An Etymological Dictionary of Astronomy and Astrophysics

A series of spheres with varying radii centred on the Earth, each rotating uniformly about an axis fixed with respect to the surface of the next larger sphere, all comprising a model in Greek astronomy to describe the motions of the heavenly bodies. The spheres turned with different speeds about axes with different orientations.
The fixed stars revolved around the Earth by the motion of the most distant sphere to which the stars were thought to be attached. Each of the five planets’ (Mercury, Venus, Mars, Jupiter, and Saturn) motion could be described using four spheres. The Sun and the Moon required three spheres each to explain their motions. Therefore, a total of 27 spheres described the behavior of the heavenly bodies in terms of circular motion.
Eudoxus was the first person to devise a model that could explain the → retrograde motion of the planets in the sky along a looped curve known as the → hippopede.

See also: → sphere; Eudoxus (Ευδοξοσ) of Cnidus (c 408 BC - c 355 BC), Greek astronomer and mathematician.