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The standard-bearer for the Earth-centered, or geocentric, model of the universe was Ptolemy, the last of the great ancient Greek astronomers who lived in Alexandria (now in Egypt) in the 2nd century A.D. Ptolemy’s system was able to predict the motions of the seven known “planets” quite accurately. (To the Greeks, planets were any objects that moved relative to the background stars, so they included Mercury, Venus, Mars, Jupiter, Saturn, the Sun, and the Moon; Uranus, Neptune, and Pluto had not yet been discovered.)
That was no small feat. If you watch one of the planets, say Mars, over the course of a year or two, you’ll notice that it moves in a rather complicated way against the background of fixed stars. Most of the time, it travels from west to east at a fairly steady speed. But at approximately two-year intervals, Mars slows down, appears to stop, and then moves from east to west for a couple of months before resuming its normal motion. Today we know this backward motion occurs because Earth, traveling in a smaller orbit around the Sun, periodically passes Mars like a faster race car on an inside track. Yet Ptolemy lived at a time when the heavens were considered perfect. He thought all heavenly bodies orbited the Earth on the most perfect path imaginable, a circle, and did so at constant speeds. That he was able to fashion a complex system of circles within circles to recreate the heavens stands as an extraordinary accomplishment.
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