaxy and the nearest neighbor, the *

Or, more

Andromeda

likely, vve

galaxy,

now know,

to a

is

about 700,000 parsecs.

neutron

star.

full-sized

It

follows

E

1 II

SIGHT OF

HOME

197

that supernova in other galaxies cannot be seen by naked eye.

supernova that Galaxy,

or, at

by naked eye must be located

visible

is

in

Any own

our

most, in the Magellanic Clouds.

Now

astronomers have studied novae which have flared up in our Galaxy. For instance there was a nova in the constellation Hercules in telescopic obscurity to the

1934

bright as the North Star)

in

2nd magnitude (say

a matter of days

and stayed near that

brightness for three months. In 1942, a nova reached (as bright as Arcturus) for a

But novae themselves }

as

first

magnitude

month.

aren’t unusual.

An

average of 20

flare

out per

ear per galaxy.

Supernovae are different breeds altogether and astronomers would love to get data on them. Unfortunatelv, thev are quite rare.

It is esti-

mated that about 3 supernovae appear per galaxy per millennium; that is, one supernova for every 7,000 ordinary- novae. Naturally, a supernova can be best studied

if

it

appears in our

own

Galaxy, and astronomers are waiting for one to appear. Actually, there

supernovae

is

a

chance that our Galaxy has had

in the course of the last

three very bright novae which

thousand

years.

At

its

expected

least there

3

were

have been sighted by naked eye

in

that interval.

The

first

astronomers.

From

these Orientals,

where

was sighted

1034 by Chinese and Japanese the position in the constellation Taurus, recorded bv

of these

in a.d.

modern astronomers had

a prettv

good notion

as

to

any remnants of the novae. In 1844 the English astronomer, William Parsons, located an odd object in the appropriate to look for

place. It

was

a tiny star barely visible in a

tually turned out to

be

a

good telescope (which even-

white dwarf*). Surrounding

it

was an

ir-

regular mass of glowing gas. Because the gas was irregular, with clawlike projections, the object was

named

the Crab Nebula.

Continued observation over decades showed the gas was expanding. Spectroscopic data revealed the true rate of expansion and that com-

bined with the apparent rate revealed the distance of the Crab Nebula

be about 1,600 parsecs. Assuming that the gas had been exploded outward at some time in the past, it was possible to calculate backward to

to

see

when

that explosion

had taken place (from the present position and

rate of expansion of the gas). It turns out the explosion took place * It

turned out to be better than that. appeared showed it to be a neutron star.

New

studies ten years after this article

first

ASIMOV ON ASTRONOMY

198

about 900 years ago. I'here seems no doubt that the Crab Nebula is what remains of the nova of 1054. For the nova to be brighter than Venus it must have had a peak

m

apparent magnitude of —5. Substituting that for

M,

1,600 for D, the value of

in the

equation and

the absolute magnitude, u'orks out to be

and from the white-dwarf remnant and the gassy explosion, there can be no doubt that the nova of 1054 was a true about —16. From

just

this

supernova and one which took place within our Galaxy.

new star appeared in the constellation outshone Venus and was visible by day. This time In 1572 a

Europeans. In

fact,

the last and most famous of

all

Cassiopeia. It also it

was observed by

naked-eye astrono-

Tycho Brahe, observed it as a young man and wrote a book about it entitled De Nova Stella (“Concerning the New Star”) and it is from that title that the word “nova” for new stars comes. In 1604 still another new star appeared, this time in the constellation mers,

Serpens.

grew

It

was not quite

as bright as

— 2.5).

It

Mars

at

its

was observed by another great astronomer, Johann Kepler,

who had been Tycho's

Now

nova of 1572 and perhaps only brightest (say an apparent magnitude of

as bright as the

assistant in the latter’s final years.

were the novae of 1572 and 1604 supernovae? Unlike the case of the nova of 1054, no white dwarf, no nebulosity, no the question

is,

anything has been located in the spots reported by Tycho and Kepler.

The

direct evidence of supernova-hood

is

missing. Perhaps they were

onlv ordinarv novae.

Well,

if

they were ordinary novae with absolute magnitudes of only

—9, then the nova no more,

if

it

of 1572

must have been about 60 parsecs

was to surpass Venus

in brightness.

The nova

distant,

of 1604

would be 200 parsecs distance. Stars that close could scarcely fail to be seen with modern telescopes, even if they were dim, it seems to me. (Of course, if they ended up as dim as “Joe” they might not be seen, but that level of dimness

is

most

Most astronomers seem were supernovae

in

astronomical history.

our

unlikely.’^)

satisfied

own

Two

and 1604

Galaxy, and this brings up an irony of

supernovae appeared

gle generation, the generation

scope,

that the novae of 1572

just

in the space of a sin-

before the invention of the

and not one supernova has appeared

in

tele-

our Galaxy in the nine

generations since. they ended up as neutron stars, very unlikely in the case of ordinary novae, they might not be seen optically, but they might be detected as “pulsars,” bodies producing very rapid pulses of radio beams. If

)

photo: yerkes observatory

NOVAE Because our

own

personal

out energy at a fixed

the Sun,

star,

a stable star that pours

day after day, age after age, we mustn't think

rate,

that stars are intrinsieally stable. It

Sun

is

is

just

our good fortune that our

well balanced. {Actually, that's putting the cart before the horse.

is

Sun were not

the

would not have developed on Earth, or if it had, it would relatively quickly have been destroyed. The mere fact that we are here, then, shows that the Sun is well balanced and there is no need to perspire thankfully over the inevitable Eaeh star is what it is, owing to two opposed factors. There is the If

well balanced,

life

.

gravity,

star's

drawing

its

internal heat, tending to

substance tightly together, and the

expand that same substance. Since

it

star's is

the

and the compression itself that produce the enormous temperatures and pressures at the center that in turn produce the expansive force, you can see that there must be a delicate balance if the star is to remain at some point neither expanding nor contracting. If for some reason a star begins expanding, it cools off, and the gravity itself

gravitational force

expand.

The

becomes stronger than the weakening tendency

star begins to contraet so that the central

temperature

to in-

and eventually overwhelms the gravitational pull with the result that the star begins to expand again. There are numerous stars in the heaven that expand and contract in periods of from a few days to a few

creases

weeks.

Even

this

is

a kind of balance. Sometimes, there

pansion and the star explodes. shot basis, that

it

It

becomes

on the

right

much

a runaway ex-

brighter,

attracts the attention of astronomers. It

The accompanying photograph shows stage

so

is

and

its

more normal

a nova at

insignificance

its

is

on a onea “nova."

exploded bright

on the

left.

ASIMOV ON ASTRONOMY

200

SUPERNOVAE Novae must have been seen for as long as men have intelligently observed the night sky. Without instruments it might be difficult to be certain that a star has grown brighter for a time, if the brightening isn't much. However, if a star which is ordinarily too dim to be seen with the unaided eye becomes bright enough to be seen easily, it requires no instruments to take note of that.

appeared in the heavens

word "nova"

The

first

To

appearance, a ''new star" has

all

—and eventually

will disappear. In fact, the

it

Latin for "new."

is

nova to be recorded was one by the Greek astronomer Hipappearance inspired him to prepare a

map

parchus in 134

B.c. Its

{the

kind) in which a thousand stars were plotted carefully,

of

first

its

new

so that

stars that

might appear in the future could

and not confused with old

tected

On

easily

be de-

stars.

the average, about one star in four billion will explode in any

given year. But what kind of an explosion?

may

ploding,

one or two percent of

lose

tating to

any planets

and keep

right

Some

stars,

their mass.

it

may

little

An

ordinary nova, in ex-

mass. That might be devasitself,

not

the worse for

however, truly explode, blowing

Such a super-explosion

more

shines

its

have, but for

on shining, very

off

so. It will recover

its

up

hiccup. to nine tenths of

residts in a supernova,

with a brief

the point where that single exploding star

increase in brightness to

nary

star

brightly than an entire galaxy of a

hundred

billion ordi-

stars.

The photograph

here

overexposing the film at

what a

fairly close

thousand

is

not of a supernova; but the ordinary nova,

its

brightest, gives

supernova

light-years

away or

may

look

one the

like.

{By

visual impression of

fairly close, I

mean

a

so.)

when a star will explode. We we follow its dimming, as here,

Unfortunately there's no way of telling see

it

only after

and not

Even

it

its initial

has exploded so that

brightening.

a small telescope could

novae more exactly and made nant could

now be

located.

have plotted the position of the superit

Then

somewhat more if

likely that the

the supernovae had appeared after

the invention of the spectroscope, things would have been rosier

happy As

little

it is,

rem-

still

for

astronomers.

supernovae have been observed since Kepler’s time, about 50

photo: lick observatory

altogether, but only in other galaxies, so that the apparent brightness

was so low that

could be

little detail

made out

in the spectra.

Tlie brightest and closest supernova to have appeared since

showed up

Andromeda

1885 in the

in

an apparent magnitude of the naked eye. As in

I

7.

(It

galaxy, our neighbor. It reached

was not quite

visible,

you

said before, only supernovae in our

will note, to

own Galaxy

the Magellanic Clouds are visible to the naked e\e.)

Andromeda

galaxy

lies

1604

or

Since the

at a distance of 700,000 parsecs, the absolute

magnitude of the supernova comes out to just a bit brighter than —17. It was about a tenth as bright as the entire galaxy that contained it. Also, since the

Andromeda

you might say that stars of the

(In fact, tually

Milky it

this

galaxy

is

considerably larger than our own,

one supernova approached the brightness of

Way put

together— for a while anyway.

was the extraordinary brightness of

made astronomers

all

realize that there

this star that even-

were nova that were thou-

sands of times brighter than run-of-the-mill nova, and thus the concept of the supernova arose.)

Well now,

telescopes

of 1885 so that

it

and spectroscopes were trained on the supernova

was better studied than were the much

1572 and 1604, but astronomers

still

closer ones of

weren’t living right. Photography

had not yet been applied to spectroscopy.

