| Ptolemy and Regiomontanus shown on the frontispiece to Regiomontanus' Epitome of the Almagest, 1496. The Epitome was one of the most important Renaissance sources on ancient astronomy. Ptolemy Ptolemy, Latin in full Claudius Ptolemaeus (fl. AD 127-145, Alexandria), ancient astronomer, geographer, and mathematician who considered the Earth the center of the universe (the "Ptolemaic system"). Virtually nothing is known about his life. Ptolemy's astronomical work was enshrined in his great book He mathematike syntaxis ("The Mathematical Collection"), which eventually became known as Ho megas astronomos ("The Great Astronomer"). During the 9th century, however, Arab astronomers used the Greek superlative Megiste to refer to the book. When the definite article al was prefixed to the term, its title then became known as the Almagest, the name still used today. The Christian Aristotelian cosmos, engraving from Peter Apian's Cosmographia, 1524 Ptolemy accepted the following order for celestial objects in the solar system: Earth (center), Moon, Mercury, Venus, Sun, Mars, Jupiter, and Saturn. He realized, as had Hipparchus, that the inequalities in the motions of these heavenly bodies necessitated either a system of deferents and epicycles or one of movable eccentrics (both systems devised by Apollonius of Perga, the Greek geometer of the 3rd century BC) in order to account for their movements in terms of uniform circular motion. In the Ptolemaic system, the plane of the ecliptic is that of the Sun's apparent annual path among the stars. The planes of the deferents of the planets were believed to be inclined at small angles to the plane of the ecliptic, while the planes of their epicycles were inclined by equal amounts to those of the deferents, so that the planes of the epicycles would always parallel that of the ecliptic. The planes of the deferents of Mercury and Venus were assumed to oscillate above and below the plane of the ecliptic, and likewise the planes of their epicycles were thought to oscillate with respect to the planes of the deferents. | |
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