Opus 200 Isaac Asimov

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The author wishes to thank the following for permission to quote

selections from the works listed:

Atonic Energy Commission: Worlds Within Worlds.

Thomas Y. Crowell: Earth: Our Crowded Spaceship. Copy-

permission of Thomas Y. Crowell.

Doubleday & Company, Inc.: Asimoo's Guide to SJwkespeare.

ACKNOWLEDGMENTS

Evening Post Company. Of Matters Great and Small. Copyright

Follett Publishing Company: Comets and Meteors. Text copy-

Isaac Asimov. Used by permission of Follett Publishing Co., a

division of Follett Corporation.

Houghton Mifflin Company: Isaac Asimov's Treasury of Hu-

1966, 1969, 1972 by the Hearst Corporation. Eyes on the Uni-

David McKay Company, Inc.: The Ends of the Earth. Copy-

David McKay Company, Inc.

William Morrow & Company, Inc.: Alpha Centauri, the Near-

mission of William Morrow & Company, Inc.

Mysterious Press: Asimoo's Sherlockian Limericks. Copyright

The Saturday Evening Post Company; "The Dream"; "Ben-

jamin's Dream"; and "Benjamin's Bicentennial Blast." Copyright

permission of The Saturday Evening Post Company.

by Isaac Asimov. How Did We Find Out About Numbers? Copy-

ACKNOWLEDGMENTS

Thirteenth Day of Christmas." First published in Ellery Queen's

Mystery Magazine and reprinted from The Key -Word and Other

Mysteries by Isaac Asimov, published by Walker and Company,

DEDICATION

To fanet

Who saw me through the second hundred

CONTENTS

INTRODUCTION

ASTRONOMY

ROBOTS

MATHEMATICS

PHYSICS

CHEMISTRY

BIOLOGY

WORDS

HISTORY

THE BIBLE

SHORT-SHORTS

HUMOB

SOCIAL SCIENCES

LITERATURE

MYSTERIES

AUTOBIOGRAPHY

MY SECOND HUNDRED BOOKS

INTRODUCTION

In October 1969, Houghton Mifflin published my

book Opus 100. It wasn't named at random. It was the

hundredth book of mine to be published.

That hundredth book took its time coming, of

course. It wasn't till I was eighteen, after all, that I

became a professional writer. (To be specific, my

first sale took place on October 21, 1938.) Then, for

eleven years after that, my only sales were to the sci-

ence fiction magazines, so. that I became a well-

known and successful writer (within the highly spe-

cialized and non-numerous ranks of the science fiction

world, anyway) without having a single book to my

name."

Then, on January 19, 1950, just after I had turned

thirty, I finally published my first book. Pebble in the

Sky. It was a science fiction novel.

After that, first slowly (two books in 1950 and two

more in 1951) and then more rapidly (eight books in

1960 and twelve books in 1966), I began to pile them

up.

What with one thing and another, I finally managed

to reach the hundredth book not quite twenty years

* In later years, these early stories were included in various

books, so they didn't go to waste forever, you may be sure.

ISAAC ASIMOV

after I had published the first one. That's an average

of five books a vear, wliich isn't bad, at least as far as

quantity is concerned.

With regard to quality, it is perhaps harder to judge,

but even if we disregard my own personal opinion

that my books are great, it remains fair to assume that,

publishers are reasonably sane and would not have

published so many of my books if they didn't think

they were good.

Once a hundred books had come boiling out of my

typewriter ribbons, I could have been forgiven if I

had then retired. I might have considered a hundred

books a reasonable life's work and spent the rest of

my existence doing other things—having a good time,

for instance.

There was a catch, though; two catches, in fact.

In the first place, when mv hundredth book came

out I was still ten weeks on the sunny side of fifty

(which mav not be much of a sunnv side, but where

age is concerned, 1*11 snatch at a hair's breadth), and I

didn't feel old enough to retire.

In the second place, I was already having a good

time and, if I retired, the only thing I would really

want to do in retirement would be to write. So why

retire only to do what I was already doing?

So I kept on working; and to such good effect—for

one gets better (or at least faster) with practice—that

in a surprisingly brief period of time I found I was

reaching my two hundredth book.

The second hundred was completed by 1979, so

that it had only taken me ten years to turn them out,

which is an average of ten books a year.

Naturally, Hougliton Mifflin (stifling who-knows-

how-many-sighs) feels honor-bound to publish Opus

200 now, and I'm perfectly content to let them do so.

OPUS 200 13

Let me emphasize now that, in publishing first

Opus 100 and then Opus 200, neither I nor Houghton

Mifflin is in any way celebrating the matter of quan-

tity. My two hundred books are far from being a

record.

According to The Guinness Book of World Records,

an Englishman named Charles Hamilton and an

American named Charles Andrews each published

about 100,000,000 words in their lifetimes, whereas

mv published output so far comes to perhaps 15,000,000

words. Even supposing I live out a reasonably long life

and continue writing at a reasonably fast clip, 1 don't

think I can possibly surpass 25,000,000 published

words at most.

Furthermore, Charles Andrews, according to the

Book of Records, wrote 100,000 words a week when at

his peak, and I think I do well if I manage a measly

15,000 words of finished material in one week.

Then, too, the British novelist John Creasey and the

Belgian novelist Georges Simenon each published

over 500 books in the course of their careers, and I

don't see that it is at all likely that even a long and

continually busy life is going to lift me past the 400

mark.

Nevertheless, I do not labor under any sense of fail-

ure because of this. Those authors who surpass me in

quantity have (as far as I know) an only limited

range. Their domain is fiction, and usually but one or

two kinds of fiction, so that they attain speed by roll-

ing down well-oiled tracks.

I, on the other hand, write not only fiction but

nonfiction. I write different kinds of nonfictlon for

different kinds of audiences, and that is the purpose

of my Opus books—to celebrate that variety.

14 ISAAC ASIMOV

For Opus 100 I took passages from my first

hundred books and carefully divided them into cate-

gories. For Opus 200 I've taken passages from my sec-

ond hundred books and divided them into the same

categories—plus several additional ones.

Nobody who reads my writings, after all, is very

likely to have read all my books, or even most of

them, and many people who do read and are, presum-

ably, fond of some of my books are not aware of some

of the other kinds of writing I do.

In these Opus books, then, the average reader will

get a chance to sample the variety to a fuller extent

than he would otherwise have a chance to do. If he

already likes part of what I write, he may find he also

likes, or is at least curious about, some other parts of

what I write. It might give him additional pleasure to

read those other parts in toto, and that would then

certainly please me.

And if he doesn't already like part of what I write

. . . then he might not buy this book in the first place,

which would be a shame, but there's no law against it.

PART1

ASTRONOMY

To anyone who got his start writing science fiction in

the days before World War II, astronomy was the sci-

ence. No one envisioned space travel outside science

fiction (except for a very few people working on rock-

ets, who were considered 1»f all the "hardheaded"

people around them to he but a half-step removed

from science fiction writers). That meant that some

facets of astronomy used to he the exclusive domain

of those who wrote and read science fiction. For in-

stance, where hut in science fiction could one de-

scribe the surface of the Moon as seen from the sur-

face of the Moon? Astronomy lost some of its

„ exclusivity, where science fiction writers were con-

cerned, by the time my second hundred hooks began

to be written in the late 1960s. Astronauts strove to

- reach the Moon, and in 1969, the year in which Opus

100 was Jmblished, they succeeded. We know the sur-

face of the Moon in great detail now and science fic-

tion has had to come to terms with that.

But we have only reached the Moon; no one has yet

actually lived on it. Therefore, the description of a

. working and viable settlement on the Moon still lies

^y within the province of fiction.

it For instance, in 1972 (for/ which time several space-

If ships had landed on the Moon and returned safely),

ISAAC ASIMOV

my science fiction novel The Gods Themselves (Book

121) was published by Doul)leday. It won both the

Nebula (the award of the Science Fiction Writers of

America) and the Hugo (the award of the fans gath-

ered in a world convention)." The third part of the

novel is set on the Moon, which is pictured as an elab-

orate human settlement. Here is a passage in which

Selene, the young woman Iwrn and bred on the Moon,

teaches Ben, who arrived from Earth but a month be-

fore, how to maneuver on the Moon's surface.

from THE GODS THEMSELVES (1972)

Selene laughed, and the sound was metallic in Deni-

son's earpiece. Her Bgure was lost in the spacesuit she

wore.

