What does epicycle mean?
An epicycle is a circle moving on another circle. The term belongs chiefly to the history of science: in the geocentric astronomy of Ptolemy, each planet was imagined to travel around a small circle — its epicycle — whose centre in turn travelled along a larger circle called the deferent. By adjusting the sizes and speeds of these circles, astronomers could reproduce the planets' observed motions, including the puzzling backward loops known as retrograde motion. As observations improved, more epicycles were added, making the system famously cumbersome; the word has therefore come to symbolize elaborate, patch-on-patch theorizing. In modern usage 'epicycle' survives mainly in historical writing about astronomy and in engineering, where 'epicyclic' describes gear trains in which one gear rotates around another, as in automatic transmissions. The word derives ultimately from Greek, combining epi ('upon') with kyklos ('circle'), literally meaning a circle set upon a circle.
A small circle whose centre moves along the circumference of a larger circle (the deferent), employed in Ptolemaic astronomy to account for irregularities such as retrograde motion in a planet's apparent path.
"To fit Mercury's motion, the astronomer introduced a second epicycle nested within the first."
Almost always encountered in plural when describing the accumulated devices of the Ptolemaic system.
(By extension, chiefly in engineering) A gear or rotating element whose axis moves around the axis of another, as in an epicyclic gear train.
"The planet gears act as epicycles within the ring gear of the transmission."
Regular plural; the plural is especially common in historical discussion of the many epicycles required by geocentric models.
"Copernicus's model still retained some epicycles, though far fewer than Ptolemy's."
Astronomers piled epicycle upon epicycle to save their Earth-centred model — until Copernicus showed the whole stack was unnecessary.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Where does epicycle come from?
Epicycle comes via Latin epicyclus from Greek epikyklos, formed from epi ('upon') and kyklos ('circle') — literally 'a circle upon a circle'. The term entered English in the late Middle English period through scholarly astronomical writing and became standard in Renaissance treatments of the Ptolemaic system. It shares its Greek root kyklos with words such as cycle, cyclical, cyclone, and encyclopedia.
Related word forms
How epicycle is actually used
Primarily a historical and technical term: it appears most often in discussions of ancient and medieval astronomy and in mechanical engineering (epicyclic gear trains). Figurative use, implying needless complexity or circular reasoning, is relatively rare and literary.
Easily confused with epicycle
An epicycle is a small circle carried on a larger one, whereas in Ptolemaic astronomy an 'eccentric' is a single circle whose centre is offset from the Earth.