Copernican Heliocentrism - Copernican Theory

Copernican Theory

Copernicus' major work, De revolutionibus orbium coelestium - On the Revolutions of the Celestial Spheres (first edition 1543 in Nuremberg, second edition 1566 in Basel), was published during the year of his death, though he had arrived at his theory several decades earlier. The book marks the beginning of the shift away from a geocentric (and anthropocentric) universe with the Earth at its center. Copernicus held that the Earth is another planet revolving around the fixed sun once a year, and turning on its axis once a day. But while Copernicus put the Sun at the center of the celestial spheres, he did not put it at the exact center of the universe, but near it. Copernicus' system used only uniform circular motions, correcting what was seen by many as the chief inelegance in Ptolemy's system.

The Copernican model replaced Ptolemy's equant circles with more epicycles. This is the main reason that Copernicus' system had even more epicycles than Ptolemy's. The Copernican system can be summarized in several propositions, as Copernicus himself did in his early Commentariolus that he handed only to friends probably in the 1510s. The "little commentary" was never printed. Its existence was only known indirectly until a copy was discovered in Stockholm around 1880, and another in Vienna a few years later. The major features of Copernican theory are:

1. Heavenly motions are uniform, eternal, and circular or compounded of several circles (epicycles).
2. The center of the universe is near the Sun.
3. Around the Sun, in order, are Mercury, Venus, Earth and Moon, Mars, Jupiter, Saturn, and the fixed stars.
4. The Earth has three motions: daily rotation, annual revolution, and annual tilting of its axis.
5. Retrograde motion of the planets is explained by the Earth's motion.
6. The distance from the Earth to the Sun is small compared to the distance to the stars.