Athene's Prophecy (Gaius Claudius Scaevola Trilogy) (24 page)

"Then as I understand it," Gaius started slowly but clearly, and while he appeared to be looking into empty space, his eyes were fixed on Timothy as he watched for reflexes, to let him know where Timothy thought his strength was. "The Earth is a very large sphere and along with the planets, which are of comparable size, it travels around the sun. Jupiter and Saturn, being so bright and so distant will be much bigger. The Moon travels around the Earth, and it is a sphere of about a third the size of the Earth. That would give it a diameter of about 4,500 kilometers, and from geometry it is somewhere between a hundred and thirty thousand to half a million kilometres away. From Aristarchus' calculation the sun is approximately twenty times bigger and twenty times further away. I believe these distances are only approximate and . . ."

"The actual distances are irrelevant," Timothy interrupted. "The problem is one of logic, so assume what you wish."

"Then the sun, being so much more massive, is at the centre. Since the remaining planets, which reflect sunlight, are seen as points they must be much further away than the moon. Now, Mercury and Venus occur both in the morning and evening, but never get far from the sun. That suggests they're closer to the sun than the Earth, and since Mercury always stays closer to the sun, presumably the order out is Mercury, Venus, Earth. To be far enough away, Aristarchus' calculation may be too small. The angle is difficult to measure and . . ."

"As I said, for the purposes of debate, if you need the sun to be further away, assume it," Timothy nodded.

"First, the Moon goes around the Earth, so some of the time gets in between the Earth and the Sun, and we get an eclipse. The Moon takes a month to go around the Earth, so every month it has the same phase. This explains the phases . . ."

"I concede the Moon goes around the Earth," Timothy interrupted. "My position is, so does everything else. Your arguments on the phases of the Moon will be my arguments."

"Then the planets go around the sun, and simple geometry shows why those closer to the sun can appear either in the morning sky or the evening sky, but not the midnight sky."

"That," Timothy interrupted again, "is probably the strongest point in favour of Aristarchus. However, it has been explained."

"In a very awkward way," Gaius suggested.

"Perhaps," Timothy shrugged.

"Then look at this," Gaius continued. "The outer planets wander across the heavens in a general eastward direction, except very occasionally they seem to stop, then go westwards for a while, then stop and resume going eastwards. This is exactly what Aristarchus predicts, because the reversal occurs when the Earth's motion catches up with the others."

"Nevertheless, that has also been explained," Timothy continued.

"In an even more awkward way," Gaius suggested, "and there's another point. According to Aristarchus the reversal occurs when the planet is at its brightest, and is in the midnight sky. It does, and that requires the epicycles to only occur where the planet has wandered onto a Sun – Earth – planet line. Why do the epicycles only occur there, and where it is almost random against the background?"

"I don't know," Timothy admitted, "but that doesn't make the epicycle model wrong."

"Perhaps, but it certainly doesn't make it right," Gaius emphasized. "Also, there are no planetary eclipses, where the planet falls in the Earth's shadow. Again, the spheres model effectively has to say there are a large number of spheres, but only one is really close. The question is, why is that one special?"

"The same goes for Aristarchus," Timothy protested, too quickly.

"Not at all!" Gaius quickly countered. "Aristarchus says Earth is a planet, and it goes around the sun just like the others, which means that the planets must be a long way away from each other. The Moon falls around the Earth so it has to be much closer. If another planet was, say, two times as far away, the Moon could switch planets, however the planets have no effect on the Moon, nor on our tides. If the other planets are far enough away, the Earth seems to be too small and cannot shade the sun, just as Venus and Mercury are too small to shade the Earth. We know they are too small because we can see them, and see the angle they subtend is far smaller than that of the sun. But for the disc model, there is only one type of disc, the planets have to be on discs a very long way away, and that means there have to be a number of these coincidences with no cause, despite the fact the planets all wander at different speeds."

"You're assuming the planets are very large," Timothy warned. "Just because Aristarchus says so doesn't make it so. They could be small and close."

