Back in the time of the Greeks, Aristotle and Ptolemy were the leading thinkers of their time. Ptolemy himself was alive in the second Century A.D., in Alexandria Egypt. He wrote a book at this time called "The Mathematical Collection," which later came to be called the Almagest. In his book he described how he believed the Universe worked and the vision of the Universe that he put forward was one that would be adopted for many centuries, until technological improvements proved his theories wrong. .
The three basic theories that ancient astronomers believed to be true are as follows:.
All motion in the heavens is uniform circular motion. .
The objects in the heavens are made from perfect material, and cannot change their intrinsic properties (their brightness). .
The Earth is at the center of the Universe. .
Though nowadays we know these to be false, at the time these theories were never even questioned. As I said above, they believed that planetary motion was in the form of perfect circles (diagram 4) and by adjusting the velocity of these concentric spheres most of the planetary motion could be explained. However, there was still the problem of varying brightness in stars and apparent retrograde motion. To solve this problem he came up with the idea that the planets weren't necessarily attached to the concentric circles themselves, but they followed a circular path about the circle (see diagram 5). These circles were called Epicycles and the concentric spheres to which they were attached were termed the Deferents. In Ptolemaic astronomy all stars move along spheres. These spheres revolve about the earth, but the earth is not always precisely at their center. Beyond the spheres of the planets, all the stars are located within a single sphere and these stars have fixed places in the sphere. This belief was also widely accepted by the church, because it left a lot of room for heaven and hell out side the sphere which the stars lied on.