deferentów

Deferents are large circular paths used in geocentric astronomical models to describe how planets move relative to Earth. In the classic system of Claudius Ptolemy, a planet is carried by an epicycle—a small circle whose center moves along a larger circle called the deferent. The combination explains apparent retrograde motion: as the epicycle turns around the deferent, the planet can briefly reverse its apparent direction from Earth’s viewpoint. In many versions, the Earth is not exactly at the center of the deferent but offset (an eccentric) to better fit observed positions. To further adjust the rate of motion, an equant point is introduced: from this point, the deferent's angular velocity is assumed uniform, reconciling discrepancies between theory and observations when viewed from Earth.

The deferent concept was developed in antiquity and became central to astronomical calculations in late antiquity and the medieval period. The most influential formalization appears in Ptolemy's Almagest, where deferents and epicycles are carefully combined with the equant. The model allowed accurate predictions of planetary positions for its time but was eventually superseded by the Copernican heliocentric framework and, later, by Kepler's elliptical orbits and Newtonian gravity.

Today the deferent is discussed primarily as a historical artifact illustrating the pre-Copernican attempt to explain planetary motion with uniform circular motion. It remains a key concept for understanding the history of astronomy and the development of celestial mechanics.