Heliocentric Theory

Nicholas Copernicus (1472-1543) revived the heliocentric theory in the sixteenth century, after hundreds of years of building on Claudius (c. A.D. 90-168) Ptolemy's geocentric cosmological model (proving Earth is at the center of the universe). In his book, On the Revolution of the Spheres of the Universe, he placed the Sun at the center of the universe with the planets revolving around it on epicycles (a circle around which a planet moves) and deferents (the imaginary circle around Earth in whose periphery moves the epicycle). He argued that the planets in order from the Sun are Mercury, Venus, Earth (with the Moon orbiting it), Mars, Jupiter, and Saturn. The celestial sphere with the stars is far beyond Saturn's orbit. The apparent daily westward rotation of the celestial sphere, the Sun, Moon, and of the planets is the result of Earth's daily eastward rotation around its axis. If one assumes that the orbital velocities decrease with increasing distance from the Sun, then retrograde (the appearance of moving backward) motion of the planets on the zodiac could be explained by Earth overtaking Mars, Jupiter and Saturn near opposition (when they are 180° from the Sun on the zodiac), and by it being overtaken by the faster moving Mercury and Venus when they pass between the Sun and Earth (inferior conjunction). Copernicus's heliocentric model achieved a simpler cosmology than did the modified Ptolemaic geocentric model that existed in the sixteenth century, although not more accurate. The major advantage in the eyes of Copernicus was the aesthetic appearance of a system of concentric orbits with ever-widening separations and, ironically, the return to some of the fundamentals of the ancient Greeks, including purely circular motions. Copernicus's heliocentric model of the solar system did not represent accurately the observed planetary motions over many centuries. His model had many critics and was generally not accepted. An interesting variant of a geocentric model of the solar system was developed at the end of the sixteenth century by the Danish planetary observer Tycho Brahe (1546-1601). Earth was the primary center for the motions of all celestial bodies in his model but Mercury and Venus revolved around the Sun, which in turn revolved with them around Earth.