The Ptolemaic System
 
The main theory of the Solar System left by the Greeks to post-Roman Europe was a geocentric one. It was given in Ptolemy’s Almagest and so bears his name. Its acceptance by astronomers lasted about 15 centuries.
The Ptolemaic theory sought to describe the apparent movement of all the heavenly bodies and indeed predict their future positions. It did so successfully; all the apparent motions of the Sun, Moon, planets and stars were adequately accounted for. In figure 1 the main features of the Ptolemaic System are sketched.
The Earth was the fixed centre of the Universe. The stars were fixed to the surface of a transparent sphere which rotated westwards in a period of one sidereal day.
The Sun and the Moon revolved about the Earth.
The large circles centred on the Earth were called deferents and the small circles centred on the large circles or on the line joining Earth and Sun were called epicycles. The planets moved in the epicyclic orbits whose centres themselves moved in the directions shown. Because Mercury and Venus were never seen far from the Sun (they were always evening or morning objects), the centres of their epicycles were fixed on the line joining Sun to Earth. To agree with observation, the radii joining Mars, Jupiter and Saturn to the centres of their epicycles were always parallel to the Earth–Sun line.
It is worth noting at this point that errors in the descriptions and diagrams of Ptolemy’s System by some modern commentators reveal an unjustified contempt for a theory that was remarkably successful in accounting for the known phenomena of the celestial sphere. This basic theory was indeed a good first approximation to the observed behaviour of the heavenly bodies. Further modifications improved the ‘fit’. For example, to account for some of the observed irregularities in the planets’ motions, it was supposed that the deferents and epicycles had centres slightly displaced from the Earth’s centre and the deferent circles respectively. Slight tilts were given to some of the circles.

Figure 1. The Ptolemaic System.
When the Arabian astronomers of the Middle Ages accumulated more accurate observations of the planets, they found that the Ptolemaic theory had to be modified still further, epicycles being added to the epicycles until the system became cumbersome and, to some at least, unconvincing.