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deferent
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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.
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deferent

astronomy
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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.

The deferent is a concept from ancient astronomy, specifically the Ptolemaic system, which placed Earth at the center of the universe. In this model, planets were thought to move in two circular paths: an epicycle, which is a smaller circle, and a larger circle called the deferent. The center of the epicycle moved along the deferent, which itself was centered near Earth. This complex system of circles was an attempt to explain the observed, irregular movements of planets, including their occasional retrograde motion. While the Ptolemaic system was highly influential for over a thousand years, it was eventually superseded by the simpler heliocentric model proposed by Copernicus.1234

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6 Britannica Sources
  1. celestial mechanics - Early theories
  2. celestial mechanics - Newton’s laws of motion
  3. astronomy - The motion of the planets
  4. Ptolemaic system
  5. astronomy - Mesopotamia
  6. Ptolemaic system

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