She said, "Now come, Ben, there's no reason to be

afraid. You're an old hand by now—you've been here a

month."

"Twenty-eight days," mumbled Denison. He felt

smothered in his own suit.

"A month," insisted Selene. "It was well past half-

Earth when you came; it is well past half-Earth now."

She pointed to the brilliant curve of the Earth in the

southern sky.

"Well, but wait. I'm not as brave out here as I am

underground. What if I fall?"

"What if you do? The gravity is weak by your stan-

dards, the slope is gentle, your suit is strong. If you fall,

just let yourself slide and roll. It's almost as much fun

that way, anyhow."

* I mention this for no reason other than that it gives me

pleasure to do so.

OPUS 200 19

Dension looked about doubtfully. The Moon lay

beautiful in the cold light of the Earth. It was black

and white; a mild and delicate white as compared

with the sunlit views he had seen when he had taken

a trip a week before to inspect the solar batteries that

stretched from horizon to horizon along the floor of

Mare Imbrium. And the black was somehow sorter,

too, through lack of the blazing contrast of true day.

The stars were supemally bright and the Earth—the

Earth was infinitely inviting with its swirls of white

on blue, and its peeping glimpse of tan.

"Well," he said. "do vou mind if I hang on to you?"

"Of course not. And we won't go all the way up. It

will be the beginners' slope for you. Just try to keep in

time with me. I'll move slowly."

Her steps were long, slow, and swinging, and he

tried to keep in synchronization. The up-sloping

ground beneath them was dustv and with each step

he kicked up a fine powder that settled quickly in the

airlessness. He matched her stride for stride, but with

an effort.

"Good," said Selene, her arm locked in his, steady-

ing him. "You're very good for an Earthie—no, I ought

to say Immie."

"Thank you."

"That's not much better, I suppose. Immie for Im-

migrant is as insulting as Earthie for Earthman. Shall

I Just say you're simply very good for a man your

age?"

"No! That's much worse." Denison was gasping a

little and he could feel his forehead moistening.

Selene said, "Each time you reach the point where

you're about to put your foot down, give a little push

with your other foot. That will lengthen your stride

and make it all the easier. No, no—watch me."

ISAAC ASIMOV

Dension paused thankfully and watched Selene take

off with low, effortless leaps Somehow, despite the

grotesquery of the suit, she appeared slim and grace-

ful when she moved. She returned and knelt at his

feet.

"Now you take a slow step, Ben, and I'll hit your

foot when I want it to shove."

They tried several times, and Denison said, "That's

worse than running on Earth. I better rest."

"All right. It's just that your muscles aren't used to

the proper coordination. It's yourself you're fighting,

you know, not gravity . . . Well, sit down and catch

your breath. I won't take you up much farther."

Dension said, "Will I do any damage to the pack if I

lie down on my back?"

"No, of course not, but ifs not a good idea. Not on the

bare ground. It's only at 120 degrees absolute—ISO de-

grees below zero, if you prefer—and the smaller the

area of contact the better. I'd sit down."

"All right." Gingerly, Denison sat down with a

grunt. Deliberately, he faced northward, away from

the Earth. "Look at those stars!"

Selene sat perpendicular to him. He could see her

face dimly through the faceplate now and then when

the Earthlight caught it at the proper angle.

She said, "Don't you see the stars on Earth?"

"Not like this. Even when there are no clouds, the

air on Earth absorbs some of the light. Temperature

differences in the atmosphere make them twinkle, and

city lights, even distant city lights, wash them out."

"Sounds disgusting,"

"Do you like it out here, Selene? On the surface?"

'I'm not crazy about it really, but I don't mind it too

much, now and then. It's part of my job to bring tour-

ists out here, of course."

OPUS 200 21

"And now you have to do it for me."

"Can't I convince you it's not tlie same thing at all,

Ben? We've got a set route for the tourists. It's very

-tame, very uninteresting. You don't think we'd take

them out here to the slide, do you? This is for Lunar-

ites—and Immies. Mostly Immies, actually."

"It can't be very popular. There's no one here but

ourselves."

"Oh, well, there are particular days for this sort of

thing. You should see this place on race days. You

wouldn't like it then, though."

"I'm not sure I like it now. Is gliding a sport for

Immies in particular^"

"Rather. Lunarites don't like the surface generally."

"How about Dr. Neville?"

"You mean. how he feels about the surface?"

"Yes."

"Frankly, I don't think he's ever been up here. He's

a real city boy. Why do you ask?"

"Well, when I asked permissioir to go along on the

routine servicing of the solar batteries, he was per-

fectly willing to have me go, but he wouldn't go him-

self- I rather asked him to, I think, so I could have

someone answer my questions, if there were any, but

his refusal was rather strong."

"1 hope there was someone else to answer your

questions."

"Oh, ves. He was an Immie, too, come to think of it.

Maybe that explains Dr. Neville's attitude toward the

electron pump."

"What do you mean?"

"Well—" Denison leaned back and kicked his legs up

alternately, watching them rise and fall slowly with a

certain lazy pleasure. "Hev, that's not bad. Look, Se-

lene, what I mean is that Neville is so intent on devel-

ISAAC ASIMOV

oping a pump station on the Moon when the solar bat-

teries are perfectly adequate for the job. We couldn't

use solar batteries on Earth, where the Sun is never as

unfailing, as prolonged, as bright, as radiant in all

wave lengths. There's not a single planetary body in

the solar system, no body of any size, that is 'more

suitable for the use of the batteries than the Moon is.

Even Mercury is too hot. But the use does tie you to

the surface, and if you don't like the surface—"

Selene rose to her feet suddenly and said, "All right,

Ben, you've rested enough. Up! Up!"

He struggled to his feet and said, "A pump station,

however, would mean that no Lunarite would ever

have to come out on the surface if he didn't want to."

"Uphill we go, Ben. We'll go to that ridge up ahead.

See it, where the Earthlight cuts off in a horizontal

line?"

They made their way up the final stretch silently.

Denison was aware of the smoother area at their

side—a wide swath of slope from which most of the

dust had been brushed.

"That's too smooth for a beginner to work up," Se-

lene said, answering his thoughts. "Don't get too am-

bitious or you'll want me to teach you the kangaroo-

hop next."

She made a kangaroo-hop as she spoke, turned

about-face almost before landing, and said, "Right

here. Sit down and I'll adjust—"

Denison did, facing downhill. He looked down the

slope uncertainly. "Can you really glide on it?"

"Of course. The gravity is weaker on the Moon than

on Earth, so you press against the ground much less

strongly, and that means there is much less friction.

Everything is more slippery on the Moon than on the

Earth. That's why the floors in our corridors and

OPUS 200 23

^ apartments seemed unfinished to you. Would you like

to hear me give my little lecture on the subject? The

one I give the tourists?"

"No, Selene."

"Besides, we're going to use gliders, of course." She

'.had a small cartridge in her hand. Clamps and a pair

of thin tubes were attached to it.

"What is that?" asked Ben.

"Just a small liquid-gas reservior. It will emit a jet

-^of vapor just under vour boots. The thin gas layer be-

tween boots and ground will reduce friction virtually

to zero. You'll move as though you were in clear

' space."

4 Dension said uneasily, "I disapprove. Surely it's

. wasteful to use t^as in this fashion on the Moon."

"Oh, now. What gas do you think we use in these

gliders? Carbon dioxide? Oxygen? This is waste gas to

jpbegin with. It's argon. It comes out of the Moon's soil

||in ton lots, formed by billions of years of the break-

|sdown of potassium-40 . . . That's-part of my lecture,

^too, Ben . . . The argon has only a few specialized

H.uses on the Moon. We could use it for gliding for a

^million years without exhausting the supply ... All

IJright. Your gliders are on. Now wait till I put mine

ton."

^ "How do they work?"

fef "It's quite automatic. You just start sliding and that

•will trip the contact and start the vapor. You've only

,got a few minutes' supply, but that's all you'll need."

| She stood up and helped him to his feet. "Face

downhill . . . Come^u^ Ben, this is a gentle slope.

Look at it. It looks perrectly level."

"No, it doesn't," said Denison sulkily. "It looks like a

cliff to me."

