Kepler’s Theory of Planetary Motion (ca. 1600–1687): Building on the astronomical observational foundations of the Babylonians, and inspired by their new quest to explain the forces of nature, the ancient Greeks were among the first recorded civilizations that attempted to mathematically explain the motions of the sun, moon, and planets. The motions of the sun and moon followed calculable patterns, and early calendars exploited the predictable nature of these heavenly objects for many uses, primarily the synchronizing of agriculture with the seasons. The planets, however, were another matter. The term “planet” is derived from the Greek word planaomai, a verb meaning to wander. And the planets were indeed wanderers of the night sky. Unlike the sun, moon, and stars, the planets appeared to be able to change not only their speed of movement but also their direction of motion in the night sky. Of special concern was Mars, whose movement consisted of stationary phases, called stations, and periods in which the planet moved backward across the sky, called retrograde motion. To the ancient Greeks, the objects of the night sky were of perfect origin, and thus the erratic motion of the planets presented a special problem.
The Milesian philosophers of ancient Greece, a group that included Thales and Anaximander, were among the first to construct a mechanical model for the heavens. This model consisted of three rings that housed the sun, moon, and stars. These rings surrounded the Earth, which itself was portrayed as a large cylinder with a flat surface on top. The rings were spaced at distances nine, eighteen, and twenty-seven times the diameter of the Earth. The Milesians, however, made no provisions for planets in their model. It was not until the time of