Ancient astronomers subscribed to the geocentric model of the Universe, believing that all heavenly objects revolved the Earth. These observers knew that five objects moved relative to the fixed stars, and they named them "planets," after the Greek word for "wanderers." These planets are now known as Mercury, Venus, Mars, Jupiter, and Saturn.

Occasionally, these planets would appear to stop their eastward movement and then back up for several weeks or months. This westward movement is called retrograde motion. Two Greek astronomers, Hipparchus (146-127 B.C.) and Ptolemy, devised a theory to account for this odd behavior. Each planet was assumed to revolve around a small circle called an epicycle, and this epicycle in turn revolved around the Earth. Ptolemy published this idea in the second century A.D. For the next thousand years, observers found this theory useful for describing the workings of the heavens...