If

the supernova of 1885 had

photo: hale observatories

THE CRAB NEBULA Some explosions in the heavenly objects may be extremely enormous and may yet be so far from us or have taken place so long ago that they cannot be studied or their effects have long vanished. Some explosions may take place quite close to us in space and time and yet be comparatively small

The

and therefore

graph. It

The ago,

have no

real effect

on

us.

best combination of an explosion that was quite forceful, quite

what has produced the object shown in this photothe Crab Nebula and there is no mistaking its nature.

close, quite recent,

7 here

will

is

is

called

is

nothing that could look more

explosion took place in a d

and the Crab Nebula

.

is

.

like

an explosion.

1054,

quite a thousand years

only 4,500 light-years away. That means

3500 b c when the first civilizations were forming in the valleys of the Euphrates and the Nile and man had not yet learned how to write. It was only in that the that the actual explosion took place about

.

.,

THE SIGHT OF HOME held on for 20 years more, or

if

it

further from Earth (so that the light to reach us)

and studied

its

203

had been located 20 light-years would have taken 20 years longer

spectrum could have been recorded photographically

in detail.

Well, astronomers can only wait!

It’s

even

money

that

ing the next century, there will be a supernova blowing

Andromeda

our Galaxy or in the

vided, of course, the next supernova

however, virtually

nil

that does bring

Still

from what

up

its

top either in

galaxy and this time cameras

heaven knows what else— radio telescopes, too)

is,

sometime dur-

is

(and

be waiting. Pro-

will

not old Sol— the chance of which

little

we know

of supernovae.

doom

a rather grisly situation— the

Earth by nova-formation on the part of the Sun.

of the

The Earth would be

puffed into gas within minutes of an explosion of the Sun.

Yet

is

What

only the Sun that need concern us?

it

if

a neighboring

exploded?

star

For instance, suppose

was Alpha Centauri that decided to blow

it

became an ordinary nova and reached an ab— 9, then its apparent magnitude would be — 13.5.

up. If Alpha Centauri solute

magnitude of

would be two and

It

a half

times as bright as the

full

Moon and

a fine

spectacle for that portion of Earth’s population that lived farther south

than Florida and Egypt.

dun

to

And

For a while,

Sun and then

it

the

mids were being

still

We see

interior

is

rich in

was the brightest object was a

left

in the sky, save

shell of gas

expanding outward, as a

it

now

as

it

was

in

2500

gamma

an explosion so close to

rays,

and

debris ex-

result of the force

b.c.,

when

the Pyra-

cosmic

rays, all

us,

we can

record radio

coming from the cloud. At

the tiny fragment of the original star, collapsed into an

object just a few miles across

So

star,

built.

result of so large

waves, X-rays,

it

faded and what was

of that explosion.

its

tourist attraction

Moon.

panding outward, and

As a

new

be seen by the unaided eye, suddenly flowered into incredi-

ble brightness. for the

a

reached us and a hitherto undistinguished

light of that explosion

too

would be

(It

—

a neutron star, or '^pulsar.”

unusual properties

is

the cloud that

it

has been said that

astronomy can be divided into two halves: the study of the Crab Nebula, and the study of everything else. all

and

ASIMOV ON ASTRONOMY

204

countries like Argentina, the

would clean up

for a

Union

of

South Africa, and Australia

few months.)

Or suppose Alpha Centauri went supernova and reached an solute

magnitude of

— 17.

current theories, but

let’s

(It

is

impossible for

suppose

it

anyway.)

it

to

Its

ab-

do so according

to

apparent magnitude

would then be —21.5 which would make it 4,000 times as bright as the full Moon and actually VJoo as bright as the Sun. Under such conditions there would be no night for any region of the Earth that had Alpha Centauri in

its

papers and you would cast a shadow. sky,

it

would

still

be

a clearly visible

light and, in the absence of clouds, fact, for a

night sky.

You

could read news-

With Alpha Centauri and blindingly

you would cast

a

in the

day

brilliant point of

double shadow. In

couple of months. Earth would be truly a planet of a double

sun.

The

total

amount

increased by as effect

of energy reaching Earth

much

on the weather.

as 0.6 per cent.

A

would (temporarily) be

This might have

large part of the

a significant

Alpha Centauri radiation

would be high-energy and that ought to play hob with the upper atmosphere. In short, although Alpha Centauri as supernova might not endanger while.

life

on Earth,

it

would

certainly

make

things hot for us for a

—

16

THE BLACK OF NIGHT

My

many of you are daughter Robyn (now in

am

myself.

I

suppose

She came to

me

which one of the older

comic

the fourth grade)’*'

is

strip

'Teanuts.”

very fond of

it,

as

I

one. day, delighted with a particular sequence in

little

“Why

sister,

familiar with the

is

characters in '‘Peanuts” asks his bad-tempered

the sky blue?” and she snaps back, “Because

it isn’t

green!”

When Robyn

was

all

through laughing,

I

thought

I

would

seize the

occasion to maneuver the conversation in the direction of a deep and subtle scientific discussion (entirely for Robyn’s

stand). So

And

I

said,

“Well,

tell

me, Robyn, why

she answered at once

“Because

it

isn’t

suppose

(I

This

are both

or

less.

article still

good, you under-

the night sky black?”

ought to have foreseen

it),

purple!”

Fortunately, nothing like this can *

I

is

own

was

first

me.

If

1964. Robyn is now at college age and we whose characters are still in the fourth grade

published in

very fond of “Peanuts,”

ever seriously frustrate

ASIMOV ON ASTRONOMY

2o6

Robyn won't Reader.

cooperate^

I

can always turn, with a

*

The

«k

story of the black of night begins with a

astronomer, Heinrich ticed

with you!

will discuss the blackness of the night sky

I

astronomy

pointment.

as a

Wilhelm Matthias hobby, and

came about

It

Toward the end

on the Helpless

snarl,

German

physician and

He

Gibers, born in 1758.

prac-

in midlife suffered a peculiar disap-

in this fashion

.

.

.

began to

of the eighteenth century, astronomers

some sort of planet must exist between the orbits of Mars and Jupiter. A team of German astronomers, of whom Gibers was one of the most important, set themselves up with the in-

suspect, quite strongly, that

among

tention of dividing the ecliptic

own

his

themselves and each searching

portion, meticulously, for the planet.

Gibers and his friends were so systematic and thorough that by rights they should have discovered the planet and received the credit of

But

life is

details,

funny

Giuseppe

planets at light

it

steadily. It it

Piazzi,

an

its

for a period of

moved

was very

such so that

Italian

While they were astronomer

who

discovered, on the night of January

all,

which had shifted

followed

so

(to coin a phrase).

still

arranging the

wasn't looking for

1,

1801, a point of

position against the background of stars.

time and found

less rapidly

it

He

was continuing to move

than Mars and more rapidly than Jupiter,

likely a planet in

an intermediate

orbit.

He

reported

was the casual Piazzi and not the thorough Gibers

it

it.

it

as

who

got the nod in the history books.

Gibers didn't lose out altogether, however. of time, Piazzi

fell

and was unable

sick

It

seems that after a period

By

to continue his observations.

the time he got back to the telescope the planet was too close to the

Sun

to

be observable.

Piazzi didn't have this

was bad.

It

enough observations to calculate an

would take months

for the

orbit

and

slow-moving planet to get to

the other side of the Sun and into observable position, and without a calculated orbit

it

might

Fortunately, a young

was

just blazing his

He had worked which made

more than

it

easily take years to rediscover

German mathematician,

way upward

it.

Karl Friedrich Gauss,

into the mathematical

firmament.

out something called the ‘'method of least squares," possible to calculate a reasonably

good

orbit

from no

three good observations of a planetary position.

Gauss calculated the orbit of

Piazzi's new^ planet,

and wTen

it

was

in

observable range once more there w'as Gibers and his telescope watch-

THE BLACK OF NIGHT ing the place where Gauss’s calculations said right and,

To

be

on January

new

sure, the

turned out to be

it

1802, Gibers found

1,

than 500 miles

known

be.

Gauss was

it.

a peculiar one, for

in diameter. It

than any other known planet and smaller than at lites

would

(named “Geres”) was

planet

less

it

207

was

far smaller

least six of the satel-

at that time.

Could Ceres be

all

Mars and Jupiter? The would be a shame to waste

that existed between

German astronomers continued

looking

(it

and sure enough, three more planets between Mars and Jupiter were soon discovered. Two of them, Pallas and Vesta, were discovered by Gibers. (In later years many more were discovered.) that preparation)

all

But, of course, the big payoff

out of

it

was the name of

second place. All Gibers got

isn’t for

a planetoid.

The thousandth

planetoid be-

tween Mars and Jupiter was named “Piazzia,” the thousand and first “Gaussia,” and the thousand and second (hold your breath, now) “Glberia.”

Nor was Gibers much in

luckier in his other observations.

comets and discovered

There

that.

that

is

Shall It

ence.

is

minor

a

a

is

specialized

them, but practically anvone can do

five of

called “Gibers’

Comet”

in

consequence, but

distinction.

we now hard to

comet

He

dismiss Gibers? tell just

Sometimes

what

By no means. win you

will

a place in the annals of sci-

a piece of interesting reverie that does

it is

it.

In 1826

Gibers indulged himself in an idle speculation concerning the black of night and dredged out of it an apparently ridiculous conclusion.

Yet that speculation became “Gibers’ paradox,” which has come to have profound significance a century afterward. In fact, we can begin with Gibers’ paradox and end with the conclusion that the only reason life exists

anywhere

in the universe

is

that the distant galaxies are reced-

ing from us.

What now and

possible effect can the distant galaxies have we’ll

work

In ancient times,

it

on us? Be patient

out.

any astronomer had been asked why the night sky was black, he would have answered— quite reasonably— that it was if

because the light of the Sun was absent. question

him why the

have answered

— again

stars did

one had then gone on to

not take the place of the Sun, he would

reasonably

and individually dim. In

If

fact,

— that all

the stars were limited in

the

stars

we can

see

number

would,

if

ASIMOV ON ASTRONOMY

2o8

lumped

together, be only a half-billionth as bright as the Sun. Their in-

fluence on the blackness of the night sky

is

therefore insignificant.

