FIG. 1. THE UNIVERSE as conceived in the Ptolemaic and Copernican systems. In the Ptolemaic, or geocentric, system (right), as presented by Petrus Apianus in his Cosmographia of 1524, surrounding the earth were air and fire, the moon, Mercury, Venus, the sun, the fixed stars, and the ninth and tenth heavens. In the Copernican, or heliocentric, system (left), as presented by Nicolaus Copernicus in his De revolutionibus Orbium Coelestium of 1543, the central stationary sun was surrounded by Mercury, Venus, the earth and the moon, Mars, Saturn, Jupiter, and the fixed stars.