proof that neither motion change, nor differences in matter can exist. His heavenly bodies, separating out with the heaviest matter towards the center, are again concentrations of fire-vapor, here regulated by "Necessity" to move them between an inner "wreath" of fire and an outer solid. Empedocles of Acragas (mid 5th cent. BCE) worked on a system to reconcile the "unchanging" universe of Parmenides' sphere with chaotic, differentiated matter by having the universe in a state of flux between harmony and strife. He also advocated an outer, hard universal sphere upon which the stars are fixed, and an inner sphere of double hemispheres, one of lighter fire for day, one of darker for nightAnaxagoras, friend of the Athenian statesmen Perikles and thus slightly younger than Empedocles, follows the usual theory of separation and condensation, but his heavenly bodies are again solid objects. His most important contribution to astronomy was the claim that the moon's light is a reflection of the and that eclipses of the moon were caused by earth's shadow, eclipses of the sun by the moon passing before. The Pythagoreans first proposed a non-geocentric system, perhaps partly on the basis of moral and religious grounds: to them, humanity and earth were imperfect, and only by sacrifice and a strict regimen of personal conduct could one strive to reach the divine. Accordingly, they placed the divine, poetically called the "Hearth of the Universe" or "Throne of Zeus", at the center of a finite, spherical universe. The sun is a glass sphere which catches and reflects this hearth-light. A counter-earth, the "antichthon," had to be invented, supposedly to make the number of planetary spheres ten. These include the five visible planets out through Saturn, earth, the moon, the sun, and the heavenly sphere on which were the stars. Also, the counter-earth was invented to account for the frequency of lunar eclipses and to solve a major problem in this view...
