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Ptolemy (fl. 146—170) Greek astronomer and geographer

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epicycle

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Ptolemaic system


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The ancient Greek geocentric model of the Solar System, as described by Ptolemy. It may be traced back through the work of, for example, Hipparchus, Apollonius, Callippus, and Eudoxus. The Earth is placed at the centre of the Universe, and around it revolve the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn, in that order; beyond Saturn is the sphere of the fixed stars. In the basic model each body moves along the circumference of a small circle, the epicycle, whose centre in turn follows the circumference of a larger circle, the deferent, centred on the Earth. In later refinements, Ptolemy introduced two points equally spaced on either side of the Earth: the eccentric and the equant. The centre of the epicycle revolved around the eccentric, not the Earth, and the orbiting body moved uniformly with respect to the equant. As a computational device the Ptolemaic system predicted planetary movements, including their retrograde motion, tolerably well, and survived with minor amendments until displaced by the Copernican system in the 16th century.


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