Taking up the hypothesis formulated by Aristarchos of Samos in the third century B.C.E., Nicolaus Copernicus developed the heliocentric theory of celestial motion. He described it in his epoch-making work, De Revolutionibus orbium coelestium.
In the Copernican system, the Sun is immobile at the center of the universe and the planets—including the Earth—rotate around it. The Earth is thus no longer a fixed hub, but revolves around the Sun in one year and on its own axis in twenty-four hours.
For Copernicus, the motion of the heavenly bodies from east to west in the course of a day is only apparent. In fact, it is caused by the Earth's rotation in the opposite direction; also apparent is the Sun's annual travel along the ecliptic, actually due to terrestrial rotation; the retrograde movement of the planets is apparent as well. For the ancients, who assumed the Earth to be immobile, each planet moved on a circle called the epicycle, whose center, in turn, moved around the Earth on a larger circle known as the deferent. In the Copernican system, instead, both motions are centered on the Sun, but only one is performed by the planet itself; the other is the revolution of the Earth. The combination of the two motions generated the celestial body's apparent retrogression.
Like his predecessors, Copernicus believed that the planets were embedded in solid crystal spheres. He also remained faithful to the axiom of the circularity and uniformity of celestial motions. He was thus forced to adopt intricate combinations of circles, which made his system technically no less complex than the Ptolemaic system. Overall, however, his new concept seemed to provide an ultimate demonstration of the perfect harmony of the cosmos, with six planets all continuously revolving in the same direction around the Sun.
Last update 28/gen/2008