Bv the nineteenth century, however, this last argument had lost its force. The number of stars was tremendous. Large telescopes revealed them by the countless millions. Of course, one might argue that those countless millions of stars were of no importance for they were not visible to the naked eye and therefore did not contribute to the light in the night sky. This, too,

argument. The

less

to

Way

the Milky

stars of

be made out, but en masse they make

The Andromeda

the sky.

Way

the Milky

galaxy

much

is

and the individual

a

is

a use-

are, individually, too faint

dimly luminous belt about

away than the stars of that make it up are not in-

farther

stars

dividually visible except (just barely) in a very large telescope. Yet, en

Andromeda

masse, the

galaxy

faintly visible to the

is

in fact, the farthest object visible to the

asks

you how

far

you can

see; tell

unaided eye; so

him 2,000,000

In short, distant stars— no matter

how

ments

if

these

Olbers,

about the

who

dim

is,

anyone ever

light-years.)

and no matter how dim, of the night sky, and this

become detectable without the

aid

of instru-

distant stars exist in sufficient density.

know about Milky Way, therefore didn’t

the

Andromeda

set

about asking himself

ought to be expected from the distant

light

if

eye. (It

distant

individually— must contribute to the light contribution can even

naked

know how much

galaxy, but did

stars altogether.

He began

by making several assumptions: 1.

2.

That the universe is infinite in extent. That the stars are infinite in number and evenly spread through-

out the universe. 3.

Tliat the stars are of uniform average brightness through

all

of

space.

Now

let’s

imagine space divided up into

onion) centering about

us,

shells

(like

those of an

comparatively thin shells compared with

the vastness of space, but large enough to contain stars within them.

Remember stars of

that the

amount

of light that reaches us

equal luminosity varies inversely as the square of the distance

A

and Star B are equally bright but Star

from

us. In

A

three times as far as Star B, Star

is

Star

A

light,

from individual

other words,

were

five

if

Star

A

delivers only Yq the light. If

times as far as Star B, Star

A

would

deliver 1,45 the

and so on.

Tliis holds for

our

shells.

The

average star in a shell 2,000 light-years

THE BLACK OF NIGHT from US would be only 14

appearance as the average star

as bright in

from

in a shell only 1,000 light-years

209

(Assumption

us.

3

us,

tells

course, that the intrinsic brightness of the average star in both shells

the same, so that distance

is

we need

the only factor

of is

to consider.)

Again, the average star in a shell 3,000 light-years from us would be only

%

appearance as the average star in the 1,000-light-year

as bright in

and so on. But as you work your way outward, each succeeding

shell,

voluminous than the one before. Since each

shell

thin

is

more enough to

shell

is

be considered, without appreciable

error, to

be the surface of the sphere

made up

we can

see that the

shells increases as the surface of the spheres

would— that

of

of the radius.

volume

the shells within,

all

The

of the

we

is,

of the

as the square

would have four times the The 3,000-light-year shell would

2,000-light-year shell

1,000-light-year shell.

have nine times the volume of the 1,000-light-year If

volume

and so on.

shell,

consider the stars to be evenly distributed through space (As-

sumption 2), then the number of stars in any given shell is proportional 1.000to the volume of the shell. If the 2,000-light-year shell is four times as voluminous stars.

If

as

th^ 1,000-light-year

the 3,000-light-year shell

light-year shell,

Well, then, stars as

if

it

shell, is

it

contains four times as

many

nine times as voluminous as the

contains nine times as

many

stars,

and

so on.

the 2,000-light-year shell contains four times as

the 1,000-light-year shell, and

if

each star in the former

is

many 14 as

3.000-

bright (on the average) as each star of the latter, then the total light delivered by the 2,000-light-year shell

4 times 14 that of the 1,000-

is

light-year shell. In other words, the 2,000-light-year shell delivers just as

much

total light as the 1,000-light-year shell.

light-year shell

is

%

9 times

The

total brightness of the

that of the 1,000-light-year shell,

and the brightness of the two shells is equal again. In summary, if we divide the universe into successive delivers as

verse

is

much

light, in toto, as

infinite in extent

infinite

number

may be

individually,

do any

(Assumption

1)

of the others.

And

and therefore

of shells, the stars of the universe,

ought to deliver an

shells,

infinite

each shell if

the uni-

consists of

an

however dim they

amount

of light to the

may

block the light

Earth.

The one of the

To

more

catch, of course,

is

that the nearer stars

distant stars.

take this into account,

let’s

no matter which direction one

look at the problem in another way. In

looks, the eye will eventually encounter a

star, if it

is

true they are infinite in

space (Assumption 2). Tlie star will contribute its bit of light

number and evenly

may be

and

distributed in

individually invisible, but

be immediately adjoined

will

it

in all

directions by other bits of light.

The

night sky would then not be black at

solutely solid solid

smear of

smear of

starlight.

but would be an ab-

So would the day sky be an absolutely

with the Sun

starlight,

all

itself invisible

against the lumi-

nous background.

Such

a sky

would be roughly

as bright as 150,000 suns like ours,

do you question that under those conditions

life

and

on Earth would be im-

possible?

However, the sky

not as bright as 150,000 suns.

is

Somewhere in the Olbers’ paradox cumstance or some logical error. black.

Olbers himself thought he found truly transparent; that

it

it.

He

there

is

The

night sky

some mitigating

is

cir-

suggested that space was not

contained clouds of dust and gas which ab-

sorbed most of the starlight, allowing only an insignificant fraction to reach the Earth.

That sounds good, but clouds in space but

if

it

is

no good

they absorbed

at

all

all.

There

are indeed dust

the starlight that

fell

upon

them (by the reasoning of Olbers' paradox) then their temperature would go up until they grew hot enough to be luminous. They would, eventually, emit as still

much

be star-bright over

But

if

light as they absorb

all its

and the Earth sky would

extent.

the logic of an argument

is

faultless

and the conclusion

is

still

mount wilson and palomar observatories

photo;

DARK CLOUDS Clouds are clouds, whether they are or in deep space.

They

are

made up

to be

found

in our

of small particles

atmosphere

and even though

those small particles are widely spaced they are efficient in absorbing

luminous objects beyond them. Clouds of water droplets obscure the Sun, and clouds of dust particles obscure light so that they obscure

many suns. The dust

many

they occur in those directions where clouds, dark

numerous

when be seen. The

clouds in space are most apparent to the naked eye

and therefore

stars are to

invisible in themselves, stand out

stars shine outside their borders

because

and none within. The

first

person to study the dark clouds thought they represented directions in

happened to exist. He was William Herschel, the discoverer of the Galaxy, and he referred to them as '‘holes’* in the

which no

stars

heavens.

But they

aren’t.

They

are vast obscuring clouds of dusts that con-

centrate in the line of vision of the Milky

seeing as

much

of that

band

of stars as

Way

we might. The photograph

a composite of a broad section of the Milky

is

that the dust

is

and prevent us from

Way

here

and you can

see

there in every direction and, of course, cannot be

swept away.

we are located far to one end of our Galaxy and if there were no dust we could see the main body of our Galaxy in the direction Actually,

of the constellation of Sagittarius

rather brighter than average). all

we can

see visually

is

our

As

{where the Milky

it is,

own end

Way

is

indeed

the dust clouds obscure that

and

and nothing more. waves which can penetrate

of the Galaxy

we have learned to record radio the clouds easily, and in this way have learned about our Galaxy’s center, even though we can never see it by eye. Fortunately,

ASIMOV ON ASTRONOMY

212

wrong, we must investigate the assumptions. for instance?

Are the

stars

What

about Assumption

2,

indeed infinite in number and evenly spread

throughout the universe?

Even

time there seemed reason to believe

in Olbers’

dimmer

the

He assumed

that,

on the average,

were more distant than the bright ones (which follows

stars

from Assumption off

assumption

William Herschel made

to be false. Tlie German-English astronomer

counts of stars of different brightness.

this

3)

and found that the density of the

space

stars in

fell

with distance.

From

the rate of decrease in density in different directions, Herschel

decided that the

stars

made up

a lens-shaped figure.

The

long diameter,

he decided, was 150 times the distance from the Sun to Arcturus (or

we would now

6,000 light-years,

would

say),

and the whole conglomeration

consist of 100,000,000 stars.

This seemed to dispose of Olbers’ paradox.

If

the lens-shaped con-

glomerate (now called the Galaxy) truly contained istence, then

be

infinite

no

contain

Assumption

in

2 breaks

down. Even

extent outside the Galaxy

stars

if

all

the stars in ex-

we imagined

(Assumption

space to it

1),

would

and would contribute no illumination. Gonsequently,

number of star-containing shells and only a finite (and not very large) amount of illumination would be received on Earth. That would be why the night sky is black. there

would be only

a finite

TTie estimated size of the Galaxy has been increased since Herschehs day. It

and

is

now

believed to be 100,000 light-years in diameter, not 6,000;

how-

to contain 150,000,000,000 stars, not 100,000,000. This change,

ever,

is

not crucial;

it still

leaves the night sky black.

In the twentieth century Gibers’ paradox

came

to

The

be appreciated that there were indeed

foggy patch in

Andromeda had been

came back

to

stars outside felt

life,

for

the Galaxy.

throughout the nine-

teenth century to be a luminous mist that formed part of our

own

Galaxy. However, other such patches of mist (the Orion Nebula, for stance) contained stars that

lit

up the

mist.

it

The Andromeda

the other hand, seemed to contain no stars but to glow of

in-

patchy on

itself.

