Introduction
Since at least the time of Hipparchus and Eudoxus the ancient world believed that the Sun, moon, planets, and stars moved around the earth in circular orbits. Aristotle’s Physics put the heavenly bodies on perfect crystal spheres. This theory was further formalized in the second century by Ptolemy in his Almagest which served the basic needs of astronomers for the next 1,500 years. Over that time, Aristotelian physics became an article of faith not to be questioned.
But there were unexplained irregularities that never quite fit the theory. The ancients knew that the seasons were of different lengths. For example, Winter is about 89 days in length while Summer is about 94 days. Why did the Sun sometimes speed up and slow down like that? They also noticed that Mars would move East across the night sky for a few years, slow down, and then reverse course and move West for a few months before looping back to its original course. (What we now call apparent retrograde motion.)