"Nonsense. Now listen to me and remember what I

ISAAC ASIMOV

told you. Keep your feet about six inches apart and

one just a few inches ahead of the other. It doesn't

matter which one is ahead. Keep your knees bent.

Don't lean into the wind because there isn't any. Don't

try to look up or back, but you can look from side to

side if you have to. Most of all, when you finally hit

level, don't try to stop too soon; you'll be going faster

than you think. Just let the glider expire and then fric-

tion will bring you to a slow halt."

"I'll never remember all that."

"Yes, you will. And I'll be right at your side to help.

And if you do fall and I don't catch you, don't try to

do anything. Just relax and let yourself tumble or

slide. There are no boulders anywhere that you can

collide with."

Denison swallowed and looked ahead. The south-

ward slide was gleaming in Earthlight. Minute un-

evennesses caught more than their share of light, leav-

ing tiny uphill patches in darkness so that there was a

vague mottling of the surface. The bulging half-circle

of Earth rode the black sky almost directly ahead.

"Ready?" said Selene. Her gauntfeted hand was be-

tween his shoulders.

"Ready," said Denison faintly.

*Then off you go," she said. She pushed and Deni-

son felt himself begin to move- He moved quite

slowly at first. He turned toward her, wobbling, and

she said, "Don't worry. I'm right at your side."

He could feel the ground beneath his feet—and then

he couldn't. The glider had been activated.

For a moment he felt as though he were standing

still. There was no push of air against his body, no

feel of anything sliding past his feet. But when he

turned toward Selene again, he noticed that the lights

OPUS 200 25

and shadows to one side were moving backward at a

slowly increasing speed-

"Keep your eyes on the Earth," Selene's voice said

in his ear, "till you build up speed. The faster you go,

the more stable you 11 be. Keep your knees bent . . .

You're doing very well, Ben."

"For an Immie," gasped Denison.

"How does it feel?*'

"Like flying," he said. The pattern of light and dark

on either side was moving backward in a blur. He

looked briefly to one side, then the other, trying to

convert the sensation of a backward flight of the sur-

roundings into one of a forward flight of his own.

Then, as soon as he succeeded, he found he had to

look forward hastily at the Earth to regain his sense of

balance. "I suppose that's not a good comparison to

use to you. You have no experience of flying on the

Moon."

"Now I know, though. Flying must be like gliding—

I know what that is."

She was keeping up with him easily.

Denison was going fast enough now so that he got

the sensation of motion even when he looked ahead.

The Moonscape ahead was opening before him and

flowing past on either side. He said, "How fast do you

get to go in a glide?"

"A good Moon-race," said Selene, "has been clocked

at speeds in excess of a hundred miles an hour—on

steeper slopes than this one, of course. You'll probably

reach a top of thirty-five."

"It feels a lot faster than that somehow."

"Well, it isn't. We're leveling off now, Ben, and you

haven't fallen. Now Just hang on; the glider will die

off and you'll feel friction. Don't do anything to help

it. Just keep going."

ISAAC ASIMOV

Selene had barely completed her remarks when Den-

ison felt the beginning of pressure under his boots.

There was at once an overwhelming sensation of

speed and he clenched his fists hard to keep from

throwing his arms up in an almost reHex gesture

against the collision that wasn't going to happen. He

knew that if he threw up his arms, he would go over

backward.

He narrowed his eyes, held his breath till he

thought his lungs would explode, and then Selene

said, "Perfect, Ben, perfect. I've never known an Im-

mie to go through his first slide without a fall, so if

you do fall, there'll be nothing wrong. No disgrace."

"I don't intend to fall," whispered Denison. He

caught a large, ragged breath, and opened his eyes

wide. The Earth was as serene as ever, as uncaring.

He was moving more slowly now—more slowly—more

slowly—

"Am I standing still now, Selene?" he asked. "I'm

not sure."

"You're standing still. Now don't move. You've got

to rest before we make the trip back to town . . .

Damn it. I left it somewhere around here when we

came up."

Denison watched her with' disbelief. She had

climbed up with him, had glided down with him. Yet

he was half-dead with weariness and tension, and she

was in the air with long kangaroo-leaps. She seemed a

hundred yards away when she said, "Here it isl" and

her voice was as loud in his ears as when she was

next to him.

She was back in a moment with a folded, paunchy

sheet of plastic under her arm.

"Remember," she said cheerily, "when you asked on

our way up what it was, and I said we'd be using it

OPUS 200

before we came down?" She unfolded it and spread it

on the dusty surface of the Moon.

"A lunar lounge is its full name," she said, "but we

just call it a lounge. We take the adjective for granted

here on this world." She inserted a cartridge and

tripped a lever.

It began to fill. Somehow Denison had expected a

hissing noise, but of course there was no air to carry

sound.

"Before you question our conservation policies

again," said Selene, "this is argon also."

It blossomed into a mattress on six stubby legs. "It

will hold you," she said. "It makes very little actual

contact with the ground and the vacuum all around

will conserve its heat."

"Don't tell me it's hot," said Denison, amazed.

"The argon is heated as it pours in, but only rela-

tively. It ends up at 270 degrees absolute, almost

warm enough to melt ice, and quite warm enough to

keep your insulated suit from losing heat faster than

you can manufacture it. Co ahead. Lie down."

Denison did so, with a sensation of enormous lux-

ury.

"Great!" he said with a long sigh.

"Mama Selene thinks of everything," she said.

She came from behind him now, gliding around

him, her feet placed heel to heel as though she were

on skates, and then let them fiy out from under her, as

she came down gracefully on hip and elbow on the

ground just beside him.

Denison whistled. "How did you do that?"

"Lots of practicel And don't you try it. You'll break

your elbow."

ISAAC ASIMOV

Some of the real findings on the Moon tended to de-

stroy a few of the more interesting science fictional

notions. For instance, to the best of our knowledge,

there have always been not more than small traces of

water on the Moon, and even these are vanishing. Our

study of the Moon rocks has shown that. Yet, in the

science fiction written before we reached the Moon, it

was often assumed that there was some water on the

Moon that might be frozen under the soil or chemi-

cally combined with the molecules of the crustal rock.

Even as late as 1972 1 held on to the hope that this

might be so despite the negative findings of the first

astronauts on the Moon. Thus, here is another scene

from The Gods Themselves. This time Selene and Ben

are inside the settlement.

Denison tried to beat down his self-consciousness.

Time and again, he made a groping motion as though i,

to hitch upward the pants he wasn't wearing. He wore :•

only sandals and the barest of briefs, which were un-

comfortably tight. And, of course, he carried the blan- ^

ket. 1

Selene, who was similarly accoutered, laughed. H

"Now, Ben, there's nothing wrong with your bare H

body, barring a certain flabbiness. It's perfectly in ^

fashion here. In fact, take off your briefs if they're

binding you."

"Nol" muttered Denison. He shifted the blanket so

that it draped over his abdomen and she snatched it

from him.

She said, "Now give me that thing. What kind of a

Lunarite will you make if you bring your Earth puri-

tanism here? You know that prudery is only the other

OPUS 200 29

side of prurience. The words are even on the same

page in the dictionary."

"I have to get used to it, Selene."

"You might start by looking at me once in a while

without having your glance slide off me as though I

were coated with oil. You look at other women quite

efficiently, I notice."

"If I look at you—"

Then you'll seem too interested and you'll be em-

barrassed. But if vou look hard, you'll get used to it,

and you'll stop noticing. Look, I'll stand still and you

stare. I'll take off my briefs."

Denison groaned, "Selene, there are people all

around and you're making intolerable fun of me.

Please keep walking and let me get used to the situa-

tion."

"All right, but I hope you notice the people who

pass us don't look at us."

"They don't look at you. They look at me all right.

They've probably never seen so old-looking and ill-

shaped a person."

"They probably haven't," agreed Selene cheerfully,

"but they'll just have to get used to it."

Denison walked on in misery, conscious of every

gray hair on his chest and of every quiver of his

paunch. It was only when the passageway thinned out

and the people passing them were fewer in number

that he began to feel a certain relief.

He looked about him curiously now, not as aware of

Selene's conical breasts as he had been, nor of her

smooth thighs. The corridor seemed endless.

"How far have we come?" he asked.

"Are you tired?" Selene was contrite. "We could

have taken a scooter. I forget you're from Earth."

