epicycle
Quick Summary
In Ptolemaic astronomy, an epicycle was a circle on which a planet moved. The center of this epicycle was, in turn, carried around the circumference of a larger circle, known as the deferent. This model, developed by ancient Greek astronomers like Apollonius of Perga and later refined by Ptolemy, was used to explain the observed complex movements of planets, including retrograde motion, where planets appear to move backward in the sky. The epicyclic model, combined with eccentric circles and the concept of an equant point, allowed for remarkably accurate predictions of planetary positions for centuries.1234
Ask Britannica AI about this topic
Britannica doesn't have a full article on this topic, so we've curated a list of our relevant coverage.
mechanicsHistory…emerged of circular orbits, called epicycles, on top of circular orbits. This system of astronomy culminated with the Almagest of Ptolemy, who worked in Alexandria in the 2nd century CE. The Copernican innovation simplified the system somewhat, but Copernicus’s astronomical tables were still based on circular orbits and epicycles. Kepler…
Read More…emerged of circular orbits, called epicycles, on top of circular orbits. This system of astronomy culminated with the Almagest of Ptolemy, who worked in Alexandria in the 2nd century CE. The Copernican innovation simplified the system somewhat, but Copernicus’s astronomical tables were still based on circular orbits and epicycles. Kepler…
Read More
celestial mechanicsEarly theories…revolved around small circles called epicycles at a uniform rate while the centre of the epicyclic circle orbited Earth on a large circle called a deferent. Other variations in the motion were accounted for by offsetting the centres of the deferent for each planet from Earth by a short distance.…
Read More…revolved around small circles called epicycles at a uniform rate while the centre of the epicyclic circle orbited Earth on a large circle called a deferent. Other variations in the motion were accounted for by offsetting the centres of the deferent for each planet from Earth by a short distance.…
Read More
Ptolemaic system…Ptolemy combined eccentricity with an epicyclic model. In the Ptolemaic system each planet revolves uniformly along a circular path (epicycle), the centre of which revolves around Earth along a larger circular path (deferent). Because one half of an epicycle runs counter to the general motion of the deferent path, the…
Read More…Ptolemy combined eccentricity with an epicyclic model. In the Ptolemaic system each planet revolves uniformly along a circular path (epicycle), the centre of which revolves around Earth along a larger circular path (deferent). Because one half of an epicycle runs counter to the general motion of the deferent path, the…
Read More
Nicolaus CopernicusCopernicus’s astronomical work…revolving, off-centre circles called eccentrics; epicycles, little circles whose centres moved uniformly on the circumference of circles of larger radius (deferents); and equants. The equant, however, broke with the main assumption of ancient astronomy because it separated the condition of uniform motion from that of constant distance from the centre.…
Read More…revolving, off-centre circles called eccentrics; epicycles, little circles whose centres moved uniformly on the circumference of circles of larger radius (deferents); and equants. The equant, however, broke with the main assumption of ancient astronomy because it separated the condition of uniform motion from that of constant distance from the centre.…
Read More
astronomyThe motion of the planets…based on eccentric circles and epicycles. (An eccentric circle is a circle that is slightly off-centre from Earth, and an epicycle is a circle that is carried and rides around on another circle.) This innovation is usually attributed to Apollonius of Perga (c. 220 bce), but it is not conclusively…
Read More…based on eccentric circles and epicycles. (An eccentric circle is a circle that is slightly off-centre from Earth, and an epicycle is a circle that is carried and rides around on another circle.) This innovation is usually attributed to Apollonius of Perga (c. 220 bce), but it is not conclusively…
Read More- astronomyThe motion of the planets
…eccentric circle, which carried an epicycle. Imagine looking down on the plane of the solar system from above its north pole. The planet moves counterclockwise on its epicycle. Meanwhile, the centre of the epicycle moves counterclockwise around the eccentric circle, which is centred near (but not quite exactly at) Earth.…
Read More…eccentric circle, which carried an epicycle. Imagine looking down on the plane of the solar system from above its north pole. The planet moves counterclockwise on its epicycle. Meanwhile, the centre of the epicycle moves counterclockwise around the eccentric circle, which is centred near (but not quite exactly at) Earth.…
Read More
mathematicsApplied geometryApollonius introduced an alternative “epicyclic” model, in which the planet turns about a point that itself orbits in a circle (the “deferent”) centred at or near Earth. As Apollonius knew, his epicyclic model is geometrically equivalent to an eccentric. These models were well adapted for explaining other phenomena of…
Read MoreApollonius introduced an alternative “epicyclic” model, in which the planet turns about a point that itself orbits in a circle (the “deferent”) centred at or near Earth. As Apollonius knew, his epicyclic model is geometrically equivalent to an eccentric. These models were well adapted for explaining other phenomena of…
Read More
HipparchusSolar and lunar theory…motion, the eccenter and the epicyclic models (see Ptolemaic system). These models, which assumed that the apparent irregular motion was produced by compounding two or more uniform circular motions, were probably familiar to Greek astronomers well before Hipparchus. His contribution was to discover a method of using the observed dates…
Read More…motion, the eccenter and the epicyclic models (see Ptolemaic system). These models, which assumed that the apparent irregular motion was produced by compounding two or more uniform circular motions, were probably familiar to Greek astronomers well before Hipparchus. His contribution was to discover a method of using the observed dates…
Read More
physical scienceAncient Middle Eastern and Greek astronomy…a geometric figure called an epicycle-deferent system, a proof probably first made by Apollonius of Perga a century earlier.
Read More…a geometric figure called an epicycle-deferent system, a proof probably first made by Apollonius of Perga a century earlier.
Read More