Peterborough said, "Yes, but-"

  Rubin overrode him. "Mario," he said, "listen carefully. An eclipse of the Sun takes place when the Moon gets exactly between the Sun and the Earth. Observers on Earth then see the Sun blacked out because the opaque body of the Moon is squarely in front of it. We on Earth are in the Moon's shadow. Now if you're on the Moon," his voice grew harsh, "how the Hell can you be in the Moon's shadow?"

  Avalon said, "Not so fast, Manny; an eclipse is an eclipse is an eclipse. There is such a thing as a Lunar eclipse, when the Earth gets between the Sun and the Moon. The Moon is in the Earth's shadow in that case and the whole Moon gets dark.

  "The way I see it, then, is that Murderer takes a beautiful photograph of an eclipse on Earth, with the Moon moving in front of the Sun. He has advanced equipment that he has invented himself so that no one can possibly take a better photo of the moon in front of the Sun. Victim, however, goes him one better by taking an even more impressive photograph of an eclipse on the Moon, where, as Mario says, there is no air, with the Earth moving in front of the Sun."

  Peterborough mumbled, "Not the same thing."

  "It sure isn't," said Halsted, who had pushed his coffee cup to one side and was doing some quick figuring. "As seen from Earth, the Moon and the Sun have the same apparent width, almost exactly. Pure coincidence, of course; no astronomical necessity at all. In fact, eons past, the Moon was closer and appeared larger, and eons future, the Moon will be-Well, never mind. The fact is that the Earth is larger than the Moon, and from the Moon you see the Earth at the same distance that you see the Moon when you're standing on Earth. The Earth in the Moon's sky is therefore as much larger than the Moon in appearance as it is in actuality. Do you get that?"

  "No," said Gonzalo, flatly.

  Halsted looked annoyed. "Well, then, don't get it. Take my word for it. The Earth in the Moon's sky is about 3% as wide in appearance as the Moon is in Earth's sky. That means the Earth in the Moon's sky is also that much bigger than the Sun, because the Sun looks just the same from the Moon as from the Earth."

  "So what's the difference?" said Gonzalo. "If the Earth is bigger, it gets in the way of the Sun that much better."

  "No," said Halsted. "The whole point about the eclipse is that the Moon just fits over the Sun. It hides the bright circle of the gleaming Sun and allows its corona-that is, its upper atmosphere-to shine all about the hidden Sun. The corona gleams out in every direction with the light of the full Moon and does so in beautifully delicate curves and streamers.

  "On the other hand, if you get a large body like the Earth in front of the Sun, it covers up the shining sphere and the corona as well. You don't see anything."

  Avalon said, "That's assuming the Earth goes squarely in front of the Sun. When you see the eclipse before or after midpoint, at least part of the corona will stick beyond the Earth's sphere."

  Peterborough said, "Part isn't the whole. It wouldn't be the same thing."

  There was a short silence; and then Drake said, "I hope you don't mind if a fellow-chemist tries his hand at this, young man. I'm trying to picture the Earth in the sky, getting in the way of the Sun. And if we do that, then there's this to consider: the Earth has an atmosphere and the Moon has not.

  "When the Moon moves in front of the Sun, as viewed from the Earth, the Moon's surface is sharp against the Sun. When the Earth moves in front of the Sun, as viewed from the Moon, the Earth's boundary is fuzzy and the Sun shines through Earth's atmosphere. Does that make a difference that you can use in the story?"

  "Well," said Peterborough, "I've thought of that, actually. Even when the Sun is completely behind the Earth, its light is refracted through the Earth's atmosphere on every side, and a red-orange light penetrates it and reaches the Moon. It's as though the Moon can see a sunset all around the Earth. And that's not just theory. When there's a total eclipse of the Moon, you can usually see the Moon as a dull brick-red circle of light. It gleams in Earth's sunset atmosphere.

  ' 'As the eclipse, as viewed from the Moon, progresses, that side of the atmosphere that has just passed over the Sun is brighter, but grows gradually dimmer while the other side grows brighter. At eclipse mid-point, if you are viewing it from a part of the Moon which sees both Earth and Sun centered with respect to each other, the red-orange ring is evenly bright all the way around-assuming there isn't too much in the way of clouds in Earth's atmosphere at the time."

  Drake said, "Well, for God's sake, isn't that a sufficiently spectacular sight for Victim to photograph? The Earth would be a black hole in the sky, with a thin orange rim all around. It would be-''

  "No, sir," said Peterborough. "It isn't the same thing. It's too dull. It would be just a red-orange ring. Once the photograph is taken the first time, that would be it. It wouldn't be like the infinitely varying corona."

  Trumbull said, "Let me try! You want the corona visible all around, is that it, Milton?''

  "Yes, sir."

  "Stop me if I'm wrong, but in my reading, I've been given to understand that the sky is blue because light is scattered by the atmosphere. On the Moon, where there is no atmosphere, the sky is black. The stars, which on Earth are washed out by the scattered light of our blue sky, would not be washed out in the Moon's airless sky. They would be visible."

  "Yes, though I suspect the Sun's glare would make them hard to see."

