Concentric spheres

[1][2] 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.

[3][4] 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.

[5] 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.

[6] Callippus, a contemporary of Eudoxus, attempted to improve his system by increasing the total number of homocentric spheres.

Unlike Eudoxus, Aristotle believed that his system represented an actual model of the cosmos.

Animation depicting Eudoxus' model of retrograde planetary motion. The two innermost homocentric spheres of his model are represented as rings here, each turning with the same period but in opposite directions, moving the planet along a figure-eight, or hippopede
Schematic model likely representing the first 4, out of a total of 27, spheres of Eudoxus' cosmic model following Giovanni Schiaparell. Each sphere has its own rotational axis that, together, generates a complex motion for the planet, in this illustration, the moon.