Astronomy Hypertext_ Ptolemy's Geocentric Cosmology

Astronomy Hypertext_ Ptolemy's Geocentric Cosmology -...

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1/15/12 Astronomy Hypertext: Ptolemy's Geocentric Cosmology 1/1«/history_ptolemyGeocentric.html Astronom\ H\perte[t Ptolem\'s Geocentric Cosmolog\ Latest Modification: July 22, 2002 Ptolem\'s geocentric s\stem, taken in part from the earlier work of Heraclides and Apollonius, presented each planet as moving uniforml\ around a small circle called an epic\cle . The center of the epic\cle in turn revolved uniforml\ around the circumference of a large circle called a deferent . B\ means of proper combinations of si]es and rates of motion for the epic\cle and deferent, planetar\ motions could be mostl\ direct and occasionall\ retrograde. Also, since a planet on an epic\cle is sometimes nearer and sometimes farther from the Earth, this accounted for the observed variations in planetar\ brightness. To represent the irregular rates of motion of the planets, Ptolem\
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Unformatted text preview: continued to emplo\ the device attributed to Hipparchus of having the deferent off center from the Earth (to produce an eccentric deferent ), so that a planet would appear to go fastest when it was closest to the Earth. Having constructed orbits for the Sun, Moon and planets out of a combination of epic\cles and eccentric deferents, Ptolem\ found that the heavenl\ bodies were moving at an even more irregular rate than could be accounted for b\ these devices. His solution to this problem was to suppose that the planets' motions were uniform not as viewed from either the Earth or even the center of the eccentric deferent, but from a point on the other side of the center of the deferent from the Earth; this point was called the equant . ± 1995, J. C. Evans Physics & Astronomy Department, George Mason University Maintained by J. C. Evans; [email protected]
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