Ancient Greek astronomers’ conception that the heavens are a series of crystal spheres and circles, centered on Earth, supporting the stars and planets. In the early fourth century B.C.E., Eudoxus of Cnidus, a pupil of Plato, suggested that each planet was attached to a separate sphere. Other philosophers believed that the planets were in fact small crystal buttons and that light shone through them, reflecting the eternal fire that existed outside the human world. These ideas were adopted and refined by Aristotle in the late fourth century B.C.E. Aristotle’s ideas, in turn, formed the basis of many ancient and medieval cosmologies, including those of Apollonius of Perga (c. 262-190 B.C.E.), Hipparchus (190-120 B.C.E.), and Ptolemy (100-165 B.C.E.)
Aristotle’s geocentric concept of the universe reigned unchallenged until the 16th century, although astronomers had noticed curious motions in the heavens that required refinements of his theory. At certain times, planets appeared to move backwards in their orbits. The movements of the Sun and the Moon were especially erratic. In 1547, a Polish monk named Nicolaus Copernicus proposed a heliocentric universe, centering on the Sun rather than on Earth. In 1607, the Austrian astronomer Johannes Kepler showed that planetary orbits were not true circles but were ellipses instead.