Some astronomers began to suspect the truth, but it wasn’t definitely established until 1924, when the American astronomer Edwin Powell Hubble turned the 100-inch able to

make out

dividually so

dim

telescope

separate stars in that

it

became

its

on the glowing mist and was outskirts.

clear at

These

stars

were

in-

once that the patch must be

THE BLACK OF NIGHT hundreds of thousands of

away from us and

light-years

Galaxy. Furthermore, to be seen, as

213

was, at that distance,

it

our entire Galaxy and be another galaxy in

in size

And

so

it is. It is

and to contain

now

its

own

it

must

rival

right.

believed to be over 2,000,000 light-years from us

at least 200,000,000,000 stars. Still other galaxies

within the observable universe there

were

we now suspect that many as 100,000,000,000

Indeed,

discovered at vastly greater distances.

galaxies,

far outside the

may be

as

and the distance of some of them has been estimated

as

high as

6,000,000,000 light-years.

Let us take Gibers' three assumptions then and substitute the word

and

'‘galaxies” for "stars”

Assumption there

is

no

see

how

they sound.

that the universe

1,

infinite,

is

sounds good. At

end even out to distances of

sign of an

least

billions of light-

years.

Assumption

that galaxies (not stars) are infinite in

2,

number and

evenly spread throughout the universe, sounds good, too. At least they are evenly distributed for as far out as

we can

see,

and we can

see pretty

far.

Assumption

galaxies (not stars) are of

3, fliat

ness throughout space,

is

uniform average bright-

harder to handle. However,

we have no

reason

to suspect that distant galaxies are consistently larger or smaller than

nearby ones, and

if

and

then

star content,

the galaxies it

come

to

some uniform average

size

certainly seems reasonable to suppose they

are uniformly bright as well.

Well, then, why

is

the night sky black?

Let’s try another tack.

luminous object spectrum (that

is is,

the object

is

is

Astronomers can determine whether a distant

its

light as spread out in a

its

rainbow of wavelengths

violet to long-wavelength red).

crossed by dark lines which are in a fixed position

motionless with respect to us.

the lines shift toward the violet.

us,

to that.

approaching us or receding from us by studying

from short-wavelength

The spectrum

We’re back

the lines shift toward the red.

From

If

If

the object

the object

is

is

if

approaching

receding from us,

the size of the shift astronomers

can determine the velocity of approach or recession. In the 1910s and 1920s the spectra of

some

galaxies (or bodies later

understood to be galaxies) were studied, and except for one or two of the

very

nearest,

came apparent

all

that

are

receding

from

us.

the farther galaxies are

In

fact,

it

soon

receding more

be-

rapidly

photo:

mount wilson and palomar

observatories

GALACTIC CLUSTERS The astronomical view

of the Universe has been steadily expanding

ever since the telescope was invented.

was the

and

plaiiets

When

it

was

their satellites and, of course, the

astronomical interest.

The

stars

first

Sun

invented,

that engaged

were dismissed as background.

However, the telescope revealed that there were many more than could be seen with the unaided eye. stars

in

huge

clusters;

it

revealed

other; variable stars; giant stars;

were billions of

stars

That seemed passed

it

them were

many

studied,

objects.

stars

revealed vast masses of

binary stars wheeling about each

pigmy

grouped together a vast

It

stars.

By 1820

it

seemed there

in a lens-shaped galaxy.

Universe,

but after another century has

began to appear that outside the galaxy we

other galaxies,

mon

like

it

live in there

were

them larger than our own. More and more of and it came to be realized that they were very comof

.

THE BLACK OF NIGHT

215

than the nearer ones. Hubble was able to formulate what

“Hubble’s Law” galaxy

is

now

called

in 1929. lliis states that the velocity of recession of a

proportional to

Galaxy B,

as

is

it is

its

distance from us.

If

Galaxy

The

receding at twice the velocity.

galaxy, 6,000,000,000 light-years

from

us,

A

twice as far

is

farthest observed

receding at a velocity half

is

that of light.

The

reason for Hubble’s

Law

taken to

is

lie in

the expansion of the

made

universe itself— an expansion which can be

from the

to follow

equations set up by Einstein’s General Theory of Relativity (which,

hereby state firmly,

I

will

I

not go into)

Given the expansion of the universe, now, how are Gibers’ assumptions affected? If,

at a distance of 6,000,000,000 light-years a galaxy recedes at half

the speed of light, then at a distance of 12,000,000,000 light-years a galaxy ought to be receding at the speed of light

holds). Surely, further distances are meaningless, for velocities greater

than that of

light.

Even

or any other “message” could reach us

and

it

would not^

in effect,

imagine the universe to be

some 12,000,000,000

be

in

if

Hubble’s

(if

we cannot have

that were possible,

from such

no

light,

a more-distant galaxy

our universe. Gonsequently,

finite after all,

Law

we can

with a “Hubble radius” of

light-years.

*

In 1973 a quasar was detected at that distance and there were immediate newspaper headlines to the effect we had seen the “end of the universe.”

They

existed not only singly (as stars do) but they existed in clusters

{as stars do).

Our own

galaxy

is

part of the “Local Cluster” which

contains at least four large galaxies

—our own,

two recently discovered galaxies called Maffei dozen or more small galaxies.

We can't

Andromeda and Maffei 2

the 1

see our Local Cluster well, though, for

can see other

clusters.

The photograph shows

galaxy,

—plus

we

are in

it,

a

but we

a cluster of galaxies in the

constellation of Hercules. These are not just galaxies that

happen

to

be in the same direction from us but are widely separated in the third dimension. They are gravitationally interconnected; that close

enough together

to be

is,

they are

moving about some common center

of

gravity.

You might

try

and count the number of

galaxies present in this very

small patch of sky {as seen with the 200-inch Palomar telescope). Anything that seems hazy or elongated

is

a galaxy.

ASIMOV ON ASTRONOMY

2i6

But that doesn’t wipe out Olbers’ paradox. Under the requirements

move

of Einsten’s theories, as galaxies observer, they

up

and

less

become

and ‘shorter

shorter

that there

less space, so

faster relative to

in the line of travel

room

is

and

faster

for larger

and

an

and take

larger

num-

bers of galaxies. In fact, even in a finite universe, with a radius of 12,-

000,000,000 light-years, there might

almost

axies;

of

all

them

still

(paper-thin)

be an

number

infinite

existing in the

of gal-

outermost few

miles of the Universe-sphere.

So Assumption by

tion 2,

2 stands

even

if

to the red

ceding galaxy delivers galaxy would deliver

A

energy at

less

shift

by the change

in position of the

the entire spectrum

a shift in the direction of lesser energy.

A

radiant energy to the Earth than the

same

were standing

still

relative to

all

us— just because

faster a galaxy recedes the less radiant

no matter how bright

but not

if it is

3

fails!

It

might

it

is

And

it

would hold true

from

might

and

falls short,

us;

is

if

the universe were

of the expected radiant

fails,

3

we

receive only a finite is

will always continue. It

may

have receded too

sion. Either way,

of

expansion

new

Galaxy (plus a

the ‘local cluster” of

in the universe. All the other galaxies will

far to detect.

so that the universe will always

will

make up

neighbors, which together

seem alone

this

continue without the production of

galaxies so that, eventually, billions of years hence, our

will

amount

black.

According to the most popular models of the universe,

galaxies)

content of

deliver.

because Assumption

its

its

subjected to a successively

more and more,

energy from the universe and the night sky

few of

it

expanding. Each succeeding shell in an expanding

successively farther

greater red shift;

energy

energy

be.

universe delivers less light than the one within because galaxies

re-

galaxy receding at the speed of light delivers no radiant

Thus, Assumption static,

is

if it

The

of the red shift. delivers.

does not; and Assump-

move only because

spectral lines, but those lines

A shift

1

can be enough to insure a star-bright sky.

itself,

But what about the red shift? Astronomers measure the red moves.

Assumption

Or new seem

galaxies

may

full of galaxies,

continuously form despite

its

expan-

however, expansion will continue and the night sky

remain black.

There

is

another suggestion, however, that the universe

that the expansion will gradually slow

down

oscillates;

until the universe

comes

THE BLACK OF NIGHT to a

moment

faster,

till it

about

a

217

of static pause, then begins to contract again, faster

tightens at last into a small sphere that explodes

new

and

and brings

expansion.

then as the expansion slows the dimming effect of the red shift will diminish and the night sky will slowly brighten. By the time the universe is static the sky will be uniformly star-bright as Olbers’ paradox If so,

required. Tlien, once the universe starts contracting, there will be a

and the energy delivered brighter and still brighter.

“violet-shift”

become This

far

will

will increase so that the sky will

be true not only for the Earth

(if it still

existed in the far

future of a contracting universe) but for any body of any sort in the universe. In a static or, worse

still,

a contracting universe there could,

by Olbers’ paradox, be no cold bodies, no solid bodies. There would be uniform high temperatures everywhere in the millions of degrees, I

—

suspect— and

So

I

life

simply could not

get back to

my

exist.

earlier statement.

Earth, or anywhere in the universe, are

moving away from In fact,

now

that

is

The

reason there

is

life

on

simply that the distant galaxies

us.

we know

the ins and outs of Olbers’ paradox, might

we, do you suppose, be able to work out the recession of the distant galaxies as a necessary

Maybe we

could

consequence of the blackness of the night sky?

amend

the famous statement of the French philoso-

pher Rene Descartes.

He said, “I think, therefore I am!” And we could add: “I am, therefore

the universe expands!”

A GALAXY AT A TIME

Four or

ago there was a small

five years

my

house.

ing

some rooms

It

wasn’t

much

in the

of a

two blocks from producing smoke and damag-

fire

fire, really,

at a school

basement, but nothing more. What’s more,

was outside school hours so that no Nevertheless, as soon as the

lives

first

were

piece of

it

in danger. fire

apparatus was on the

scene the audience had begun to gather. Every idiot in town and half the idiots from the various contiguous towns

came

racing

down

to see

They came by auto and by oxcart, on bicycle and on Tliey came with girl friends on their arms, with aged parents on the

fire.

shoulders,

and with infants

They parked fire

engine had

added to

it

all

their

at the breast.

the streets solid for miles around and after the

come on

foot.

the scene nothing

first

more could have been

except by helicopter.

