Concept

Deferent and epicycle

Overview

In the Hipparchian, Ptolemaic, and Copernican systems of astronomy, the epicycle (from Ancient Greek ἐπίκυκλος (epíkuklos) 'upon the circle', meaning "circle moving on another circle") was a geometric model used to explain the variations in speed and direction of the apparent motion of the Moon, Sun, and planets. In particular it explained the apparent retrograde motion of the five planets known at the time. Secondarily, it also explained changes in the apparent distances of the planets from the Earth.

The model is often attributed to Apollonius of Perga, who was active at the end of the 3rd century BC. It was developed by Apollonius of Perga and Hipparchus of Rhodes, who used it extensively, during the 2nd century BC, then formalized and extensively used by Ptolemy in his 2nd century AD astronomical treatise the Almagest.

Epicyclical motion is used in the Antikythera mechanism, an ancient Greek astronomical device, for compensating for the elliptical orbit of the Moon, moving faster at perigee and slower at apogee than circular orbits would, using four gears, two of them engaged in an eccentric way that quite closely approximates Kepler's second law.

Epicycles were highly accurate at predicting apparent planetary motion, because, as Fourier analysis later showed, any smooth curve can be approximated to arbitrary accuracy with a sufficient number of epicycles. However, they fell out of favor with the discovery that planetary motions are largely elliptical from a heliocentric frame of reference, which led to the discovery that gravity obeying a simple inverse square law could better explain all planetary motions.

Overview adapted from Wikipedia (CC BY-SA 4.0; see the article history for authors).

Possible passages

Found by search and not yet checked by a person; they may only mention the idea in passing.

4. There is another aphya derived from fish, for that which is called cobitis is derived from small and inferior gobii, which bury themselves in the earth. The membrades are produced from the phalerica. The trichides come from these, and the trichiæ from the trichides; from one kind of aphya, which inhabits the port of Athens, the encrasicoli are derived.
Aristotle, Aristotle's History of Animals In Ten Books
States what Deferent and epicycle is; the author is tied to Deferent and epicycle in the knowledge graph.
Read the chapter · Open in the reader
Augustine appears to be of the same opinion when he expresses his belief that all corporeal things are ruled by God through the spirit of life (De Trin. iii, 4). From what has been said, then, it is clear that the heavenly bodies are not living beings in the same sense as plants and animals, and that if they are called so, it can only be equivocally. It will also be seen that the difference of opinion between those…
Aquinas Thomas, Summa Theologica, Part I
States what Deferent and epicycle is; the author is tied to Deferent and epicycle in the knowledge graph.
Read the chapter · Open in the reader
The orbit of the Moon is not precisely in the same Plane with that of the Earth; but makes a very small angle with it. The points of intersection with those two Planes, are called, the Nodes of the Moon. These Nodes of the Moon are in continual motion, and in eighteen or nineteen years, revolve backwards, from east to west, through all the different points of the Ecliptic. For the Moon, after having finished her…
Adam Smith, The Essays of Adam Smith
States what Deferent and epicycle is.
Read the chapter · Open in the reader

Philosophers who discussed Deferent and epicycle

Jan of Głogów

Related ideas

Place in the tradition

Part of Cosmology.

Explore Deferent and epicycle in the interactive graph →