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The Scientific Revolution

The Problem of the Planets

The night sky as it appears to the naked eye is a generally orderly place. The stars form patterns, move through the heavens in lockstep, and even the complex movement of the sun and moon can be charted. The planets, however, presented a problem to ancient observers.

Five stars moved about in odd motions that simply did not fit well with the movement of everything else -- the Greeks called them "wandering stars"—and one of the great challenges of ancient astronomy was accounting for their movements.

The fellow who did the best job was a Greek working in second- century Alexandria, named Ptolemy. His system placed the earth at the center of the universe and it did a pretty good job of accounting for the movements of the lights up in the sky.

Ptolemy's model of the universe was accepted throughout the Middle Ages, though not without revision. His model was a little ragged at the edges and more accurate observations revealed discrepancies, particularly in regard to the movement of the planets. Specifically, using tables based on Ptolemy's model, medieval astronomers made predictions regarding the position of this or that planet and the planets did not show up on time.

Even Ptolemy had known that the simplest model, which had each planet moving in a circular orbit about the Earth, led to gross inaccuraces. To compensate, he invented the notion of epicycles; that is, a circular orbit whose center in turn moved in a circular orbit.

For example, Venus did not move directly around the Earth, but rather moved in its own orbit. The center of this orbit, however, did move around the Earth. Everything moved in perfect circles, of course, because a circle was a perfect shape and Heaven was a place of perfection.

By the later Middle Ages, increasingly accurate observations had led to increasing elaborations of Ptolemy's systems. Epicycles were added to epicycles until the planets were clanking about in a ludicrous contraption of scores of intersecting circles.

Many among the learned were uncomfortably aware that the situation was downright embarassing. With as many as 200 and more epicycles wheeling about, the whole system was looking less and less divine. And, to make matters worse, the planets still were not showing up on time. The invention of accurate timekeeping devices were, by the 15th century, badly fraying the fabric of the Ptolemaic universe.

One easy way out, long known in the West, was to make the sun rather than the Eart the center of the universe. Such a solution brought its own train of problems, however. For one thing, the mathematical calculations required to create the tables were much more complex. For another, certain passages in the Bible seemed to imply clearly that the sun went around the Earth; the Church had been teaching this as dogma for some time, and a heliocentric theory would have to fight a theological battle as well as a scientific one.