The Ptolemaic System
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The geocentric model of Plato could not explain the retrograde motion of the planets. Around 140 A.D. Ptolemy proposed his refined geocentric model. In the Ptolemaic universe, a planet moves in a small circle called an epicycle, and the center of the epicycle moves along a larger circle around the Earth. The centers of the epicycles of Mercury and Venus must lie on the line joining the Earth and the Sun. Stars are fixed on an outermost sphere. This model gives predictions on the positions of the planets within a few degrees from the actual positions. This was generally accepted and the Ptolemaic model dominated the western world for 1500 years. It was, in fact, superior to the Copernican system propsed in the 16th century. Although Copernicus idea that the earth rotates around the sun was correct he still assumed that the planets move in perfect circles, instead of ellipses. Therefore the Copernican system predicted the positions of the planets less precisely than the - incorrect - geocentric model of Ptolemy. |