“Pure assumption, unproved.”
“But astronomers make predictions, eclipses and such, based on that assumption — and they work out.”
“You’ve got it backwards. The Chinese were predicting eclipses long before the theory of the invariability of natural law was popular. Anyhow, at the best, we notice certain limited similarities between events in the sky and events on earth. Which has nothing to do with the question of a back side of the moon which we’ve never seen and may not be there.”
“But we’ve seen a lot of it,” Morrie pointed out.
“I get you,” Cargraves agreed. “Between librations and such — the eccentricity of the moon’s orbit and its tilt, we get to peek a little way around the edges from time to time and see about 6o per cent of its surface — if the surface is globular. But I’m talking about that missing 40 per cent that we’ve never seen.”
“Oh,” said Ross, “you mean the side we can’t see might just be sliced off, like an apple with a piece out of it. Well, you may be right, but I’ll bet you six chocolate malts, payable when we get back, that you’re all wet.”
“Nope,” Cargraves answered, “this is a scientific discussion and betting is inappropriate. Besides, I might lose. But I did not mean anything of the slice-out-of-an-apple sort. I meant just what I said: no back side at all. The possibility that when we swing around the moon to look at the other side, we won’t find anything at all, nothing, just empty space-that when we try to look at the moon from behind it, there won’t be any moon to be seen — not from that position. I’m not asserting that that is what we will find; I’m asking you to prove that we will find anything.”
“Wait a minute,” Morrie put in, as Art glanced wildly at the moon as if to assure himself that it was still there — it was! “You mentioned something of that sort on earth — a thing with no back. What was it? I’m from Missouri.”
“A rainbow. You can see it from just one side, the side that faces the sun. The other side does not exist.”
“But you can’t get behind it.”
“Then try it with a garden spray some sunny day. Walk around it. When you get behind it, it ain’t there.”
“Yes, but Doc,” Ross objected, “you’re just quibbling. The cases aren’t parallel. A rainbow is just light waves; the moon is something substantial.”
“That’s what I’m trying to get you to prove, and you haven’t proved it yet. How do you know the moon is substantial? All you have ever seen of it is just light waves, as with the rainbow.”
Ross thought about this. “Okay, I guess I see what you’re getting at. But we do know that the moon is substantial; they bounced radar off it, as far back as `46.”
“Just light waves again, Ross. Infra-red light, or ultra-shortwave radio, but the same spectrum. Come again.”
“Yes, but they bounced.”
“You are drawing an analogy from earth conditions again. I repeat, we know nothing of moon conditions except through the insubstantial waves of the electromagnetic spectrum.”
“How about tides?”
“Tides exist, certainly. We have seen them, wet our feet in them. But that proves nothing about the moon. The theory that the moon causes the tides is a sheer convenience, pure theory. We change theories as often as we change our underwear. Next year it may be simpler to assume that the tides cause the moon. Got any other ideas?”
Ross took a deep breath. “You’re trying to beat me down with words. All right, so I haven’t seen the other side of the moon. So I’ve never felt the moon, or taken a bite out of it. By the way, you can hang on to the theory that the moon is made of green cheese with that line of argument.”
“Not quite,” said Cargraves. “There is some data on that, for what it’s worth. An astronomer fellow made a spectrograph of green cheese and compared it with a spetcrograph of the moon. No resemblance.”
Art chortled. “He didn’t, really?”