Apparently

this

happens every time. At every

disaster, big or small,

the two-legged ghouls gather and line up shoulder to shoulder and chest to back.

They do

this, it

seems, for two purposes: a) to stare goggle-

eyed and slack-jawed at destruction and misery, and b) to prevent the

A

GALAXY AT

approach of the proper authorities

A

TIME

219

who

are attempting to safeguard

who

rushed to see the

life

and property. Naturally,

I

wasn’t one of those

very self-righteously noble about I

will confess

that this

destructive instinct.

my

It’s

However

it.

(since

messy

just that a

and

I

felt

we

are

all

I

am

free of the

not necessarily because

is

fire

little fire in a

friends),

basement

idea of destruction; or a good, roaring blaze at the munitions

isn’t

dump,

either. If a star

Come

were to blow up, then we might have something.

to think of

veloped after

all,

or

it,

I

my

must be well defascinated by the subject

instinct for destruction

wouldn’t find myself so

of supernovas, those colossal stellar explosions.

Yet

them,

in thinking of

I

have,

turns out, been a piker.

it

Here Fve

been assuming for years that a supernova was the grandest spectacle the universe had to offer (provided you were standing several dozen light-

away) but, thanks to certain 1963 findings, it turns out that a supernova taken by itself is not much more than a two-inch firecracker.

years

This realization arose out of radio astronomy. Since World War II, astronomers have' been picking up microwave (very short radio wave) radiation from various parts of the sky,

and have found that some of it comes from our own neighborhood. The Sun itself is a radio source and so are Jupiter and Venus.

The

radio sources of the Solar System, however, are virtually in-

We

would never spot them if we weren’t them. To pick up radio waves across the vastness of

significant.

we need something the Solar System

better.

is

we can

stellar distances

For instance, one radio source from beyond

the Crab Nebula.

been diluted by spreading out for reaching us,

right here with

Even five

after

its

radio waves have

thousand light-years before

up what remains and impinges upon our instruments. But then the Crab Nebula represents the remains of a supernova that blew

still

pick

itself to

kingdom come—-the

first

light of the ex-

plosion reaching the Earth about 900 years ago.

But

a great

number

of radio sources

lie

outside our Galaxy altogether

and are millions and even billions of light-years distant. Still their radiowave emanations can be detected and so they must represent energy sources that shrink mere supernovas to virtually nothing.

For instance, one particularly strong source turned out, on investigation, to arise from a galaxy 200,000,000 light-years away. Once the large telescopes zeroed in

on that galaxy

it

turned out to be distorted in

ASIMOV ON ASTRONOMY

220

shape. After closer study at

all,

but two galaxies

When

two

However,

became quite

clear that

it

is little

a galaxy

likelihood of actual

(which are too small and too widely spaced).

stars

the galaxies possess clouds of dust (and

if

was not

in the process of collision.

galaxies collide like’ that, there

between

collisions

it

many

galaxies, in-

cluding our own, do), these clouds will collide and the turbulence of the collision will set up radio-wave emission, as does the turbulence (in order of decreasing intensity) of the gases of the

Crab Nebula, of

our Sun, of the atmosphere of Jupiter, and of the atmosphere of Venus.

more and more radio sources were detected and pinpointed, the number found among the far-distant galaxies seemed impossibly high. There might be occasional collisions among galaxies but it seemed most unlikely that there could be enough collisions to account for all But

as

those radio sources.

Was

there any other possible explanation?

some cataclysm

just as vast

and intense

as that represented

one that involved a

colliding galaxies, but

from the necessity of supposing

Wliat was needed was

collisions

a pair of

Once freed any number

single galaxy.

we can

by

explain

of radio sources.

But what can

do alone, without the help

a single galaxy

of a sister

galaxy?

Well,

it

can explode.

A

But how?

galaxy isn’t really a single object.

aggregate of up to a couple of hundred billion

explode individually, but

how

It

stars.

simply a loose

is

These

stars

can

can we have an explosion of a whole

galaxy at a time?

To

answer

that,

an aggregation

really as loose

our

own may

most of that

begin by understanding that a galaxy

let’s

outskirt of our

own Galaxy

its

A

galaxy like

longest diameter, but

so

more than a thin powdering of stars We happen to live in this thinly starred we accept that as the norm, but it isn’t.

its

nucleus, a dense packet of stars roughly

be ignored.

of a galaxy

spherical in shape is

to think.

consists of nothing to

volume

we might tend

stretch out 100,000 light-years in

—thin enough

The nub

as

isn’t

is

and with

a diameter of, say, 10,000 light-years. Its

then 525,000,000,000 cubic light-years, and

100,000,000,000

stars,

that

means there

is

1

star per

if

it

contains

5.25 cubic light-

years.

With

stars

massed together

like that, the

average distance between

A stars in

GALAXY AT

the galactic nucleus

is

A

TIME

221

1.7 light-years— but that’s the average

over the entire volume. Tlie density of star numbers in such a nucleus

one moves toward the center, and I think it is entirely fair to expect that toward the center of the nucleus, stars are not separated by more than half a light-year. increases as

F,ven half a light-year

is

something

like

3,000,000,000,000 miles or

400 times the extreme width of Pluto’s orbit, so that the stars aren’t actually crowded; they’re not likely to be colliding with each other, and yet

.

.

.

Now suppose that, What happens? In If

most

somewhere

supernova

in a galaxy, a

nothing (except that one

star

the supernova were in a galactic suburb

— in

cases,

is

lets go.

smashed to

our

flinders).

own neighborhood,

— the stars

would be so thinly spread out that none of them would be near enough to pick up much in the way of radiation. The incredible quantities of energy poured out into space by such a supernova would simply spread and thin out and come to nothing. for instance

In the center of a galactic nucleus, the supernova to dismiss.

A

good supernova

at

its

height

is

10,000,000,000 times the rate of our Sun.

is

not quite as easy

releasing energy at nearly

An

object five light-years

away would pick up a tenth as much energy per second as the Earth picks up from the Sun. At half a light-year from the supernova it would pick up ten times as much energy per second as Earth picks up from the Sun.

This

isn’t

good.

If

a supernova let go five light-years

from us we

would have a year of bad heat problems. If it were half a light-year away I suspect there would be little left of earthly life. However, don’t worry. There is only one star-system within five light-years of us and it is

in

not the kind that can go supernova.

But what about the effects on the stars themselves? If our Sun were the neighborhood of a supernova it would be subjected to a bath of

own temperature would have to go up. After the supernova is done, the Sun would seek its own equilibrium again and be as good as before (though life on its planets may not be). However, energy and

its

in the process,

it

would have increased

tion to the fourth in

power

of

its

its

fuel

consumption

absolute temperature.

Even

in propor-

a small rise

temperature might lead to a surprisingly large consumption of fuel. It is by fuel consumption that one measures a star’s age. When the

ASIMOV ON ASTRONOMY

222 fuel supply shrinks

low enough, the

plodes into a supernova. slightly for a year

A

expands into a red giant or

star

it

move

to

(several billion years),

it

would mean

Some

and

a

a century, or ten centuries

Sun has

closer to such a crisis. Fortunately, our

of

warming the Sun

distant supernova by

might cause

ex-

a long lifetime

ahead

few centuries or even a million years

little.

stars,

however, cannot afford to age even

They

slightly.

are

already close to that state of fuel consumption which will lead to drastic

changes, perhaps even supernova-hood. Let’s

on the brink, pre-supernovas.

How many

of

call

such

which are

stars,

them would

there be per

galaxy?

has been estimated that there are an average of

It

supernovas per

3

eentury in the average galaxy. That means that in 33,000,000 years there are about a million supernovas in the average galaxy. Considering

that a galactic

life

span

may

easily

be a hundred

billion years,

any

only a few million years removed from supernova-hood

that’s

star

may

reasonably well be said to be on the brink.

out of the hundred billion

If,

million stars are

on the brink, then

means

nova. This

an average galactie nucleus, a

stars in

star

1

out of 100,000

a pre-super-

is

that pre-supernovas within galactic nuclei are sep-

arated by average distances of 80 light-years.

Toward the

center of the

nucleus, the average distance of separation might be as low as 25 lightyears.

But even

from a supernova would be only

that which the Earth receives from the Sun, and

%5o be

at 25 light-years, the light

trifling.

And,

up one galaxy

as a

matter of

or another

if

we

is

then as

the average galaxy has

1

effect

would

frequently see supernovas light

and nothing happens. At

slowly dies out and the galaxy

However,

fact,

its

it

least,

the supernova

was before.

pre-supernova in every 100,000

may be poorer than that in supernovas— or galaxy may be particularly rich and 1 star out of

particular galaxies

stars,

richer.

An

every 1,000

occasional

may be

a pre-supernova.

In such a galaxy, the nucleus would contain 100,000,000 pre-super-

Toward the

novas, separated by an average distance of 17 light-years. center, the average separation If a

might be no more than

supernova lights up a pre-supernova only

shorten years

its life

significantly,

and

from explosion before,

plosion afterward.

it

if

5

3

light-years

that supernova

had been

light-years.

away a

it

will

thousand

might be only two months from

ex-

GALAXY AT

A

Then, when its

lets go, a

it

few months

On

like a

we end up with is

first,

may have

by the second and closer supernova, and

bunch

a galaxy in

one

several million perhaps,

There

by the

which has had its

after

blasts.

it

and on

223

distant pre-supernova

lifetime shortened, but not so drastically,

lifetime shortened again a

more

TIME

A

of tumbling

which not

dominoes

a single

this

would

supernova

lets

go, until

bang, but

after the other.

the galactic explosion. Surely such a tumbling of dominoes

would be suEcient to give birth to a coruscation of radio waves that would still be easily detectable even after it had spread out for a billion light-years.

