cular orbit about the Earth. 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. However many questions arose about this theory. 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 embarrassing. With as many as 200 and more epicycles wheeling about, the whole system was looking less and less divine. The invention of accurate timekeeping devices was, by the 15th century, badly fraying the fabric of the Ptolemaic universe. (Shapin)
             The first bold step in the Scientific Revolution was taken by Nicolaus Copernicus (1473-1543). In De Revolutionibus Orbium Coelestium, published in the year of his death, Copernicus suggested a new explanation of the apparent motions of heavenly bodies. Following the hypothesis of Aristarchus, Copernicus put the sun in the center of the motionless sphere of the fixed stars and had the planets (including the earth) move in concentric circles around it. The moon circled the earth, which rotated around its own axis and also slowly changed the direction of its axis. The heliocentric system of Copernicus challenged (and eventually replaced) the Ptolemaic system that had stationary earth as its center. The heliocentric theory gave modern astronomy a new direction but it did not remove the complexity that cumbered the Ptolemaic system. To reconcile the circular and uniform planetary motion wi...