"In which case they should be eclipsed!" Gaius countered.

"I see," Timothy mused. "They are really good points. In fact, Gaius, you've excelled so far, and if it wasn't for the fact that the Aristarchus model is physically wrong, your arguments on planetary motion would be reasonably convincing."

"You keep saying it's wrong. How?"

"We'll come to that," Timothy assured him. "First, everything goes around the Earth once a day. I assume you will concede that?"

"Everything else more or less stays put," Gaius interposed. "Day/night is explained by the sun being at the centre, and the Earth rotating."

"So let me get this right," Timothy said, a little pedantically, Gaius thought, until he realized that Timothy was overdoing the importance of this. It meant that this would not be his main line of approach. "You are saying that while everything seems to be going around the Earth, it is not, and it is the Earth that is rolling."

"I believe that explains everything quite well," Gaius shrugged. "As Aristotle said, when anything moves, it moves with respect to something else."

"Which is why the Earth has to be immovable," Timothy protested.

"The sun would work just as well," Gaius smiled. "Look at it this way. According to you, all the stars rotate around the Earth at a terrifying velocity?"

"Around an immovable Earth," Timothy confirmed.

"So, supposing something removed the Earth?" Gaius smiled. "What happens? Does everything else just stop? And if so, how does whatever is driving the rotating spheres know when to stop?"

"What?" Timothy gasped. "You can't just remove the Earth."

"You can in the abstract," Gaius said, "and in any case, the problem is merely one of size. The quality is conceptually different from the quantity, as Aristotle noted."

"That may be," Timothy said, as he struggled to recover, "but the fact remains, the Earth is quite different in quality from the rest of the universe."

"You don't know that," Gaius smiled.

"Oh, yes I do!" Timothy responded. "The Earth undergoes continual change, but the other bodies have remained constant for as long as we have been observing them."

"They could be too far away," Gaius replied. "If a body as big as the Earth is reduced to the size of a pin head, you wouldn't see such changes."

"Suppose you see a large storm," Timothy replied. "You can measure the wind speed, and estimate the size of the clouds from the time it take them to pass. If there were clouds that big on the Moon, then we would be able to see them. We would also see them on the edges, but the edges of the Moon are very sharp."

Gaius thought about this for a moment. While this was a point he had never considered, for some reason it should support his position, but he could not for the life of him think how. To gain time he decided to get the conversation back to where it should be, so he said, "We are being distracted. My point is, and I repeat it, that the Moon seems to go slightly slower is because it really does go around, the Earth." He paused, then said in a challenging way, "It explains everything, and there is nothing contradicting it. Therefore, as the
great
Aristotle would say, it must be true."

"Then let's have some contrary evidence! If the surface of the Earth is moving, there should be a contrary wind," Timothy said firmly, "and if the Earth goes around the sun, there should be a steady wind for that too, different between night and day as the surface is either going in the same direction, or the opposite direction, to its path around the sun."

"Not if the contraries come from the medium the motion is in," Gaius intervened in a triumphant tone.

"Oh?" Timothy was puzzled.

"The motion around the sun is eternal," Gaius smiled, "therefore there is no contrary. If the motion receives its contraries from the medium, and the motion is eternal, then there is no medium. The motion is in a void."

"The spinning Earth is in air," Timothy frowned.

"I have here a bucket of water and I'll put it on this potter's wheel and spin it," Gaius offered. He had been ready for this argument, and had brought the potter's wheel and put it in the corner of the room. "Watch! The water quickly catches up with the bucket."

"So?"

"Aristotle made the excellent point that everything, no matter how light, falls to the centre. So does air, but because of Archimedes' principle, it's on top. Outside that, there is void, and the bodies and air move with eternal motion. There's no contrary wind because there is nothing on the outside and the air has caught up with the Earth, just as the water does in that bowl. Furthermore," he added, and wagged a finger at Timothy, "we can prove there's nothing out there."

