<P> To summarize, Ptolemy devised a system that was compatible with Aristotelian philosophy and managed to track actual observations and predict future movement mostly to within the limits of the next 1000 years of observations . The observed motions and his mechanisms for explaining them include: </P> <Table> The Ptolemaic System <Tr> <Th> Object (s) </Th> <Th> Observation </Th> <Th> Modeling mechanism </Th> </Tr> <Tr> <Td> Stars </Td> <Td> Motion of entire sky E to W in ~ 24 hrs ("first motion") </Td> <Td> Stars: Daily motion E to W of sphere of stars, carrying all other spheres with it; normally ignored; other spheres have additional motions </Td> </Tr> <Tr> <Td> Sun </Td> <Td> Motion yearly W to E along ecliptic </Td> <Td> Motion of Sun's sphere W to E in year </Td> </Tr> <Tr> <Td> Sun </Td> <Td> Non-uniform rate along ecliptic (uneven seasons) </Td> <Td> Eccentric orbit (Sun's deferent center off Earth) </Td> </Tr> <Tr> <Td> Moon </Td> <Td> Monthly motion W to E compared to stars </Td> <Td> Monthly W to E motion of Moon's sphere </Td> </Tr> <Tr> <Td> The 5 planets </Td> <Td> General motion W to E through zodiac </Td> <Td> Motion of deferents W to E; period set by observation of planet going around the ecliptic </Td> </Tr> <Tr> <Td> Planets </Td> <Td> Retrograde motion </Td> <Td> Motion of epicycle in same direction as deferent . Period of epicycle is time between retrograde motions (synodic period). </Td> </Tr> <Tr> <Td> Planets </Td> <Td> Variations in speed through the zodiac </Td> <Td> Eccentric per planet </Td> </Tr> <Tr> <Td> Planets </Td> <Td> Variations in retrograde timing </Td> <Td> Equants per planet (Copernicus used a pair of epicycles instead) </Td> </Tr> <Tr> <Td> Planets </Td> <Td> Size of deferents, epicycles </Td> <Td> Only ratio between radius of deferent and associated epicycle determined; absolute distances not determined in theory </Td> </Tr> <Tr> <Td> Interior planets </Td> <Td> Average greatest elongations of 23 ° (Mercury) and 46 ° (Venus) </Td> <Td> Size of epicycles set by these angles, proportional to distances </Td> </Tr> <Tr> <Td> Interior planets </Td> <Td> Limited to movement near the Sun </Td> <Td> Center their deferent centers along the Sun--Earth line </Td> </Tr> <Tr> <Td> Exterior planets </Td> <Td> Retrograde only at opposition, when brightest </Td> <Td> Radii of epicycles aligned to Sun--Earth line </Td> </Tr> </Table> <Tr> <Th> Object (s) </Th> <Th> Observation </Th> <Th> Modeling mechanism </Th> </Tr> <Tr> <Td> Stars </Td> <Td> Motion of entire sky E to W in ~ 24 hrs ("first motion") </Td> <Td> Stars: Daily motion E to W of sphere of stars, carrying all other spheres with it; normally ignored; other spheres have additional motions </Td> </Tr>

An earth-centered system of planets is known as a(n) model