ISAAC ASIMOV

"I should hope you do. Isn't that the ideal for an

immigrant? I'm not the least bit tired. Hardly the least

bit tired at anv rate. What I am is a little cold."

"Purely your imagination, Ben," said Selene firmly.

"You just think you ought to feel cold because so

much of vou is bare. Put it out of vour head."

"Easv to sav," he sighed. "I'm walking well, I hope."

"Very well. I'll have vou kangarooing yet."

"And participating in glider races down the surface

slopes. Remember, I'm moderately advanced in years.

But really, how far have we come?"

"Two miles, I should judge."

"Good Lord! How many miles of corridors are there

altogether?"

"I'm afraid I don't know. The residential corridors

make up comparatively little of the total. There are

the mining corridors, the geological ones, the in-

dustrial, the mvcological . . . I'm sure there must be

several hundred miles altogether."

"Do you have maps?"

"Of course there are maps. We can't work blind."*

1 mean you, personally."

"Well, no, not with me, but I don't need maps for

this area; it's quite familiar to me. I used to wander

about here as a child. These are old corridors. Most of

the new corridors—and we average two or three miles

of new corridors a-year, I think—are in the north. I

couldn't work my wav through them, without a map,

for untold sums. Mavbe not even with a map."

"Where are we heading?"

"I promised you an unusual sight—no, not me, so

don't say it—and you'll have it. It's the Moon's most

unusual mine and it's completely off the ordinary

tourist trails."

"Don't tell me you've got diamonds on the Moon?"

OPUS 200 31

''Better than that."

The corridor walls were unfinished here—gray rock,

dimly but adequately lit by patches of electrolumi-

nescence. The temperature was comfortable and at a

steady mildness, with ventilation so gently effective

there was no sensation of wind. It was hard to tell

here that a couple of hundred feet above was a sur-

face subjected to alternate trying and freezing as the

Sun came and went on its grand biweekly swing from

horizon to horizon and then underneath and back.

"Is all this airtight?" asked Denison, suddenly un-

comfortably aware that he was not far below the bot-

tom of an ocean of vacuum that extended upward

through infinity.

"Oh, yes. Those walls are impervious. They're all

booby-trapped, too. If the air pressure drops as much

as ten percent in any section of the corridors there is a

hooting and howling from sirens such as you've never

heard and a flashing of arrows and blazing signs di-

recting you to safety such as you've never seen."

"How often does this happen?"

"Not often. I don't think anyone has been killed

through air-lack in at least five years." Then, with sud-

den defensiveness, "You have natural catastrophes on

Earth. A big quake or a tidal wave can kill thou-

sands."

"No argument, Selene." He threw up his hands. "I

surrender."

"All right," she said. "I didn't mean to get excited

. . . Do you hear that?"

She stopped in an attitude of listening.

Denison listened, too, and shook his head. Sud-

denly, he looked around. "It's so quiet. Where is every-

body? Are you sure we're not lost?"

"This isn't a natural cavern with unknown passage-

ISAAC AS1MOV

ways. You have those on Earth, haven't you? I've seen

photographs."

"Yes, most of them are limestone caves formed by

water. That certainly can't be the case on the Moon,

can it?"

"So we can't be lost," said Selene, smiling. "If we're

alone, put it down to superstition."

To what?" Denison looked startled and his face

creased in an expression of disbelief.

"Don't do that." she said. "You get all lined. That's

right- Smooth out. You look much better than you did

when you first arrived, you know. That's low gravity

and exercise."

"And trying to keep up with nude young ladies who

have an uncommon amount of time off and an un-

common lack of better things to do than to go on bus-

men's holidays."

"Now you're treating me like a tourist guide again,

and I'm not nude," Selene retorted.

"At that, even nudity is less frightening than Intui-

tionism . . . But what's this about superstitition?"

"Not really superstition, I suppose, but most of the

people of the city tend to stay away from this part of

the corridor complex."

"But why?"

"Because of what I'm going to show you." They

were walking again. "Hear it now?"

She stopped and Denison listened anxiously. He

said, "You mean that small tapping sound? Tap—tap.

Is that what you mean?"

She loped ahead with the slow-motion movement of

the Lunarite in unhurried flight. He followed her, at-

tempting to ape the gait.

"Here—here—"

Denison's eye followed Selene's eagerly pointing fin-

OPUS 200 33

ger. "Good Lord," he said. "Where's it coming from?"

There was a drip of what was clearly water; a slow

dripping, with each drip striking a small ceramic

trough that led into the rock wall.

"From the rocks. We do have water on the moon,

you know. Most of it we can bake out of gvpsum;

enough for our purposes, since we conserve it pretty

well."

"I know- I know. I've never yet been able to man-

age one complete shower. How you people manage to

stay clean I don't know."

"I told you. First, wet yourself- Then turn off the

water and smear just a little detergent on you. You

rub it— Oh, Ben, I'm not going through it yet again.

And there's nothing on the Moon to get you all that

dirty anyway . . . But that's not what we're talking

about. In one or two places there are actually water

deposits, usually in the form of ice near the surface in

the shadow of a mountain. If we locate it, it drips out.

This one has been dripping since the corridor was

first driven through, and that was eight years ago."

"But why the superstition?"

"Well, obviously, water is the great material re-

source on which the Moon depends. We drink it, wash

with it, grow our food with it, make our oxygen with

it, keep everything going with it. Free water can't

help but get a lot of respect. Once this drip was dis-

covered, plans to extend the tunnels in this direction

were abandoned till it stopped. The corridor walls

were even left unfinished."

"That sounds like superstition right there."

"Well—a kind of awe, maybe. It wasn't expected to

last for more than a few months; such drips never do.

But after this one had passed its first anniversary, it

began to seem eternal. In fact, that's what it's called:

ISAAC ASIMOV

The Eternal. You'll even find it marked that way on

the maps. Naturally people have come to attach im-

portance to it, a feeling that if it stops it will mean

some sort of bad fortune."

Denison laughed.

Selene said warmly, "No one really believes it, but

.everyone part-believes it. You see, it's not really eter-

nal; it must stop sometime. As a matter of fact, the

rate of drip is only about a third of what it was when

it was first discovered, so that it is slowly drying. I

imagine people feel that if it happened to stop when

they were actually here, they would share in the bad

fortune. At least, that's the rational way of explaining

their reluctance to come here."

"I take it that you don't believe this."

"Whether I believe it or not isn't the point. You see,

I'm quite certain that it won't stop sharply enough for

anyone to be able to take the blame. It will just drip

slower and slower and slower and no one will ever be

able to pinpoint the exact time when it stopped. So

why worry?"

"I agree with you."

At the start, my writing consisted almost entirely of

science fiction. Of my first hundred books, nearly one

third is science fiction. That fell off with time, how-

ever. Of my second hundred books, only thirteen can

be considered science fiction under even the most lib-

eral interpretation.

That did not end my concern with astronomy, how-

ever, for 1 continued to deal with it in my nonfiction

and for every age level.

I wrote some picture books for Walker 6- Company,

for instance, at the suggestion of Beth Walker. They

OPUS 200

were ABC hooks, actually, in which two words were

defined for each letter of the alphabet. The idea was

that an ei^ht-year'old could read the definitions {or,

at least, have an adult read it to him) and then be

fascinated by the pictures.

The first and mo-vt successful of these was ABC's of

Space (Book 10J), which was published in 1969.

Here, for instance, are the definitions of the two

words under 0:

from ABC's OF SPACE (1969)

0 is for Ocean of Storms

a dark, smooth area on the Moon where the first

unmanned spaceship landed in 1966. It is not

really an ocean, because tliere is probablv no wa-

ter on the Moon. There are no storms either, but

we still use the name.

o is for orbit

the path a small world takes around a larger one.

The Moon moves in an orbit around the Earth.

The Earth moves in an orbit around the Sun.

Both orbits are almost like circles. An orbit is also

the path a spaceship takes around the Earth or

Moon.

I was not particularly fond of the ABC books, of

which three others were ]mblished by Walker by 1972.

These wereABC's of the Ocean (Book 107), ABC's of

the Earth (Book 117), and ABC's of Ecology (Book

124). The ABC format didn't leave me enough scope.

ISAAC ASIMOV

I did, however, start another series of books for

Walker is- Company with which I had a good deal

more fun.