  Trumbull said, "That's not important. All you would have to do is cut an opaque circle of metal and hold it up in the air at the proper distance from your photographic equipment in order to just block out the Sun's blazing disc. You can't do that on Earth, because even if you blocked out the Sun, the scattered light of the sky obscures the corona. On the Moon, there's no scattered light in the sky and the corona would shine out."

  Peterborough said, "In theory, that's possible. In fact, it can even be done on Earth on mountain tops, making use of a coronagraph. It still wouldn't be the real thing, though, for it's not just a matter of light scattered by the atmosphere. There's light scattered and reflected by the ground.

  ' 'The Lunar surface would be very brightly lit up and light would be coming in from every angle. The photographs you would take would not be good ones. You see, the reason the Moon does the good job it does here on Earth is that its shadow doesn't just fall on the telescope and camera. It falls on all the surrounding landscape. The shadow of the Moon can, under ideal conditions, be 160 miles wide and cover 21,000 square miles of the Earth's surface. Usually, it's considerably smaller than that; but generally it's enough to cover the immediate landscape-that is, if it happens to be a total eclipse."

  Trumbull said, "A bigger opaque object, then-"

  "It would have to be quite big and quite far away," said Peterborough, "to achieve the effect. That would be too cumbersome."

  Halsted said, "Wait, I think I have it. You would need something big for the purpose, all right. Suppose there were spherical space settlements in the Moon's orbit. If Victim is in a spaceship and gets the space settlement between himself and the Sun, that would be exactly what he wants. He could arrange to be close enough to have the shadow-which, of course, is conical and narrows to a point if you get far enough away-to be just thick enough to enclose his entire ship. There would be no world-surface to reflect light, and there you are."

  Peterborough said, uneasily, "I hadn't thought of that. It's possible."

  Halsted grinned, and a flush of pleasure mounted to the hairline he had once had. "That's it, then."

  Peterborough said, "I don't want to be troublesome, but- but if we introduce the space motif, it's going to create some problems in the rest of the story. It's sort of important that everything stay on or near the Earth and yet that there be something so startling and unexpected that it would-"

  He paused and Rubin completed the sentence for him, "So startling and unexpected that it would drive Murderer to rage and vengeance."

  "Yes."

  "Well," said Rubin, "since I'm the master of mystery here, I think I can work it out for you without leaving Earth very far behind, just as soon as I get some points straightened out.-You said that Murderer is taking the photographs from a plane. Why?"

  "Oh. That's because the Moon's shadow, when it falls on Earth, moves quickly-up to 1440 miles an hour or about 0.4 miles a second. If you're standing in one place on Earth, the longest possible duration of a total eclipse is seven minutes and then the shadow has moved beyond you. That's when the Earth is as deep into the Moon's shadow as it ever gets. When the Earth isn't as deep in and is nearer the final point of the shadow, the total eclipse may last only a couple of minutes, or even only a few seconds. In fact, more than half the time, the Moon's shadow during an eclipse doesn't reach the

  Earth's surface at all; and when the Moon is squarely in front of the Sun, the Sun overlaps it on all sides. That's an 'annular eclipse' and enough sunlight then slips past the Moon to wash out everything. An annular eclipse is no good at all."

  "But in the airplane?" prompted Rubin.

  "In an airplane, you can race along with the shadow and make the total eclipse last for an hour or more even if it would only endure a very short time on one position on Earth. You have a great deal more time to take photographs and make scientific observations. That's not science-fictional; it's done right now."

  "Can you take very good pictures from the plane?" asked Rubin. "Does it allow a steady enough basis for photography?"

  "In my story," said Peterborough, "I've got a computer guiding the plane, allowing for wind movements, and keeping it perfectly steady. That's one of the places where the science fiction comes in."

  "Still, the Moon's shadow eventually leaves the Earth's surface altogether, doesn't it?"

  "Yes, the eclipse track covers a fixed portion of the Earth's surface, and it has an overall starting point and an overall ending point."

  "Exactly," said Rubin. "Now Murderer is confident that his photographs taken from the stratosphere are going to include the best views of an eclipse ever seen, but he doesn't count on Victim's having a spaceship. Don't worry, there's no need to leave Earth very far. It's just that the spaceship follows the Moon's shadow after it leaves the Earth. Victim has a still longer chance to take photographs, a steadier base, and no atmospheric interference whatever. Murderer is hoist on his own petard for he sees that poor simp, Victim, do exactly what he does but go him one better. He snaps and becomes a killer."

  Gonzalo waved both arms in the air in excitement. "Wait! Wait! We can do even better than that. Listen, what about that annular eclipse you mentioned a while ago? You said the shadow doesn't reach the Earth."

  "It doesn't reach the surface. That's right."

  "How high off the surface is it?"

  "That depends. Under extreme conditions, the end point of the shadow could miss the Earth by hundreds of miles."

  "Yes," said Gonzalo, "but could that end point miss Earth by, say, ten miles?"

  "Oh, sure."

  "Would it still be annular, and no good?"

