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A model of the solar system in which the planets orbit around the central Sun, proposed by Nicholas Copernicus and published in his book De revolutionibus in 1543. The theory did not find favour at first since it could predict the planetary positions no more accurately than the Ptolemaic system, which had been used for hundreds of years, and it displaced the Earth from the centre of the universe, which was regarded as religiously unacceptable by many. In Copernicus's heliocentric model, the basic planetary orbits were circles and he had to invoke epicycles to reproduce the observed movements of the planets. Nevertheless, Copernicus's ideas marked a turning point and stimulated work that was later to lead to the development by Johannes Kepler of a more accurate heliocentric model in which the planetary orbits are elliptical rather than circular.
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