Ptolemy’s theory of the solar system placed the Earth at the center with each planet on an orbit around it and the stars on a celestial sphere. Ptolemy believed that astronomical elements existed in circular, rotating motions.
Ptolemy’s solar system was geocentric, meaning the Earth was located on a fixed point at the center of the universe. If the Earth was stationary, then the other planets were on an epicenter. The epicenter allowed the various planets to rotate at a different speed. Each planet had its own epicenter along the deferent, the single circle around the Earth that all celestial objects followed. The stars were at a fixed point and further away than the planets. These stars numbered to 1,022 in the solar system. The order of the solar system started with the Earth at the center followed by Mercury, Venus, the Sun, Mars, Jupiter, Saturn and then the stars.
Ptolemy published his theories on the solar system along with all known astronomical knowledge in a book entitled “The Mathematical Collection.” This became the most popular and extensively researched book on astronomy for that era. Modern astronomers still use the idea of circular spherical motions, especially in designing equipment for planetariums.
