Portal:History of science/Article/28
The celestial spheres, or celestial orbs, are the fundamental element of Earth-centered (geocentric) astronomies and cosmologies developed by Plato, Aristotle, Ptolemy, and others. In these geocentric models the stars and planets are carried around the Earth on spheres or circles. Note that the spheres carry the planets and thus are not to be confused with the modern concept of the spherical planets themselves.
The spheres were most commonly arranged outwards from the center in this order: the sphere of the Moon, the sphere of Mercury, the sphere of Venus, the sphere of the Sun, the sphere of Mars, the sphere of Jupiter, the sphere of Saturn, the starry firmament, and sometimes one or two additional spheres. The order of the lower planets was not universally agreed on. Plato and his followers ordered them Moon, Sun, Mercury, Venus, and then followed the standard model for the upper spheres; there were further disagreements on the relative place of the spheres of Mercury and Venus.