"We can?" Timothy gave an even more perplexed look.

"Yes! Watch stars come up from the horizon. They really shimmer, and that must be because of the air. Now, as the ancients noted, stars go behind the Moon quite sharply. The reason there're no clouds on the Moon is simply because there's no air there. The Moon is not in air, so its motion is eternal!"

Timothy gave Gaius a look that seemed as if it was true respect, then he nodded and said, "That was good logic. I concede that if there is a void, the argument of the winds is not valid, and there's no way to prove there is air around the Moon." He paused, then added, "How about this?

"Let's suppose for a moment the Earth travels around the sun. It must therefore move with two motions. Think of going around a table in a circle. You go left to right and front to back"

"So?"

"Think of people scattered around a field, and you walk a circle. Sometimes, someone will seem to be to the left of someone else, then as you keep walking they may appear to pass in front and end up on the right. That does not happen with the stars. No matter what, they are always in the same position relative to each other."

"If you're far enough away, and the stars are far enough apart, you wouldn't notice," Gaius said. He had seen this argument, and was prepared. "If Democritus is correct, and the stars are other suns, to get that dim they have to be very far away." He paused, then continued, "Added to which, something like that does happen to the paths of the planets."

"First, the planets," Timothy nodded. "They are on separate spheres, which travel around the Earth at different speeds and at times meet epicycles, so yes, your argument about the planets is correct, but in different forms it is the same for both explanations."

"Except for one point," Gaius frowned.

"Which is?"

"If the planets are on different spheres, one inside the other, how come you can see through them to the outer spheres? Why is nothing shaded?"

"The spheres are made of material you can see through," Timothy shrugged.

"There's nothing like that on Earth," Gaius shrugged, "which is clear enough and strong enough."

"Firstly, there's glass, and secondly there's no reason why the material of the heavens can't be different from on Earth."

"Firstly, as you put it," Gaius smiled, "there's good reason to believe the material of the heavens are the same as those of Earth. Bits of meteors have been found, and they comprise stone and iron. In your model, these fall to the centre, and would break anything like glass, or would never reach the Earth."

"They could fall through holes in the lowest sphere," Timothy pointed out.

"That's little better than resorting to magic," Gaius countered, "and there's no reason to resort to that when there's the perfectly simple explanation of everything falling to the centre, in this case the sun."

"But the Moon falls around the Earth?"

"While both of them are falling around the Sun," Gaius replied. "Smaller falls around bigger. And there's more. Let's suppose the Sun moves around the Earth on your spheres, and the Moon does also. There is now the issue of eclipses."

"I thought we'd dealt with them," Timothy frowned. "We agreed that eclipses happen when the Moon, being closer, crosses the Earth – Sun line, with a solar eclipse when the Moon is on the sun side and . . ."

"We agree the cause of eclipses," Gaius interposed. "It's just that this can't happen with your spheres."

"And why not?" Timothy said in a puzzled tone. "This is usually thought to be the stronger point of the argument about the spheres."

"Because if the Moon falls around the Earth, and does so to also pull on the tides, it must move with two motions."

"So?" Timothy opened is hands as if he could not understand.

"Once the bodies get back into the same positions, the same thing should happen, but it doesn't. Eclipses are rather rare," Gaius pointed out. "No frequent repeating cycle means your moon has to move with three motions, one of which has to be like a pendulum. There's no physical cause for that, but on the other hand, if the Moon goes around the Earth on a plane different from that on which the Earth goes around the Sun, then eclipses occur when the three bodies are fortuitously in line."

"Interesting," Timothy shrugged, then said, "Let's deal with this spinning Earth. For the Earth to spin, it needs a continual applied force and . . ."

"That assumes there is a contrary to overcome," Gaius replied. "If it is spinning in a void, nothing will slow it down. It would be like a top, going on forever."

"Then if it were spinning," Timothy continued, "there would be a natural tendency for light things like leaves not to be able to keep up . . ."

"If the air keeps up by friction, so do they," Gaius countered.