The geocentric model as described by Ptolemy in his work, the Almagest, placed the planets in circular orbits around the stable earth. The symmetry of the circle was largely argued as necessary for a universe that would be aesthetically pleasing, and later Christians adopted the circle as a representation of the perfection of God.
In order to describe the erratic motion of the planets, specifically retrograde motion, and any speed discrepancies, Ptolemy devised a series of secondary circles linked to each planets circular path. I think these were called the deferent circles, and this model was so successful that it was followed by astronomers for hundreds of years.
Johannes Kepler was the first to present the idea of ellipses as a model for planetary movement, and he did so after studying the detailed observations taken by Tycho Brahe which revealed the planetary paths on an unparallelled scale. Much of the resistance to Kepler's model was based on the reluctance to abandon the ever-perfect circle.

• Re: Re: but what about planet speed? - libbie468 - February 16, 2001 - 21:06 UTC
  • Re: Re: Re: but what about planet speed? - alexander - February 17, 2001 - 11:21 UTC
    • but what about planet speed? - llama - February 17, 2001 - 17:14 UTC