Is this just

speculation?

observational data It

made

it

To

it

was, but in late 1963

some

appear to be more than that.

involves a galaxy in Ursa

number 82 on

begin with,

Major which

a list of objects in the

is

called

M82

because

it is

heavens prepared by the French

astronomer Charles Messier about two hundred years ago. Messier was a comet-hunter and was always looking through his telescope and thinking he had found a comet and turning handsprings

and then finding out that he had been fooled by some foggy object which was always there and was not a comet. Finally, he decided to map each of 101 annoying objects that were foggy but were not comets so that others would not be fooled as he was. It was that

The

first

on

list

his

globular clusters

of annoyances that list.

Mi,

is

made

his

name immortal.

the Crab Nebula. Over two dozen are

(spherical conglomerations of densely strewn stars).

Ml 3

being the Great Hercules Cluster, which

Over

thirty

is

the largest known.

members of his list are galaxies, including the Andromeda Galaxy (M31) and the Whirlpool Galaxy (M51). Other famous objects on the list are the Orion Nebula (M42), the Ring Nebula (M57), and the Owl Nebula (M97). Anyway, M82 is a galaxy about 10,000,000 light-years from Earth which aroused

interest

when

it

proved to be a strong radio source.

Astronomers turned the 200-inch telescope upon it and took pictures through filters that blocked all light except that coming from hydrogen ions. exist

There was reason to suppose that any disturbances that might would show up most clearly among the hydrogen ions.

They

did!

A

three-hour exposure revealed

jets

of hydrogen

up to

a

thousand light-years long, bursting out of the galactic nucleus. The

ASIMOV ON ASTRONOMY P

*

%

«

« « I

« \

photo;

mount WILSON and PALOMAR

OBSERVATORl

.

GALAXY AT

A total

TIME

A

225

mass of hydrogen being shot out was the equivalent of at

5,000,000 average stars.

From

the rate at which the

jets

least

were traveling

and the distance they had covered, the explosion must have taken place about 1,500,000 years before. (Of course, it takes light ten million years to reach us from M82, so that the explosion took place 11,500,000 years ago. Earth-time— just at the beginning of the Pleistocene Epoch.)

M82,

then,

is

The

the case of an exploding galaxy.

energy expended

equivalent to that of five million supernovas formed in rapid suc-

is

cession, like

though on

uranium atoms undergoing

a vastly greater scale, to

be

in

fission

sure.

an atomic

bomb—

feel quite certain that if

I

EXPLODING GALAXY When

it

was

first

understood that galaxies were gigantic collections

and that they were located at unprecedented distances from us {and this was only half a century ago) there was little one could do

of stars

with them but look at them through a telescope. And, except for the closest, there was little one could see but shapes of light some spheri-

—

cal or elliptical blobs,

was the

some

spiral

forms seen at various angles.

methods of detecting radio wave radiation from the sky that gave us a chance to do more. For various reasons, it It

discovery^ of

possible to detect radio waves

is

from greater distances than light-waves

can be detected. Furthermore, our attention objects.

The number

hundreds of If iar

attracted to far fewer

is

of optical objects seen in our telescopes

billions; the

we concentrate on

number

of radio objects

radio galaxies,

we

is

see that

is

in the

in the thousands.

we

ones; objects that have shapes that are not typical

deal with pecul-

and that seem

to

be radiating unusually large amounts of energy. One particularly startling consequence of paying attention to radio waves came when a galaxy known as M82 was studied. It quickly turned out that it had under-

gone an enormous explosion still

at

some time

in the past

and that

it

exploding {at least in the sense that the light reaching us right

is still

was

now

the light of an exploding galaxy )

The photograph

here shows the explosion. It

is

a negative, rather

than a positive, but astronomers routinely study negatives since detail better seen

is

in

black-against-white,

rather than white-against-black.

Here you can see the dainty black filaments sticking out on either side huge masses of gas still being hurled away from that exploding

—

center.

And what made

a

whole galaxy explode?

We don’t know.

226

ASIMOV ON ASTRONOMY

there had been any

life

anywhere

in that galactie nucleus, there isn’t

V

any now. In fact,

be examples of prime

Which Wliat

brings

up

longer

real estate.

a horrible

the nucleus of our

if

may no

suspect that even the*outskirts of the galaxy

I

thought

own

.

.Yes, you guessed

.

dear Galaxy explodes?

it!

It

very likely

SPIRAL GALAXIES Stars exist in collections ranging

Such

billions to a

few

a

few

trillions.

collections are called galaxies.

The

simplest form such a collection can take

ellipsoidal ball of stars

and

from

and the

that of a spherical or

is

largest galaxies are

are called '‘elliptical galaxies.’'

indeed of this shape

They make up about one

we can see. Much more common, making up about

fifth of all

the galaxies

which do indeed have a nucleus resembling small

are those

but along the outer edges of these

galaxies,

three fourth of the galaxies,

flat spiral

Astronomers do not as yet know what forms these galaxies or less there

what keeps them

in being

elliptical

streamers of

spiral

stars.

arms of such

once they are formed. Neverthe-

they are.

we see galaxies at all angles and a spiral galaxy seen edge on would show as a spherical ball of stars with a thin line stretching out on either side. The line would tend to be rather dark, for the spiral Naturally,

arms, unlike the galactic nucleus, are

filled

with dark clouds of dust.

The best-known

galaxy seen in this fashion

Galaxy” because

it

is

called the

rather looks like a sombrero with a

"Sombrero

crown on each

side of the rim.

A

would look like an oval patch of light, dark lines marking the spaces between

spiral galaxy seen obliquely

but there would be the spiral arms.

By

all

The Andromeda Galaxy

is

spiral galaxy

and one ends ond nucleus. It was

in a large first

looked

like

and

"Whirlpool” Galaxy.

startling galactic appear-

clump

The

The one spiral

of stars that

is

in the photo-

arms are

almost

clearly

like a sec-

studied well over a century ago by an Irish

astronomer, the Earl of Rosse. it

and

seen broadside.

the best-known example of that.

visible

what

looks like that.

odds, though, the most beautiful

ance would be a graph

elliptical

it

is

He

still

didn’t called

know what it was but he saw by the name he gave it the

—

photo: hale observatories

ASIMOV ON ASTRONOMY

228

among

won’t, of course (I don’t want to cause fear and despondency

uncom-

the Gentle Readers), for exploding galaxies are probably as

mon among

going to happen, exercise, to

To

among

galaxies as exploding ‘stars are it is all

the

more comfortable then,

wonder about the consequences

begin with,

we

stars. Still, if it’s

as

not

an intellectual

of such an explosion.

are not in the nucleus of our Galaxy but far in

the outskirts and in distance there

is

a

modicum

of safety. This

is

between ourselves and the nucleus are vast clouds of

epecially so since

dust that will effectively screen off any visible fireworks.

Of and years

course, the radio waves

all,

and

would come spewing

would probably ruin radio astronomy

this

might

rise

else.

we might be caught in the fallout of that Suppose, though, we put cosmic radiation

words,

extent of

its

formation

is

for millions of

Worse still would be the cosmic high enough to become fatal to life. In other

by blanking out everything

radiation that

out, through dust

galactic explosion. to

one

since the

side,

uncertain and since consideration of

ence would be depressing to the

spirits.

Let’s also abolish

its

pres-

the dust

clouds with a wave of the speculative hand.

Now we

What

can see the nucleus.

does

it

look like without an ex-

plosion?

Gonsidering the nucleus to be 10,000 light-years in diameter and 30,000 light-years away from us,

a patch

Its total light

about

^5

would be

When

area about 20° in diameter.

make up

it

entirely

paratively dim.

above the horizon

would be about 30 times that given

An

roughly spherical it

would

of the visible sky.

brightest, but spread out over so large

its

visible as a

an area

it

off

by Venus

at

would look com-

area of the nucleus equal in size to the full

Moon

would have an average brightness only 1/200,000 of the full Moon. It would be visible then as a patch of luminosity broadening out of the Milky

Way

than the Milky off

in the constellation of Sagittarius, distinctly brighter

Way

itself;

brightest at the center, in fact,

and fading

with distance from the center.

But what

would take

if

the nucleus of our Galaxy exploded?

place,

I

feel certain, in

The

explosion

the center of the nucleus, where

the stars were thickest and the effect of one pre-supernova on

its

neighbors would be most marked. Let us suppose that 5,000,000 super-

novas are formed, as in M82. If

the nucleus has pre-supernovas separated by

5

light-years

in

its

central regions (as estimated earlier in the chapter, for galaxies capable

of explosion), then 5,000,000 pre-supernovas

would

fit

into a sphere

GALAXY AT

A

TIME

A

229

about 850 light-years in diameter. At a distance of 30,000 light-years, such a sphere would appear to have a diameter of 1.6°, which is a little more than three times the apparent diameter of the full Moon.

We

would therefore have an excellent view.

Once

the explosion started, supernova ought to follow supernova at

an accelerating

rate. It

would be

a chain reaction.

we were to look back on that later, we could say (and be roughly

vast explosion

nucleus had

this

If

all

watch the explosion

actually

that the center of the

correct)

exploded at once. But

in process,

is

millions of years

only roughly correct.

we

will find

it

If

we

will take con-

siderable time, thanks entirely to the fact that light takes considerable

time to travel from one

When

a

star to another.

supernova explodes,

it

can’t affect a neighboring pre-super-

nova (5 light-years away, remember) until the radiation of the first reaches the second— and that would take years. If the second 5

was on the 5 years first.

the

far side of the first (with respect to ourselves),

would be

W

e

lost

star

star

an additional

while the light traveled back to the vicinity of the

would therefore see the second supernova 10

years later than

first.

Since a supernova will not remain visible to the naked eye for

more

than a year or so even under the best conditions (at the distance of the Galactic nucleus), the second supernova would not be visible until long after the

first

had faded

off to invisibility.

In short, the 5,000,000 supernovas, forming in a sphere 850 lightyears in diameter,

would be seen by us

equal to roughly a thousand years.

