Geocentric Theory
The geocentric theory of the universe describes the universe with Earth at its center and puts the other celestial bodies in circular orbits around it. The usual ordering placed the Moon, Mercury, Venus, the Sun, Mars, Jupiter, Saturn, and the fixed stars on a series of concentric spheres further and further away from the stationary Earth.
The heavens revolved around Earth once every 24 hours. While an outer sphere carried the Sun around Earth every day, it rotated on an inner sphere about an axis attached to the outer one. This second effect accounted for the Sun's yearly transit along the ecliptic, a plane held at a 23.5° angle with respect to the celestial equator (taking the North Star as the North Pole). Because the Sun's tilted orbit placed it at different angles relative to the equator at various times of the year, this model provided a natural explanation for the origin of seasons. Three more spheres are required to make the Sun's motion consistent with the dates on which the solstices and equinoxes fall. The motion of the Moon and the planets are treated in a similar fashion, and there were 27 nested spheres in all. All the celestial bodies traveled around Earth with uniform velocity, and the spheres were all transparent since the stars could be seen on the outermost one. This geocentric model, although shaped by the philosophical precepts of Plato, was largely designed by Eudoxus of Cnidus (c.400-353 B.C.).
Aristotle, another pupil of Plato, devised his own geocentric theory with 55 crystalline spheres. (The additional spheres were necessary to refine Eudoxus's model so that it better matched the observational data.) He also introduced an additional sphere belonging to the Prime Mover, which dwelt beyond the sphere of the fixed stars. The rotation of this sphere imparted motion to the other spheres.
Centuries later, Ptolemy dispensed with the crystalline spheres of Aristotle and introduced such modifications as epicycles and deferents to explain the retrograde motion of the outer planets. His classic compendium of astronomy, Almagest (The Greatest), served as the authoritative textbook on celestial mechanics for over a thousand years. Though there were in fact several geocentric theories, when historians or philosophers write about geocentrism, it is usually Ptolemy's system they infer.
Nicolas Copernicus supplanted the geocentric theory with a heliocentric model, which was much like one proposed by Aristarchus of Samos (310-c.230 B.C.). This new model, coupled with the elliptic orbits postulated by Johannes Kepler, explained the observations of the night sky without resorting to more and more ad hoc hypotheses.
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