Wednesday, February 13, 2008

Aristotle Lays Down the Law

Fortunately for the hard-pressed astronomers of yore, total eclipses of the sun are relatively rare events. More immediately, they had nightly occurrences they couldn’t explain.

They could watch the sky all night and see the stars glide predictably across the sky, as if affixed to a moving celestial sphere. Likewise, the moon made its traversal with perfect regularity. But there were (so far as the ancients could see) five heavenly bodies that didn’t behave with this regularity. Mercury, Venus, Mars, Jupiter, and Saturn looked pretty much like stars, but, in contrast to stars, they wandered across the sky and so were called by the Greeks planetes, or “wanderers.” While the planets always remain near the ecliptic, they have strange motions. Relative to the stars, planets seem to slow down and speed up, moving (from an earthbound observer’s perspective) generally eastward (prograde motion), but sometimes westward (retrograde motion) with respect to the background stars.

How did the ancients explain this strange observation? The great Greek philosopher Aristotle (384–322 B.C.E.), tutor of Alexander the Great, formulated a picture of the solar system that put the earth at its center with all the other heavenly bodies orbiting around it. The orbits were described as perfect circles. Now, Aristotle was a smart man. He provided observational evidence that the earth was spherical rather than flat. Moreover, his descriptions of the orbits of the moon and the sun accorded very well with what people actually observed. As far as the stars went, the celestial sphere idea explained them well enough. The problem was the planets. Aristotle’s simple geocentric (earth-centered) model did not explain why the planets varied in brightness, why they moved at varying speeds, and why their motion was sometimes prograde and sometimes retrograde.

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