The title of each book in the series, which was orig-

inally suggested by mi/ editor, Millicent Selsam, was

to begin How Did We Find Out. They were to deal

with science history on a Junior high school level.

The first one of these was How Did We Find Out

the Earth Is Round? (Book 133), which Walker pub-

lished in 1973. Writing the book was sheer pleasure,

and I knew I had something I would continue. In-

deed, of my second hundred hooks, no fewer than

thirteen are members of fhe How Did We Find Out

series.

One of the things that made the series pleasurable

for me was that the books varied widely in subject

matter_Three of them dealt with astronomy, four with

physics, two with biology, one with mathematics, one

with chemistry, one with geology, and one with an-

thropology.

One of the "astronomicaU" was How Did We Find

Out About Comets? {Book 162}, which was pub-

lished in 1975. Millie requested that topic during the

hullabaloo concerning the then forthcoming comet

Kohoutek. Though, alas, the comet fizzled, the book

certainly remained valid. Here's how I handled the

way in which cometary orbits were finally worked

out.

from How Dro WE FIND Our ABOUT COMETS (1975}

A German astronomer, Johannes Kepler, who had

been one of Tycho's assistants, disagreed with part of

Copemicus's theory. After studying the motions of the

OPUS 200 37

planets in the sky, Kepler said, in 1609, that the plan-

ets moved around the sun in orbits that were not

circles. Each planet moved around the sun in an

"ellipse."

An ellipse looks like a flattened circle. It can be so

slightly flattened that you cannot tell it from a circle.

It can be more flattened, so that you can see at a

glance that it is not a circle. Or it can be very flat-

tened, so that it looks long and thin, something like a

cigar.

The orbit of the earth around the sun is an ellipse

that is only very slightly flattened. It is almost circu-

lar. The moon's orbit around the earth is more flat-

tened, and Mercury's orbit around the sun is still more

flattened. Even Mercury's orbit, which is more flat-

tened than that of any other planet known in Kepler's

time, is not very flattened. Its orbit still looks like a

circle.

The sun is not at the very center of the elliptical

orbits of the planets around it. The flatter the ellipse,

the closer one end of it is to the sun.

When the earth moves around the sun, it is only

91,500,000 miles from the sun at one end of its orbit, but

94,500,000 miles from the sun at the other end. The

farther distance is less than 4 percent greater than the

nearer distance.

Mercury's orbit around the sun is more elliptical, so

there is a bigger difference. When Mercury is at the

end of the ellipse nearer the sun, it is only 28,000,000

miles away. At the other end, it is 44,000,000 miles

from the sun. The farther distance is about 50 percent

greater than the nearer distance.

Kepler was able to work out elliptical orbits for all

the planets, but what about the comets? If thev were

heavenly bodies, did that mean they had orbits, too?

38 ISAAC ASIMOV

Kepler carefully studied the reports he had about

the changing positions of comets in the sky. Finally,

he decided that comets must move in straight lines.

He thought they came from far out in space, passed

near the sun, then traveled onward far out in space in

the other direction.

They could only be seen when they were close to

the sun and reflected its light. Before they came close

enough to the sun, they could not be seen. After they

moved far enough from the sun, they again could not

be seen. According to Kepler's view, comets were not

part of the solar system. Each comet just passed

tlirough the solar system once and was never seen

again.

An Italian astronomer, Giovanni Alfonso Borelli,

carefully studied the positions of a comet that ap-

peared in the sky in 1664. He found he had to dis-

agree with Kepler.

The only way to make sense out of the path the

comet took across the sky, Borelli said, was to suppose

that it changed direction as it passed the sun. It came

closer and closer to the sun, along a line that was

nearly straight. Then it moved around the sun, and

left along a line that was again nearly straight but had

changed direction.

The way Borelli explained this was to point out that

ellipses could be very flattened indeed. They could be

so flattened that they would resemble a very long,

thin cigar. In fact. if you imagined an ellipse that was

more and more flattened, and longer and longer, you

could eventually imagine one that was so flattened it

Just went on and on forever. Such an ellipse would be

closed only at one end. In the other direction, it would

never be closed, but would just go on and on. A one-

OPUS 200 39

ended ellipse that goes on and on forever is called a

"parabola."

Borelli decided that a comet's orbit was a para-

bola, with the sun very near the closed end. The com-

et came in at one side of the parabola, went whizz-

ing around the sun, and theu moved outward along

the other side of the parabola.

Borelli's view was like that of Kepler, except that

the orbit he conceived was not a straight line. Like

Kepler, Borelli thought a comet was originally so far

away it could not be seen. As it came closer and closer

to the sun, it grew bright enough to be seen, and then

as it went farther and farther from the sun, it once

more became too dim to be seen. In Borelli's view, as

in Kepler's, the comets were not members of the solar

system. Each comet just passed through the solar sys-

tem once and never returned.

Kepler's notion of elliptical orbits worked very well

for the planets, but there were fots of questions left.

Why did the planets go around the sun in ellipses in-

stead of circles (or some other curve)? Why did plan-

ets move faster when they were nearer the sun than

when they were farther away?

These questions and many others were answered by

the English scientist Isaac Newton. In 1687, he pub-

lished a book in which he described his theory of uni-

versal gravitation. According to this theory, every

body in the universe attracted every other body. The

strength of the attraction between two particular bod-

ies depended on. the "mass" or each body (how much

matter it contained) and on how far apart the two

bodies were. The strength of the attraction could be

calculated by a simple mathematical equation.

Newton showed how to use the equation to work

ISAAC ASIMOV

out the exact orbit of the moon around the earth and

of the planets around the sun.

The same equation explained why each planet

moved quickly at some times and slowly at other

times, and why some planets moved faster than oth-

ers. It explained little changes in the motion of tne

planets that were produced by the tiny pulls of one

planet on another even as all were caught in the gi-

gantic pull of the much larger sun. It explained the

tides on the earth and many other things, too.

But comets were the one set of heavenly bodies that

remained puzzling. If comets traveled in orbits that

were parabolas, Newton's theory could account for

that fact Suppose, though, the orbits were not quite

parabolas. Suppose the orbits were Just very long

ellipses and were closed at the other end.

We can only observe the comet at the end of the

orbit near the sun. The shape of that small part of the

enormous orbit would be a narrow curve if the ellipse

were very long. The shape would be slightly wider, if

the ellipse were even longer, and still wider if the el-

lipse never closed at all and were a parabola.

The differences in the shapes of the small bit of

orbit we could see, as predicted by Newton's theory,

were so tiny that astronomers in Newton's time could

not tell them apart. They couldn't really say whether

the orbit or a comet was a very long ellipse or

whether it was a parabola,

It made a difference. If a comet's orbit were a para-

bola, it would visit the solar system once and would

never be seen again. If the orbit were a very, very

long ellipse, then eventually the comet would come to

the other end of the ellipse, turn around, and begin to

approach the sun again. The comet would return.

In fact, if astronomers could calculate the exact

OPUS 200

length of the orbit, they could even predict when the

comet would return. That would be a big victory for

Newton's theory.

Newton had a young friend, Edmund Halley, who

had helped Newton publish his book and who was in-

terested in the comet problem.

In 1682, a comet appeared and Halley very care-

fully studied its positions and the way it moved across

the sky. From the part of the orbit he could see, he

couldn't tell whether it would ever return.

It seemed to him, though, that if a comet did return

it should do so at regular periods—every so many

years—and that it should always trace the same curve

across the sky. He therefore began to collect all the

reports on the positions of earlier comets that he could

find- By 1705, he had collected good reports on two

dozen comets of the past and began to compare them.

He noticed that the comet of 1682, which he had

himself observed, followed the -same curve across the

sky that the comet of 1607 had. The same curve had

also been followed by the comet of 1532 (which Fra-

castoro and Apian had studied) and the comet of

1456.

These comets had come at seventy-Hve- or seventy-

six-year periods. Could it be that it was a single comet

that returned every seventy-five years or so? Could it

be that it was a "periodic comet"?

Halley worked out the orbit for a comet that re-

turned every seventy-five years and followed the same

curve in the sky that the comet of 1682 had followed.

