 Philolaus’ astronomical system is famous as the first to move the earth from the center of the universe and make it an orbiting planet instead. Copernicus referred to Philolaus as his precursor in the preface to his De Revolutionibus (see Kahn 2001, 26 for references). Unlike Copernicus, however, Philolaus did not replace the geocentric with a heliocentric universe. The central fire rather than the sun is at the center of Philolaus’ cosmos. The heavenly bodies are arranged in ten concentric circles around this central fire. Beginning from the outside, the fixed stars come first, then the five planets, the sun, the moon, the earth and a mysterious counter-earth. The system can account for the basic phenomena with elegant simplicity. The fixed stars serve as the backdrop against which the other heavenly bodies move. All of the bodies have only one circular motion around the central fire, from west to east. The moon completes its orbit in about a month, the sun in a year and each of the planets has its own period of revolution. Thus, bodies move more slowly, the farther removed they are from the central fire,and the fixed stars probably did not move at all. These motions account for the basic apparent movement of the sun, moon and planets through the zodiac, although there is no attempt to explain more sophisticated planetary phenomena such as retrograde motion, where the planet appears to stop and move backward before continuing its progress through the zodiac. Retrograde motion was only first addressed almost a century after Philolaus by Eudoxus. The earth has by far the fastest motion, completing a circuit of the central fire in 24 hours. It is the motion of the earth that accounts for the apparent daily motion of the sun across the sky and hence for night and day. The much faster moving earth, which rotates on its axis so as to keep our side always turned away from the central fire and counter-earth (see below), leaves the sun behind so that day lasts as long as the earth is facing the sun, and night occurs when the earth turns away from the sun as it moves around the central fire.