Copernican heliocentrism

Copernican heliocentrism is the name given to the astronomical model developed by Nicolaus Copernicus and published in 1543. It positioned the Sun near the center of the Universe, motionless, with Earth and the other planets rotating around it in circular paths modified by epicycles and at uniform speeds. The Copernican model departed from the Ptolemaic system that prevailed in Western culture for centuries, placing Earth at the center of the Universe, and is often regarded as the launching point to modern astronomy and the Scientific Revolution.

As a university-trained Catholic priest dedicated to astronomy, Copernicus was acquainted with the Sun-centered cosmos of the ancient Greek Aristarchus. Although he circulated an outline of the heliocentric theory to colleagues decades earlier, the idea was largely forgotten until late in his life he was urged by a pupil to complete and publish a mathematically detailed account of his model. Copernicus's challenge was to present a practical alternative to the Ptolemaic model by more elegantly and accurately determining the length of a solar year while preserving the metaphysical implications of a mathematically ordered cosmos. Thus his heliocentric model retained several of the Ptolemaic elements causing the inaccuracies, such as the planets' circular orbits, epicycles, and uniform speeds, while at the same time re-introducing such innovations as:

• Earth is one of seven ordered planets in a solar system circling a stationary Sun
• Earth has three motions: daily rotation, annual revolution, and annual tilting of its axis
• Retrograde motion of the planets is explained by Earth's motion
• Distance from Earth to the Sun is small compared to the distance to the stars