The results were quite amazing. Saturn, the planet

farthest from the sun (as far as was known in Halley's

time) was never farther from the sun than 930,000,000

miles. The comet of 1682, however, moved out as far

ISAAC ASIMOV

as 3,200,000,000 miles from the sun before it reached

the other end of its ellintical orbit and began moving

inward again. The comet moved over three times as

far awav from the sun as Saturn ever moved.

On the other hand, when the comet passed along

the end of the ellipse that was near the sun, it came as

close as 54,000,000 miles from the sun. This was only

about half of earth's distance from the sun.

After Halley had calculated the orbit, he announced

that the comet of 1682 would return some time in 1758

and would follow a particular path across the sky.

Halley did not live long enough to see the comet's

return. He was eightv-six years old when he died in

1742, but that was much too soon to see the return.

There were. however, others who were watching for

it. A French astronomer, Alexis Claude Clairault, con-

sidered the orbit as outlined bv Hallev. He realized

that the gravitational pull of the large planets, Jupiter

and Saturn, would delav the comet a little bit. It

would not pass around the sun till some time in 1759.

In 1758, astronomers eagerly watched that part of

the sky in which Hallev had said the comet should

appear. They did not have to depend only on their

eyes as Tycho and earlier astronomers had done. The

telescope had been invented in 1609.

On December 25, 1758, Christmas Day, a German

farmer named Johann Georg Palitzch, who was an

amateur astronomer, spotted the comet. The comet of

1682 appeared in the sky where Halley had said it

would and proceeded to move along the path Halley

had predicted for it. It moved around the sun quite

close to the time Clairaulrtiad predicted.

There was no question that it was the comet of 1682

and that it had returned. That meant that some of the

OPUS 200 43

mystery of comets was cleared up. They followed the

same rules -as the other bodies of the solar system ex-

cept that their oibits were more elliptical.

Naturally, the comet of 1682 that returned and

passed around the sun in 1759 came to be called "Hal-

ley's comet."

Halley's comet is the most famous comet there is.

It happens to be the one that was in the sky in 1066

when William of Normandy was preparing to invade

England. It was also in the sky in 11 B.C., about the

time when Jesus may have been born. Some people

think it may have been the Star of Bethlehem.

Halley's comet has returned twice since Palitzch

saw it. It came back in 1835 and was glowing in the

sky when Mark Twain was born. Then it came back

in 1910 and Mark Twain died when it was glowing in

the sky. It will come back yet again in 1986.

Writing for different age levels has its problems, of

course, .-since the boundaries (ire not clear. I let myself

be guided by instinct, and if I must err, I prefer to err

on the side of difficulty. I like to think that the kind of

youngster who is interested in my books would rather

stretch a little and stand on his mental tiptoes than

stoop to something he might consider babyish.

Thus, for Follett Publishing Company, I did a series

of eight books on science that were intended for an

age level higher than that .of my ABC books and

lower than that of my How Did We Find Out books.

The first four of the Follett series were published

among my first hundred books, but the second four,

including three on astronomy, were in my second

hundred books. They are Comets and Meteors (Book

ISAAC ASIMOV

134), The Sun {Book 735), and The Solar System

{Book 160). Here is how I handled the matter of com-

etary orbits in Comets and Meteors:

from COMETS AND METEOBS (1873)

Comets go around the sun the way planets do, but with

a difference. Planets move in paths, called "orbits,"

that are nearly circles. Thev stav almost the same dis-

tance from the sun all the time. Comets move in orbits

that are long and narrow. Both comets and planet or-

bits are "ellipses."

At one end of the orbit, comets pass near the sun,

perhaps only a few million miles away. At the other

end, they are much farther awav, sometimes farther

than any planet. At this point, they are billions of

miles away from the sun.

A comet has no light of its own. To be seen, it must

be near a large bright object, like the sun. Sunlight

makes a comet shine.

Comets get very little sunlight at the far end of

their orbits. They are small and dim then. They can-

not be seen even with a telescope- As they move

closer to the sun, they get more sunlight. They be-

come bright enough to be seen.

People see comets only at the end of their orbits

close to the sun. Tlien thev are close to the earth, too.

Centuries ago, people believed that comets came

from nowhere. They couldn't tell when another comet

might come.

About three hundred years ago, an English astrono-

mer, Edmund Hallev, studied records of comets that

had been seen. He found that every seventy-six years

OPUS 200 45

or so, a comet crossed a certain part of the sky. He

decided it must be a single comet that came close to

the sun every seventv-six vears.

Hallev said the comet would come back in 1758 and

cross the same part of the skv- Bv then, Halley was

dead. But the comet returned just as he said it would.

It is known as Halley's comet tor that reason.

This business of aiming high for each age group

means that almost no effort is involved if I aim for the

teenage market. I always assume that a teenager is as

intelligent as an adult and has the vocabulary of one.

What he lacks is merely the opportunity to have read

as widely as an adult. {Naturally, I am talking of an

intelligent, well-read adult.)

Consequently, in writing for teenagers, I take par-

ticular care to make no assumptions of precious

knowledge and to explain everything that doesn't

come within the range of common experience—hut I

make sure I use a full vocabulary to do so. Teenagers

are sensitive {and rightly so) to any hint of conde-

scension.

Included among my second hundred books are

three on astronomy for teenagers, which I wrote at the

suggestion of Chaucy Bennetts of I^othrop, Lee 6-

Shepard Company. She is a very capable editor who,

coincidentally, became my cousin by marriage after

the series started. The three books are Jupiter, the

Largest Planet {Book 139); Alpha Centauri, the Near-

est Star {Book 179); and Mars, the Red Planet {Book

188). Here are two excerpts from Alpha Centauri:

46 ISAAC ASIMOV

from ALPHA CENTAUM, THE NEAREST STAB (1976)

In the case of the Alpha Centauri system, the average

separation of the two stars Alpha Centauri A and Al-

pha Centauri li is greater than that of Uranus and the

sun, and less than that of Neptune and the sun. If the

Alpha Centauri sv.stem were suoerimposed on the so-

lar system, however, with Alpha Centauri A in place

of our sun, Alpha Centauri B would not take up a cir-

cular orbit between those of Uranus and Neptune.

Things would be a little more complicated than that.

If the orbit of an object moving around a star were

an exact circle, the star would remain at the precise

center of the orbit and that would represent a very

simple situation. Actually, the orbit is always an el-

lipse, a kind of flattened circle. An ellipse has a major

axis (its longest diameter) and a minor axis (its short-

est diameter). The center of the ellipse is at the point

where the two axes cross.

There are two focus points, or foci, in the ellipse.

They are located on the major axis, one on each side

of the center and at an equal distance from it. The

more flattened the ellipse, the farther the foci are

from the center and the closer they are to the ends.

These foci are located in such a way that if a

straight line is drawn from one focus to any point on

the ellipse, and another straight line is drawn from

that point to the other focus, the sum of the lengths of

the two straight lines is always the same and always

equal in size to the major axis.

As it happens, when an object moves about a star in

an elliptical orbit, the star is always at one of the foci

and is, therefore, nearer to one end of the orbit than

to the other. If the ellipse is very flattened, the star is

OPUS 200 47

far to one end and the orbiting object is very close to

tlie star at that end of the orbit and very far from it at

the other end.

The point of closest approach is called the "peri-

astron," from Greek words meaning "near the star." The

farthest point is the "apastron," from Greek words

meaning "awav from the star."

In a binary system both stars, under the pull of

gravity, move in orbits around a point between them

called the "center of gravity." As they move, both

stars always remain on opposite sides of the center

of gravity, and the larger star is always closer to it

This means tliat although both stars have orbits that

are ellipses of the same shape, the larger star always

moves through the smaller orbit

When one object in a binary system is very much

larger than the other, it makes such a small ellipse

about the center of gravity that it is practically sta-

tionary. This is true of the sun and Earth, for instance,

where the sun scarcely moves at'all while tiny Earth

moves in a large ellipse.

It is always possible, however, to suppose that the

larger of two obiects in a binarv system is standing

still and to calculate the orbit of the smaller about it.

This distorts the situation relative to observers in

other planetary systems—relative to us, for instance.

However, if we could imagine ourselves observing the

binary system from the larger of the two stars, what

we would observe would be the smaller star moving

about a motionless larger one.

