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."