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.