If

to

appear over a stretch of time

the explosions started at the near

edge of that sphere so that radiation had to travel away from us and return to set off other supernovas, the spread might easily be 1,500 years. If it started at the far end and additional explosions took place as the light of the original explosion passed

ourselves, the time-spread

On to

the pre-supernovas en route to

might be considerably

less.

the whole, the chances are that the Galactic nucleus would begin

show

individual twinkles.

At

first

there might be only three or four

twinkles a decade, but then, as the decades and centuries passed, there

would be more and more visible at

one time. And

until finally there

finally,

they would

might be

all

several

hundred

go out and leave behind

dimly glowing gaseous turbulence.

How reach a

bright will the individual twinkles be?

maximum

absolute magnitude of

A

single supernova

—17. That means

if

it

can

were

ASIMOV ON ASTRONOMY

230

at a distance of 10 parsecs

(32.5 light-years)

have an apparent magnitude of —17, which

from ourselves,

it

would

1/10,000 the brightness

is

of the Sun.

At

apparent magnitude of such a

a distance of 30,000 light-years, the

supernova would decline by 15 magnitudes. The apparent magnitude would now be —2, which is about the brightness of Jupiter at its brightest. Tliis

is

At the distance of the nucleus, no seen with the naked eye. The hundred

quite a startling statistic.

ordinary star can be individually billion stars of the nucleus just

under ordinary conditions. For

make up

a

luminous but featureless haze

a single star, at that distance, to fire

to the apparent brightness of Jupiter

is

up

simply colossal. Such a super-

nova, in fact, burns with a tenth the light intensity of an entire non-

exploding galaxy such as ours.

Yet of

it

unlikely that every supernova forming will be a supernova

is

maximum

brilliance.

Let’s

be conservative and suppose that the

supernovas will be, on the average, two magnitudes below the maxi-

mum. Each

will

then have a magnitude of

o,

about that of the

star

Arcturus.

Even

so,

the “twinkles” would be prominent indeed.

were exposed to such a sight

If

humanity

in the early stages of civilization, they

would never make the mistake of thinking that the heavens were eternally fixed ular

and unchangeable. Perhaps the absence of that

misconception

until early

(which, in actual

modern times) might have

fact,

partic-

mankind labored under

accelerated the development of

astronomy.

However, we can’t see the Galactic nucleus and

that’s that. Is there

anything even faintly approaching such a multi-explosion that

we can

see?

There’s one conceivable possibility. Here and there, in our Galaxy, are to be found globular clusters. It

of these per galaxy.

(About

a

is

estimated there are about 200

hundred of our own

clusters

have been

observed, and the other hundred are probably obscured by the dust clouds.)

These globular

clusters are like

light-years or so in

detached

bits of galactic nuclei,

100

diameter and containing from 100,000 to 10,000,000

stars— symmetrically scattered about the galactic center.

The

largest

known

globular cluster

is

the Great Hercules Cluster.

GALAXY AT

A

Ml 3,

but

it

is

not the

Centauri, which

is

closest.

The

naked

nearest globular cluster

22,000 light-years from us and

the naked eye as an object of the light to the

TIME

A

fifth

magnitude.

is

It

is

Omega

clearly visible to is

only a point of

eye, however, for at that distance even a diameter of

100 light-years covers an area of only about

1.5

minutes of arc

in

diam-

eter.

Now

us say that

Omega

Centauri contained 10,000 pre-supernovas and that every one of these exploded at their earliest opportunity. let

There would be

few^er twinkles altogether,

but they would appear over

time interval and would be, individually, twice as bright. would be a perfectly ideal explosion, for it would be unobscured

a shorter It

by dust clouds;

enough to be

And

yet,

spectacle,

Omega

it

would be small enough to be quite

sufficiently spectacular for

now

not

is

visible in

quite a bit southward

—and

I

Hmm

about

just

if I

New

nil.

my

.

.

Oh

well,

And

of viewing an explosion in

even

England and

expected to see

anyone

for a

large

sense of excitement over the

it

if I

it

happened.

high in the sky in

neighborhood

Omega

would have to

don’t like to travel. .

and

anyone.

must admit that the chances

I

Centauri are

Centauri

worked up

that I’ve

safe;

fire?

travel

full glory

I

•4

I

•4

4

INDEX

Absolute magnitude, 193

Baade, Walter, 188, 189 Barnard, Edward Emerson, 161 Barnard’s star, 161, 162

Aehilles (asteroid), 57 Achilles (legend), 150

Adams, John Couch, 105 Adams, Walter Sydney, 1 56

Barnard’s Star B, 162

Algol, 177

Barringer, Daniel Moreau, 22

Alpha Centauri,

128, 138,

32,

i42ff., 155, 161,

motion

140,

absolute magnitude of, 193 distance of, 144, 193

181,

Betelgeuse, 172, 173 Biela, Wilhelm von, 133

Andromeda

201

in,

nebula, 186

comet, 133 Big Dipper, 90, 91 Binary stars, 103 Binomial theorem, 39 Bode, Johann, 102 Biela’s

186,

188, 212, 213, 223, 226

supernova

144,

Beta Centauri, 143

Amaltheia, 123 galaxy,

+4^4048, 193 Wilhelm,

Bessel, Friedrich

Alpha Centauri A, 143 mass of, 143 Alpha Centauri B, 143 mass of, 143 Alpha Centauri C, 139

Andromeda

162

Barringer crater, 22

BD

203

of,

187,

Bode’s Law, 102

Bond, Ceorge Phillips, 47 Brahe, Tycho, 129, 198

distance of, 187, 188

Angular measure, Apollo 1 1, 4, 20

i67ff.

Calculus, 39

Calendar, 98

Aristarchus, 131

Canopus, 143

Aristotle, 20, 129, 131

Capella, 193, 197 Cassini, Ciovanni

Asteroids, 84

Bode’s

Law

discovery

formation

of,

and, 103 103, 103

of, 109,

no

surface area of, 83, 83

Trojan, 37, 38 Asteroid zone, 44!!. Astronauts, 40

Astronomical unit, 30 Atlantis, 23

Domenico,

50^ Cassini’s division, 47, 30 Celestial North Pole, 91

Center of gravity, 1 34 Cepheid variables, 181

Andromeda nebula and, 188 distance and, 184 Cerberus, 1 1

Ceres, 103, 206, 207

INDEX

234

Ether, 20

Charon, 114 Clark, Alvan Graham, 156 Comets, i28ff. composition of, 135 death

of,

34 Flamsteed, John, 103

135

distance of, 129, orbits of,

First law, Kepler’s,

1

1

Galactic clusters, 214, 215 Galaxy, Milky Way, 105, 182

30

32ff.

center of, 185 dust clouds in, 185

origin of, 136

outer shell

of,

136

226, 228

perturbations of, 134

nucleus

tails of,

size of, 176, 177, 185,

135

supernovas

Constellations, 142

Continental

drift,

of,

17

Cooper, L. Gordon, Jr., 40 Copernicus, Nicholas, 130, 152 Crab Nebula, 197, 202, 203, 219, 223

187

198

in,

Galaxy (ies), other, 214, 215 clusters of, 214, 215 collision of, 220 distance of, 190

exploding, 22off. nuclei of, 220, 221

Dachille, Frank, 17

number

Daedalus, 84 Day, length of, 92, 93 Degrees of arc, 167 on Earth, 170

radio,

Deimos, 121, 122 Delta Cephei, 178 Duodecimal system, 165 Dust clouds, galactic, 185, 211

size of, 188, 190,

of,

213

225

recession of, 215 satellite,

shapes

182

of,

226 191

226

spiral,

Galileo,

20, 39, 49, 55, 92, 179 Gases, kinetic theory of, 52 3,

Gauss, Karl Friedrich, 206 Earth, craters on,

Gaussia, 207

i6ff.

geocentric theory and, 150, 151 imaginary double sun and, i45ff.

Lagrangian points and, 59, 60 mass of, 39 orbital velocity of, 33 rotation of, 9ff., 9off. tidal effect

on

Moon

field,

Elliptical galaxies,

of,

solar,

Eros, 26

Escape

velocity, 3 5

177,

180,

230

Andromeda

galaxy and, 188

distribution of, 177 position of, 185

theory

of,

105

52ff.,

36

Gravitational constant, 116

52

226

86

10

105,

tides and, 3

Enceladus, 48 Encke, Johann Franz, 139 Encke's comet, 1 39

Energy,

clusters,

Gravitation,

Earth-grazers, 26, 84 Einstein, Albert, 215

Electromagnetic

Globular

Great Hercules Gluster,

230 Greenhouse Halley,

effect,

Edmund,

181,

223,

71

132, 133, 142

Halley’s comet, 112, 130, 133, 134,

135

Harmonic Law,

Kepler’s, 29ff.

