   Classical astronomy followed principles established by Aristotle. Aristotle accepted the idea that there were four physical elements – earth, water, air, and fire. He put the earth in the center of the universe and contended that these elements were below the moon, which was the closest celestial body. There were seven planets, or wandering stars, because they had a course through the zodiac in addition to traveling around the earth: the moon, Mercury, Venus, the sun, Mars, Jupiter. Beyond that were the fixed stars. The physical elements, according to Aristotle moved vertically, depending on their ‘heaviness’ or ‘gravity’; the celestial bodies were not physical but a ‘fifth element’ or ‘quintessence’ whose nature was to move in perfect circles around the earth, making a daily rotation. Aristotle envisioned the earth as the true center of all the circles or ‘orbs’ carrying the heavenly bodies around it and all motion as ‘uniform,’ that is, unchanging.