epicycle

1 9.9

A small circle that moves around a larger circle to show a planet's orbit

noun
/ˈɛpɪˌsaɪkəl/

Specialized

: epiciclo

To understand retrograde motion, astronomers used the concept of epicycles, where a smaller circle moves along a larger orbit.
epicycle and deferent

So you, you keep adding epicycles in order to to cover things that have gone wrong.

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Examples

The model of planetary motion included multiple epicycles to accurately predict celestial positions.
Would these theoretical advances occur under an epicycle system?

Blog text (27)
At one point, astronomy consisted of a series of epicycles; as new information was obtained, a new epicycle was added.

Blog text (15)
The planets move in elliptical orbits rather than in circles and epicycles.

Academic text (2003)
In order to account for these confusing departures from observation, Ptolemy introduced the concept of epicycles—smaller spheres on which planets orbited as they simultaneously moved around Earth.

Academic text (2004)
In ancient astronomy, the planets were thought to move in circular paths called epicycles around the Earth.

Surface Forms

epicycle singular

epicycles plural

Morphology

epicycle = cycle (semi-transparent) = epi + cycle

Formally analyzable as 'epi-' + 'cycle', but 'epi-' is a specialised combining form and the full technical astronomical meaning is not obvious to typical learners.

Etymology

Epicycle comes from the Greek parts epi- meaning 'on' or 'upon' and kyklos meaning 'circle', the same root as cycle. So an epicycle is literally a small circle 'on' a larger circle, which is why ancient astronomers used it to explain the strange paths of the planets.