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Ptolemaic system
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Diagram illustrating Ptolemy's equant model, showing a stationary Earth at the center with a planet moving in a small circle called an epicycle, which itself moves along a larger circle, the deferent. The equant point is marked off-center from Earth, and the positions of apogee and perigee are labeled. Small stars are scattered in the background. Geocentric system
How did Ptolemy explain retrograde motion?
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Nicolaus Copernicus. Nicolas Copernicus (1473-1543) Polish astronomer. In 1543 he published, forward proof of a Heliocentric (sun centered) universe. Coloured stipple engraving published London 1802. De revolutionibus orbium coelestium libri vi.
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Diagram illustrating Ptolemy's equant model, showing a stationary Earth at the center with a planet moving in a small circle called an epicycle, which itself moves along a larger circle, the deferent. The equant point is marked off-center from Earth, and the positions of apogee and perigee are labeled. Small stars are scattered in the background.
Ptolemaic system In Ptolemy's geocentric model of the universe, the Sun, the Moon, and each planet orbit a stationary Earth. For the Greeks, heavenly bodies must move in the most perfect possible fashion—hence, in perfect circles. In order to retain such motion and still explain the erratic apparent paths of the bodies, Ptolemy shifted the centre of each body's orbit (deferent) from Earth—accounting for the body's apogee and perigee—and added a second orbital motion (epicycle) to explain retrograde motion. The equant is the point from which each body sweeps out equal angles along the deferent in equal times. The centre of the deferent is midway between the equant and Earth.

Ptolemaic system

astronomy
Written by
Alexander Raymond Jones
Professor of Classics, University of Toronto. Author of Pappus of Alexandria.
Alexander Raymond Jones
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Ptolemaic system, mathematical model of the universe formulated by the Alexandrian astronomer and mathematician Ptolemy about 150 CE and recorded by him in his Almagest and Planetary Hypotheses. The Ptolemaic system is a geocentric cosmology; that is, it starts by assuming that Earth is stationary and at the centre of the universe. The “natural” expectation for ancient societies was that the heavenly bodies (Sun, Moon, planets, and stars) must travel in uniform motion along the...

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