first modern European theory of planetary motion that was heliocentric, i.e., that placed the sun motionless at the center of the solar system with all the planets, including the earth, revolving around it. Copernicus developed his theory in the early 16th cent. from a study of ancient astronomical records. He retained the ancient belief that the planets move in perfect circles and therefore, like Ptolemy, he was forced to utilize epicycles to explain deviations from uniform motion (see Ptolemaic system). Thus, the Copernican system was technically only a slight improvement over the Ptolemaic system. However, making the solar system heliocentric removed the largest epicycle and explained retrograde motion in a natural way. By liberating astronomy from a geocentric viewpoint, Copernicus paved the way for Kepler's laws of planetary motion and Newton's embracing theory of universal gravitation, which describes the force that holds the planets in their orbits.

See E. Rosen, Copernicus and His Successors (1995); T. S. Kuhn, The Copernican Revolution (1997).

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