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Ancient Greek geocentric cosmological model
The cosmological model of concentric (or homocentric) spheres, developed by Eudoxus, Callippus, and Aristotle, employed celestial spheres all centered on the Earth. In this respect, it differed from the epicyclic and eccentric models with multiple centers, which were used by Ptolemy and other mathematical astronomers until the time of Copernicus.

Eudoxus of Cnidus was the first astronomer to develop the concept of concentric spheres. He was originally a student at Plato's academy and is believed to have been influenced by the cosmological speculations of Plato and Pythagoras. He came up with the idea of homocentric spheres in order to explain the perceived inconsistent motions of the planets and to develop a uniform model for accurately calculating the movement of celestial objects. Many historians of science, such as Michael J. Crowe, have argued that Eudoxus did not consider his system of concentric spheres to be a real representation of the universe but thought it was merely a mathematical model for calculating planetary motion.

Callippus, a contemporary of Eudoxus, attempted to improve his system by increasing the total number of homocentric spheres. He added two additional spheres for the sun and the moon as well as one additional sphere for Mars, Mercury, and Venus. These additional spheres were supposed to fix some of the calculation problems in Eudoxus’ original system. Callippus’ system was able to better predict the motions of certain celestial objects but his system still had many problems and was not able to account for many astronomical observations. Aristotle developed his own system of concentric spheres in Metaphysics and De Caelo (On the Heavens). He thought that both Eudoxus and Callippus had too few spheres within their models and added more spheres onto Callippus’ system. He added three spheres to Jupiter and Mars as well as four spheres to Venus, Mercury, the sun, and the moon for a total of fifty five spheres. He later doubted the accuracy of his results and stated that he believed there were either forty seven or forty nine concentric spheres. Historians are unsure about how many spheres Aristotle thought there were in the cosmos with theories ranging from 43 to 55. Unlike Eudoxus, Aristotle believed that his system represented an actual model of the cosmos.