Aristotle and Ptolemy

But most Greek scientists and philosophers, including the influential Aristotle and Plato, held other beliefs. They thought the Earth was at the center of everything, with the sun, stars, and planets circling about it affixed to transparent celestial spheres. As these vast spheres rotated, the heavenly bodies attached to them traced out circular orbits around the Earth.

According to Aristotle, who summed up the astronomical thinking of his day, the outermost sphere contained all the stars. But neither he nor any of the other ancient Greeks had any idea how huge the universe is. They estimated that the stars were a few thousand, or perhaps a few million, miles away. The nature of the stars was something they did not try to explain.

In the A.D. 100's, the basic scheme outlined by Aristotle was taken even further by the most famous of the early astronomers, Ptolemy, a Greek who lived in Alexandria, Egypt. Because the invisible celestial spheres helped explain the movement of heavenly bodies, Ptolemy retained them in his view of the universe, though he rejected the idea that they were actual, solid objects. But despite that simplification, Ptolemy's system added new complications to account for the motions of the planets across the sky.

Although it was cumbersome, the Ptolemaic system accounted quite well for the observed motions of the heavenly bodies, so it went unchallenged for nearly 1,500 years. But by the late Middle Ages, it had become evident to careful observers of the skies that Ptolemy's universe did not provide a truly accurate description of celestial phenomena. The time was ripe for a new view of the cosmos.