  "That's right," said Peterborough. "The Moon would come just barely short of covering the Sun. There would be just the thinnest sliver of Sun around the Moon, and that would give enough light to spoil things. If you took photographs, you'd miss the prominences, the flares, and the corona."

  "But what if you went ten miles up into the atmosphere?" said Gonzalo. "Then you'd see it total, wouldn't you?"

  "If you were in the right spot, yes."

  "There it is, then. One of those annular eclipses comes along, and Murderer thinks he'll pull a fast one. He gets into his stratoplane, goes ten miles up to get into the point of the shadow or just over it, and follows it along. He's going to make a total eclipse out of an annular one-and Victim, the usual loser, does the same thing, except he uses a spaceship and follows it out into space and gets better pictures. What can get old Murderer more torn up than having him play his ace-and getting trumped."

  Avalon nodded his head. "Good, Mario. That is an improvement."

  Rubin looked as if he had unexpectedly bitten into a lemon. "I hate to say it, Mario-"

  "You don't have to say it, Manny," said Gonzalo. "I see it all over you.-There you are, kid. Write the story."

  Peterborough said, with a sigh, "Yes, I suppose that is the best that can be done."

  "You don't sound overjoyed," said Gonzalo.

  "I was hoping for something more-uh-outrageous, but I don't think it exists. If none of you could think up anything-"

  "May I interrupt, sir?" said Henry.

  "Huh? Oh-no, I don't want any more coffee, waiter," said Peterborough, absently.

  "No, sir. I mean, concerning the eclipse."

  Trumbull said, "Henry's a member of the club, Martin. He broke the tie on the matter of the discussion. Remember?''

  Peterborough put a hand to his forehead. "Oh, sure. Ask away-uh-Henry.''

  "Actually, sir, would the photographs be that much better in a vacuum than in the thin air of the stratosphere? Would the difference in quality be enough to result in murder, unless Murderer was a close approach to a homicidal maniac?"

  "That's the thing," said Peterborough, nodding. "That's what bothers me. That's why I keep saying I need a motive. These differences in quality of photos aren't big enough."

  "Let us consider, then," said Henry, "Mr. Rubin's dictum that in telling a story one should look backward."

  "I know the ending," said Peterborough. "I have the backward look."

  "I mean it in another sense-that of deliberately looking in the other direction, the unaccustomed direction. In an eclipse, we always look at the Moon-just the Moon in a Lunar eclipse, and the Moon covering the Sun in a Solar eclipse-and that's what we take photographs of. What if we take a backward look at the Earth?"

  "What's to see on Earth, Henry?" asked Gonzalo.

  "When the Moon moves into the Earth's shadow, it is always in the full phase and it is usually completely darkened. What happens to the Earth when it moves into the Moon's shadow? It certainly doesn't darken completely."

  "No," said Peterborough emphatically. "The Moon's shadow is thinner and shorter than the Earth's, and the Earth itself is larger than the Moon. Even when Earth passes as deeply as it can into the Moon's shadow, only a tiny bit of the Earth is darkened, a little dot of darkness that makes up, at most, about 1/600 of the Earth's circle of light."

  ' 'Could you see it from the Moon?'' asked Henry.

  "If you knew where to look and especially if you had a good pair of binoculars. You would see it start small, move west to east across the face of the Earth, getting bigger, then smaller, and then vanish. Interesting, but certainly not spectacular."

  "Not from the Moon, sir," said Henry. "Now suppose we reverse the positions of the characters. It is Victim who has the airplane and who can get a photograph from the stratosphere. It is Murderer who intends to trump his opponent's ace by taking a better photograph from space-a marginally better photograph. Suppose, though, that Victim, against all expectations, from his airplane over-trumps Murderer in his spaceship."

  Avalon said, "How can he do that, Henry?"

  "Victim, in his plane, suddenly realizes he needn't look at the Moon. He looks backward at the ground and sees the Moon's shadow, racing toward him. The Moon's shadow is just a dark dot when seen from the Moon; it's just the coming of temporary night as seen from the Earth's surface-but from a plane in the stratosphere, it is a racing circle of darkness moving at 1440 miles an hour, swallowing up the land and sea-and clouds, for that matter-as it goes. The plane can move ahead of it, and it is no longer necessary to take single snapshots. A movie camera can produce the most dramatic film. In this way, Murderer, having fully expected to outdo Victim, finds that Victim has captured world attention even though he had only an airplane to Murderer's spaceship."

  Gonzalo broke into loud applause, and Trumbull said, "Right on!" Even Rubin smiled and nodded.

  As for Peterborough, he fired up at once saying, "Sure! And the approaching shadow would have a thin red rim, for at the moment the shadow overtakes you, the red prominences cast their light unmasked by the Sun's white light. That's it, Henry! The backward look does it!-If I write this one properly, I don't care even if it doesn't sell.-I won't care even" (his voice shook) "if-uh-she doesn't like it and doesn't go out with me. The story is more important!"

  Henry smiled gently and said, "I'm glad to hear that, sir. A writer should always have a proper sense of priorities."

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  Isaac Asimov, The Backward Look

 

 

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