When astronomers observe a binary system, they

are not at all likely to be viewing it from directly

above, so to speak, so as to see the elliptical orbits

marked out exactly as they are. They usually view the

ISAAC ASIMOV

orbits from a tilted position, so that the ellipses they

see are not the ellipses marked out bv the orbiting

stars. What thev see are ellipses that are more flat-

tened, sometimes verv much more flattened. In these

distorted ellipses, however, the larger star, which is

supposed to be stationary, is not at the focus of the

smaller star's orbit. If astronomers tilt the orbit, in imag-

ination, until the star moves into the focus, they get

the true ellipse.

The degree of flattening of an ellipse is measured as

its "eccentricity" (from Greek words meaning "out of

center"), since the greater the eccentricity, the farther

the foci are from the center. The eccentricity of a cir-

cle, which is not flattened at all, is 0. For an ellipse,

the eccentricity is alwavs between 0 and 1. If an el-

lipse has a low eccentricity, say, less than 0.1, it is so

slightly flattened that to the eye it looks very much

like a circle. The flatter an ellipse is, the more it ap-

proaches a value of 1. An orbit with an eccentricity of

0.9, then, looks quite cigar-shaped,

An example of a high degree of eccentricity in a

binary system is Gamma Virginis, where the eccen-

tricity is 0.88. This means that the distance from the

center of the ellipse to the focus is 0.88 times the dis-

tance from the center of the ellipse to the end. With

the larger star at one focus, the end of the orbit of the

other star in the direction of that. focus (the peri-

astron) is only 0.12 times the distance from the center

and only 0.06 times the entire width of the ellipse

from end to end. The other end of the ellipse (the

apastron) is distant from the larger star by an amount

equal to 0.94 times the entire width of the ellipse.

In the case of Gamma Virginis, then, although the

average distance separating the two stars of the

OPUS 200 49

binary is 6,800,000,000 kilometers (4,200,000,000

miles), at periastron the distance of separation is onlv

810,000,000 kilometers (500,000,000 miles) while at

apastron it is 12,800,000,000 kilometers (7,900,000,000

miles).

In other words, the two stars of Gamma Virginis, as

they circle each other, swoop together to a separating

distance equal to that of Jupiter and the sun, and then

move apart to a distance more than twice that be-

tween Pluto and the sun. (The system was at apastron

in 1920 and the two stars have been moving closer

ever since. Thev will be at periastron in 2006.)

In general, stars separated by quite a large average

distance are likely to have large eccentricities, A bi-

nary like Caoella with an average separation of only

84,000,000 kilometers (52,000,000 miles) has quite a

low eccentricity, one of only 0.0086. This means that

the distance between the two stars of the Capella svs-

tem varies from 83,300,000 , kilometers (51,600,000

miles) at periastron to 84,700,000 kilometers (52,400,000

miles) at apastron.

This is so small a change that from the standpoint

of one of the stars of the Capella system, the other

would scarcely seem to change in brightness during

the 104-day period of revolution. In the case of

Gamma Virginis, on the other hand, an observer near

one of the stars would see the other as 250 times

brighter at periastron than at apastron.

The eccentricities of the planetary orbits of the so-

lar system, by the way, are much more like those of

the Capella stars than those of the Gamma Virginis

stars. -The eccentricities of the orbits of Venus and

Neptune are just about those of the Capella system,

while that of Earth (0.017) is only a little higher. This

15 a good thing, too, for a highly eccentric orbit would

ISAAC ASIMOV

introduce such changes in temperature in the course

of the vear that a planet with even a suitable average

distance from its sun might prove uninhabitable.

Let us take, now, a group of binaries that have av-

erage separations of about 3.0 to 3.5 billion kilometers

(1.9 to 2.2 billion miles), a group that includes the

Alpha Centauri system. In the table below, the eccen-

tricity and the distances at periastron and apastron are

given tor this group.

Eccentricities of Binary Systems

KILO- OF KILO- OF

METERS MILES METERS MILES

70 Ophiuchi Zeta Sagittarii Alpha Centauri Eta Ophiuchi Zeta Cancri 0.50 0.2 0.521 0.90 0.31 1750 2700 1700 320 2200 11001700 1000200 1350 52504300 53006080 4100 3300 2700 34003800 2570

Sinus 0.575 1280 800 4720 3000

Xi Scorpii 0.74 780 500 5200 3300

As you see, the apastrons are not extraordinarily dif-

ferent, varying from 4100 to 6080 million kilometers

(2570 to 3800 million miles), a difference of only

about 50 percent. The periastrons differ, however,

from 320 to 2700 million kilometers (200 to 1700

miles), a difference of 800 percent.

The Alpha Centauri system is rather intermediate

OPUS 200 51

with respect to eccentricity. The orbits of the two

stars Alpha Centauri A and B are more eccentric than

those or the planets of our solar system, but less ec-

centric than those of some of the comets, asteroids,

and satellites of our solar system.

If Alpha Centauri A were in the place of our

sun, then Alpha Centauri B at its farthest would be

5,300.000,000 kilometers (3,400,000,000 miles) away,

or just about at the average distance of Pluto from our

sun. From Earth's position -near Alpha Centauri A,

Alpha Centauri B would seem a starhke point, but it

would be far brighter than anv star we see in our own

sky. It would shine with a brilliance about 100 times

greater than our full moon, though it would still be only

1/4500 as bright as Alpha Centauri A or our sun.

From its farthest point, however, Alpha Centauri B

would slowly decrease its distance to Alpha Centauri

A (and ourselves) as it moved along its orbit, until

after forty years it would be at periastron and only

1,700,000,000 kilometers (1,000;000,000 miles) from

Alpha Centauri A. At that point it would be a little

farther from Alpha Centauri A than Saturn is from the

sun. And when Earth would be on the side of its orbit

toward Alpha Centauri B, th6 companion star would

be only 1,550,000,000 kilometers (900,000,000 miles)

from us.

At that distance. Alpha Centauri B would be a little

over 14 times as bright as at apastron. It would be

1400 times as bright as the full moon, but still only

1/326 as bright as Alpha Centauri A.

Suppose Alpha Centauri B were in place of our sun,

and that we calculated the orbit of Alpha Centauri A

on the assumption that Alpha Centauri B was motion-

less. Alpha Centauri A would then seem to move in

ISAAC ASIMOV

the same orbit that Alpha, Centauri B had in the other

case."

Viewed from an Earth that was circling Alpha Cen-

tauri B instead of our own sun, Alpha Centauri A

would go through the same period of brightening as it

moved from apastron to periastron, and the same pe-'

riod of dimming as it moved back to apastron. How-

ever, since Alpha Centauri A is 3)1 times as bright as

Alpha Centauri B, Alpha Centauri A would seem that

much brighter at everv point in its orbit. At its bright-

est, it would be 5000 times brighter than our full

moon now, and 1/100 as bright as our sun appears to

us. Since Alpha Centauri B would appear dimmer

than the sun, if we imagined the former in the latter's

place, Alpha Centauri A at its closest approach would

appear 1/30 as bright as Alpha Centauri B.

* Because Alpha Centauri B is the smaller of the two stars,

it seems to move in the larger orbit of the two when viewed

from outside the system. When viewed from inside the sys-

tem, however, an observer on each star would see the other

moving in the same orbit. Thus, on Earth, if we pretend that

the Earth is motionless, the sun moves in an orbit about the

Earth that is just like the orbit that the Earth (in reality)

moves in as it circles the sun.

OPUS 200 53

The Orbit of Alpha Centauri B

superimposed on our solar system

If we were circling Alpha Centauri A instead of the

sun, the presence of Alphil Centauri B would cause us

no trouble. Despite the eccentricity of its orbit, which

allows Alpha Centauri B to swoop in and pull out in

forty-year alternations, it would remain so far away at

all times that its gravitational pull would never be

strong enough to affect Earth's orbit seriously. What's

more, its addition to the light and heat delivered by

Alpha Centauri A would never be more than a third

of 1 percent. And think of what a marvelous spectacle

it would make in the sky.

ISAAC ASIMOV

If we were circling Aipha Centauri B, the superior

brightness of Alpha Centauri A would be more dis-

turbing. but if we imagined Eaith pulled in closer to

Alpha Centauri B in order to receive as much heat and

light from that smaller sun as we receive from our own

sun, the interference of Alpha Centauri A would net

be too disturbing.

