Copernicus, Brahe, Kepler, and Galileo - a New Theory of the Universe

The old theory of the Universe, the Ptolemaic system, had the earth at the center of a universe made up of concentric spheres within spheres that held the moon, sun, planets and stars. To explain the observable, non-circular patterns (retrograde motion) of inner planets, Ptolemaic thinkers had to make many clumsy adjustments in their systems. The planets had to follow a small circle (epicycle) around a larger circle (deferent). Ptolemy used the spherical model because of the Aristotelian ideal of circles being the perfect shape.
…