3

1

INDEX

235

Henderson, Thomas, 144 Hera, 178 Hermes, 26

Lagrange, Joseph Louis, 56 Lagrangian points, 56, 57

Hersehel, Caroline, 104 Herschel, John, 182

Land

Hersehel,

William,

102,

Lalande 21185, 161 4 Large Magellanic cloud, 104,

105,

176, 181, 211, 212

High

tides,

life,

182,

distance of, 187 Lea\’itt, Henrietta Swan, 182

Leningrad, 23

7

Hipparehus, 77, 152, 200 Hooke, Robert, 133 Hubble, Edwin Powell, 188, 212 Hubble’s law, 215

Huygens, Christian, 46, 47, 50, 55 Hydrosphere, 67

Urbain

Leverrier,

105

J, J.,

Light, 35

Newton

and, 39 speed of, 34, 52 Light-year, 32

Lithosphere, 67 Local cluster, 215

Louis XIV, 50

Icarus (asteroid), 26, 84, 85 Icarus (legend), 85

Lunar day, 93 Luyten, William

Icecaps, Martian, 19

J.,

137

lo, 11

Janus, 47, Jupiter,

1

Ml, 223 M3, 181 M13, 181, 223, 231 M31, 223 M42, 223 M31, 223 M37, 223 M82, 223, 223 M97, 223 Mach numbers, 33 McLaughlin, Dean B., 29

19

67

asteroids and, 45, 46

atmosphere of, 66 comets and, 1 39 diameter

of,

63

heat radiation life

of,

163

on, 72ff.

outer satellites

121

of,

radio waves from, 12

Maffei

rotation of, 12, 50, 65, 68 satellites

of,

108,

109,

120,

121,

171, 172

of,

tidal effect of,

Kapteyn,

1

177. 185 Kepler, Johannes,

Cornelis,

3,

198 Kirkwood, Daniel, 46 Kirkwood’s gaps, 46 Kordylewski, K., 60 Kronos, 150

2,

21

187

Mariner II, 14 Mariner IV, 13, 19 Mariner 9, 19 Mars, 67

71

Jacobus

and

Magellan, Ferdinand, 183 Magellanic clouds, 182, 183 size of,

Sun and, 155 temperature

1

30,

34,

176,

craters on, 14, 13, 19

130,

distance of, 30 life on, 16 rotation of, 30 satellites of, 48, 30,

121

surface of, 19 Maxwell, J. Clerk, 47, 52, 53 Mean solar day, 94

196

1

INDEX

236

Mean

Nereid, 122

Sun, 94 Mediterranean Sea, 2 Mercury, 139 rotation of,

orbit of, 108,

Neutron Newton,

98, 99 Messier, Charles, 180, 181, 223

Meteor

1

Meteor showers, 136 Milky Way, 105, 176,

197, 198, 203

Isaac,

3,

36,

38,

39,

Noah’s flood, 24 North Star, 181 Novae, 196, 199, 200

22

falls,

19

113, 132, 133, 132

22

crater,

Meteorite

3,

stars,

1

177,

178,

frequency

of,

197

179 dust clouds size of,

211

in,

Olberia, 207

Mimas, 47 Minutes of arc, 167 on Earth, 170 angular size

Omega

Orion Nebula, 33, 182, 212, 223 Owl Nebula, 227

colonization of, 75ff. craters on, 13, 16,

distance of,

Earth and,

Centauri, 230

169

of,

of,

Pallas,

20

77 Parsons, William, 197

77

Patroclus, 37 Peleus, 1 30

Parallax,

78 6, 10,

4ff.,

207

93

Kepler's laws and, 39

Persian Gulf, 24 Perturbations, 43

mass

Phobos, 30

flights to,

of,

orbit of,

40

78 1

10, 123,

126

size of, 81

star-motion from, 100

origin of, i24fF,

phases

of,

M., 206

Olbers’ paradox, 207ff.

Moon, 4

diameter

W.

Olbers, Heinrich,

182

Phoebe, 87

97

planetary nature of, 123, 126 rotation of, 10, 898?., 93ff.

Phoenicians, 91

star-motion from, 100

Giuseppe, 103, 206 Piazzia, 207 Pioneer 10, 43

Sun and, 123

Planets, 171

size of, 4,

78

Piazzi,

surface of, 20

Bode’s law and, 111

surface area of, 79

distances of, 31, 101, 102 escape velocity from, 33, 36

tides and, 2, 6ff.

water on, 73 Music of the spheres, 30

orbital velocity of, 33, 34

Neap

period of revolution of, 31 radio waves from, 220

tides, 2,

masses

7

Neptune, 63

118

rings about, 123

Bode’s law and, 106 discover)’ of, 103,

rotation of, 99 surface area of, 80

106

distance of, 106 satellites of, 108, 118,

tidal effect of,

of, 40,

1

Planetoids, 103, 207

119

Planets, giant, 68

atmospheres

of, 63,

66

33,

1

INDEX density of, 64

mass

of,

Lagrangian points and, 59 rings of, 46!?, 53, 55, 123 rotation of, 68

63, 64

structure of, 68ff.

temperatures

volume

of,

of,

yi

108, 109,

47, 48, 1

108

of, 81,

distance of, 106, 107, 128 orbit of, 107, 108

Pons, Jean Louis, I

and

1

39

II,

189

absolute magnitude of, 196

Seconds of

arc,

34

167

on Earth, 170 Sexagesimal notation, 166 Shapley, Harlow, 163, 185 Siberia, 22

Ptolemy, Claudius, 152

Sidereal day, 92 Sidereal month,

203

Pytheas, 2

Sirius, 143,

97

194

absolute magnitude

Radioactive wastes, 60, 61

Radio galaxies, 225 Radio sources, 219

Red Red

dwarfs, 159 giants,

1

Reinmuth, 26 215

Rigel, 195

Ring nebula, 223

155

Sirius B,

1

rotation of, 47

Roche, E., 122 Roche’s limit, 122

53

156 156 temperature of, 156 61 Cygni, 144 of,

of,

59 61 Cygni C, 159, 163 Sky, brightness of, 2o8ff. 1

Small Magellanic Cloud, 182, 184 distance of, 187 Solar day, 93

Rosse, Earl of, 226

Lyrae variables, 181, 182, 185

Solar energy, 86 Solar

Sagan, Carl, 62, 72 S Andromedae, 187 Satellites, captured,

month, 97

Solar system,

121

life in,

62

Solar wind, 135 Sombrero galaxy, 226

gravitational force on, 116

Space

rotation of, 99 surface, 42

Spinoza, Benedict, 55

surface area of, 80, 81

Spring

Saturn, 48, 49 density of, 64, 65 diamr'ter of, 63

195

of,

planet

Rings, Saturn’s, 46ff., 53, 55, 123

RR

Sirius A,

mass

Relativity, theory of,

of,

companion of, 155 motion of, 155

density

59

105,

Trojan asteroids and, 59 S Doradus, 182

Prometheus, 148, 149, 150 Proserpina, 114 Proxima Centauri, 159, 162 Pulsars, 198,

55,

1

Seas, lunar, 4 Second law, Kepler’s,

origin of, 109

50,

120

19,

tidal effect of,

Pluto, 106

Population

satellites of,

63

Plato, 25

diameter

237

suits,

40

Spiral galaxies, tides, 2,

Stars, angular

226 7

diameter

apparent motion

of,

brightness of, 143

of, 172,

173 90, 91, 100

5

1

INDEX

238 clusters of, 180, 181

Tigris-Euphrates, 24

^

distance of, 144 Earth’s rotation and, 92

Titan, 50, 55

mass limits of, 163 Milky Way and, 179

Tombaugh, Clyde, 106 Toro, 27, 59 Triton, 118, 119

Moon’s rotation and, 97 motion of, 133, 161 neighborhood,

orbit of, 109

158,

157,

161,

end

215 expanding, 215

177

Struve, Friedrich

Wilhelm

von, 144

of,

oscillating, 216, size of, 208,

Sub-stars, 163

Sun, absolute magnitude

cometary

shell and,

57, 58

contracting, 217

pulsating, 178, 199 variable,

1

Universe, age of, 188, 191

189

of,

tideal effect of,

Trojan asteroids,

162, 194 parallax of, 144 planets of, 159!?.

populations

Johann Daniel, 101

Titius,

of,

217

215

Uranus, 63 Bode’s law and, 103

193

136

Earth’s rotation and, 93

discovery of, 102, 103, 104

energy

satellites of, 105,

mass

of,

of,

86

19

154

planetary mass and, 117, 118 position in Galaxy of, 177,

211 radio waves from, 220

and, ii6ff.

stability of,

185,

Van de Kamp, Van Maanen’s

star,

Variable

177

stars,

Peter, 157

157

Venus, 96, 97 brightness of, 67 clouds on, 96, 97

rotation of, 99 satellites

1

conditions on, 97 distance of, 77, 96, 144

199

tides and, yff., 12

Sun-Earth system, 1 54 Sun-planet systems, 154 Supernovae, 196, 200, 219, 221 Synodic month, 98 Synodic period, 148

mass

of,

phases

40

of,

97

rotation of, 13, 98, 99

star-motion from, 100 surface temperature of,

1

tides and, 9

Vesta, 207

comet, 135 Tarantula nebula, 182

Tails,

Volcanoes, Martian, 19

Telescopes, 39

Tethys, 48

Whipple, Fred, 135 Whirlpool galaxy, 223, 226

Thetis, 150

White

7 ’hird law, Kepler’s, 30 Three-body problem, 54

Wildt, Rupert, 68 Witt, Gustav, 26

Tides,

Wolf, Max, 57

Tenth

planet, niff.

2ff.,

95

Earth’s rotation and,

land

life

and, 4

dwarf, 136, 197

9!?.

Zeus, 148, 150, 178

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Asimov has been described as a ‘natural wonder” (over 125 books in print) and a “national resource” (a Isaac

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dedicated not only to science, but

also to history

and

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He

has said that he “likes to be a writer.” His daily work regimen of eight to

five,

seven days a week, surely

evidences his dedication. Dr. Asimov now lives in

New

City.

JACKET BY DAVID

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ISBN: 0 - 385-041 11-x

Printed in the U.S.A.

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Asimov probes mysteries that bewilder youngsters and that even fortyyear-olds never seem to have gotten around to resolving for themselves —all in an intriguing style yet with the utmost professionalism.

For instance:

Even

if

there were no oceans on the earth, there

tidal friction

The

would

still

caused by the moon.

and motions of the planets are intricately related and given some you can calculate the rest.

If

positions

there

be

is life

on other planets,

it is

more

likely to

inter-

be Jupiter

rather than the popularized Mars.

The moon always is

presents one face to us on the earth, but

nevertheless rotating at the time.

There

is

a tug-of-war within our solar system, and the earth

loses out in the battle for the

What would

How

it

be

like to

The reason

there

simply that the

from

moon.

have more than one sun

far could our astronauts travel in space

“home, sweet home’7

is

it

us.

is

1

in the sky?

and

still

see

370 -AAF-IO 3 fSl

13 S anywhere

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constantly

dis •

AStnibV ON ASTRONOMY

moving away