Copernicus, Nicolaus

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Polish astronomer who believed that the Sun, not the Earth, is at the centre of the Solar System, thus defying the Christian church doctrine of the time. For 30 years, he worked on the hypothesis that the rotation and the orbital motion of the Earth are responsible for the apparent movement of the heavenly bodies. His great work De Revolutionibus Orbium Coelestium/On the Revolutions of the Heavenly Spheres was the important first step to the more accurate picture of the Solar System built up by Tycho Brahe, Kepler, Galileo, and later astronomers.

Copernicus proposed replacing Ptolemy's ideas with a model in which the planets (including the Earth) orbited a centrally situated Sun. He proposed that the Earth described one full orbit of the Sun in a year, whereas the Moon orbited the Earth. The Earth rotated daily about its axis (which was inclined at 23.5° to the plane of orbit), thus accounting for the apparent daily rotation of the sphere of the fixed stars.

This model was a distinct improvement on the Ptolemaic system for a number of reasons. It explained why the planets Mercury and Venus displayed only ‘limited motion’; their orbits were inside that of the Earth's. Similarly, it explained that the planets Mars, Jupiter, and Saturn displayed such curious patterns in their movements (‘retrograde motion’, loops, and kinks) because they travel in outer orbits at a slower pace than the Earth. The movement of the Earth on its axis accounted for the precession of the equinoxes, previously discovered by Hipparchus.

Copernicus's model represents a complete reformation of astronomy by replacing the anthropocentric view of the universe with the heliocentric viewpoint. Unable to free himself from the constraints of classical thinking, however, Copernicus was able to imagine only circular planetary orbits. This forced him to retain the system of epicycles, with the Earth revolving around a centre that revolved around another centre, which in turn orbited the Sun. Kepler rescued the model by introducing the concept of elliptical orbits. Copernicus also held to the notion of spheres, in which the planets were supposed to travel. It was Brahe who finally rid astronomy of that concept.