And what about Alpha Centauri C—Proximo Cen-

tauri—which is the distant companion of the Alpha

Centauri A/B binary? Even though it would be far

nearer to us, if Earth were circling either Alpha Cen-

tauri A or Alpha Centauri B, than any star is to us in

our own solar system, it would not be at all bright. It

would be a fairly dim star of magnitude 3.7. What's

more, its proper motion, as a result of its 1,300,000-

year-long revolution around the center of gravity of

the system, would be just about exactly 1 second o£

arc per year.

Neither its brightness nor its proper motion would

attract much attention, and stargazers might look at

the sky forever and not suspect this dim star of be-

longing to their own system. The only giveaway

would come when astronomers decided to make a rou-

tine check of the parallaxes of the various visible stars

in the sky. After a month or so, they would begin to

get a hint of an extraordinarily large parallax and in

the end they would measure one of 20 seconds of arc,

which would be so much higher than that of any other

star that they would at once suspect it of being a

member of their own system.

Can there be a dim star somewhere out there that

belongs to our own solar system? Can it be that we

remain unaware of it because astronomers haven't

happened to study it closely enough to detect an un-

OPUS 200 55

usually high parallax? It isn't very likely—but it is con-

ceivable.

In general, the hotter a star is, the brighter it is. It's no

surprise, therefore, that so manv of the bright stars in

the sky are hotter than the sun is, or that so many of

the dim stars we see are cooler than the sun is.

What is surprising is that some stars are cool and yet

are very bright. The two prime examples of this are

Antares and Betelgeuse. Both are in spectral class M

and are therefore possessed of a surface temperature

of only 3000° C or so and, what's more, neither one is

particularly close to us—and yet each is among the

brightest stars in the sky.

In 1905 a Danish astronomer, Ejnar Hertzsprung,

reasoned that a cool star must have a dim surface, but

if it had a very large surface, the dimness of each bit

would add up to a great total brightness. In other

words, a bright star that was cool and red had to be a

very large star indeed in order to be bright.

Hertzsprung published this idea in a Journal of pho-

tography, and astronomers didn't notice it. Then, in

1914, the American astronomer Henry Norris Russell

had the same idea independently, and this time the

idea stuck. Both astronomers are usually given credit.

The Hertzsp rung-Russell reasoning led to the con-

cept of "red giants" among the stars. When attempts

were made to calculate hov^ large these red giants

would have to be in order to be as bright as they were

despite their low surface temperature, the results

seemed almost unbelievable. In 1920, however, the

German-American physicist Albert Abraham Michel-

son was able to check the matter directly.

To do this, he made use of an instrument he had

ISAAC ASIMOV

invented twenty years earlier, an instrument he called

an interferometer. It was capable of measuring, with

great delicacy, the manner in which two trains of light

waves, which were not quite parallel to each other,

interfered with each other. When such trains of light

waves were not quite parallel, the waves as they

merged sometimes reinforced each other and some-

times canceled each other, setting up patterns of alter-

nate light and dark. From the details of such an inter-

ference pattern, the exact angle at which the light

waves met could be deduced.

Such an instrument can be applied to the stars. A

star is so small, as seen from Earth, that it is virtually

a dot of light. The light rays coming from the two

opposite edges of so tiny a dot seem to come to us

almost from the same direction, and are therefore al-

most parallel—almost, but not quite. The light rays

come from very slightly different directions as they

reach us from opposite sides of a star; they converge

just a tiny bit, enough to produce an interference pat-

tern if the interferometer is large enough.

Michelson made use of a twenty-foot interferome-

ter, the largest he had constructed up to that time. He

attached it to the new hundred-inch telescope that

had just been put into use at Mount Wilson in Califor-

nia, and which was then the largest telescope in the

world. He turned this instrument on the star Betel-

geuse.

From the nature of the interference pattern, Michel-

son could determine the apparent diameter of Betel-

geuse. It turned out to be 0.045 seconds of arc. This is

a very small width, for it would take 41,500 little dots

of reddish light |ust like Betelgeuse, placed side by

side, to stretch across the width of the moon.

OPUS 200 57

Yet, Betelgeuse has the largest apparent diameter of

any star. Anv star that has a true size greater than

Betelgeuse is so far away as to have a smaller appar-

ent size. Then, too, any star that is closer than Betel-

geuse is so much smaller in true size that its apparent

size never comes up to the Betelgeuse mark.

To be even 0.045 seconds in diameter—tiny though

that angle is—at the vast distance of Betelgeuse, the

star must have an enormous real diameter. In fact, it

turns out that the diameter of Betelgeuse is at least

800 times that of the sun-

The interferometer result showed that the reasoning

of Hertzsprung and Russell was correct and there

really were red giant stars, with Betelgeuse, large as it

is, not the largest in actual size. In the table on the next

page, the diameters of some of the giant stars are given.

The large red giants would seem to be impressive

objects indeed. Imagine Betelgeuse in place of our

sun. We could not see it from Earth, because there

would be no Earth. The place where Earth would be,

if it existed, would be within Betelgeuse. The diame-

ter of Betelgeuse is so large that, if substituted for the

sun, it would include the orbits of Mercury, Venus,

Earth, Mars, and Jupiter.

Epsilon Aurigae B would do better than that. It

would swallow up the orbit of Satum as well, and its

surface would be nearly at the orbit of Uranus. What's

more, that supergiant Epsilon Aurigae B is part of a

binary system, with the other star, Epsilon Aurigae A,

considerably smaller but still large enough to swallow

up the orbit of Mars. What a view those stars must be

from not too nearby.

ISAAC ASIMOV

Giant Stars

DIAMETER

STAB

MILLIONS OF MILLIONS OF

KILOMETEBS MILES SUN = 1

Epsilon Aurigae B 2800 1700 2000

W Cephei A 1700 1200 1400

Betelgeuse 1100 700 800

Mira (Oinicron Ceti) 550 350 400

Antares 550 350 400

Xi Aurigae A 420 260 300

Epsilon Aurigae A 280 170 200

Beta Pegasi 150 95 110

Aldebaran 61 38 44

Arctums 37 23 27

Another way of emphasizing the size of the red

giants is to imagine a hollow sphere the size of Beta

Pegasi, which is only a moderate-sized giant. It would

still be large enough to hold 1,300,000 objects the size

of our sun. A hollow sphere the size of Betelgeuse

would hold nearly 43,000,000 objects the size of our

sun, and one the size of Epsiloq Aurigae B would hold

8,000,000,000 suns.

And yet, for all that, the, red giants are perhaps not

as impressive as they seem from their size alone. They

are more massive than the sun, but not very much

more massive. Betelgeuse might take up 43,000,000

times as much space as the sun does, but the red giant

is only about 20 times as massive as the sun; it con-

tains only 20 times as much matter.

If the mass of Betelgeuse (not so very great) is

spread over the enormous volume taken up by Betel-

geuse that mass must be spread very, very thin.

The sun's average density is 1.41 grams per

OPUS 200 59

cubic centimeter, but Betelgeuse's average density is

1/10,000,000 of that. If the sun were only as dense,

on the average, as Betelgeuse is, it would have a mass

of not more than 1/30 that of the Earth, and only 2.7

times that of the moon.

Epsilon Aurigae B would be far less dense. The red

giants are thin collections of gas that stretch out over

enormous distances and glow red-hot, but on an

earthly scale they are almost vacuums. The average

density of Epsilon Aurigae B is only 1/1000 that of

Earth's atmosphere, and in its outer regions the dens-

ity is far less even than that. (Like all objects, red

giants get denser as one approaches their centers, and

in the core they can get very dense indeed. This must

be true of all stars, since only in a very dense core can

the nuclear conflagration that powers them be ig-

nited. )

A situation the reverse of the red giants' arose in

connection with Sirius B. That was known to be a

very dim star with a magnitude of 10 and a luminosity

only 1/130 that of our sun. It was taken for granted

that it had to be both small and cool to deliver only

1/130 as much light as our sun.

In 1915, however, the American astronomer Walter

Sydney Adams succeeded in taking the spectrum of

Sirius B and found it to be just as hot as Sirius A and,

therefore, considerably hotter than our sun.

Yet, if Sirius B were that hot, its surface should

blaze with white light, and the only way of explaining

its dimness was to suppose that it had very little sur-

face.

Sirius B had to have so little surface as to be a

dwarf star, far smaller than anyone then had believed

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