The Ptolemaic System:

The theoretical n'mdel of the universe given by the Greeks had a stationary Earth at its centre, around which the Sun, the Moon and the planets moved in circular orbits.  I,n this model of  the universe, stars merely acted as a bedrop, much iike a painted screen  hung by a photographer at a village fair! But, doesn't this seem  to be the most natural idea in  the world? The Earth seems steady, solid, unmoving, while we can see the heavenly bodies rising and setting each day. 

Most  of  the models constructed by  the Greeks to explain the movement of planets consisted of perfect concentric spheres or circles. They held that each planet was attached to an invisible sphere or a circle that rotated around the Earth at a different speed from  .the rest of the spheres. You may recall Eudoxus' model of  27 spheres, shown partly. The astronomical ideas of many earlier Greeks were gathered by Ptolemy who published  them in his Almagest. This series of thirteen volumes contained the ideas of such men as Aristotle, Apollonius, Hipparchus,  in addition to  his own ideas. This combined picture of  the r~niverse  is called  the Ptolemaic system. There were some exceptions to this model. Notable among these was the argument of Aristarchus of Samos, that the Earth was one of the several planets, which like them orbits the Sun which was at the centre of  the universe. He also argued  that  the Sun was much bigger  than  the-Earth and stars were enormously far away. However, we do not know how he  reached these conclusions, each of which is correct.