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epicycle

[ep-uh-sahy-kuh l] /ˈɛp əˌsaɪ kəl/
noun
1.
Astronomy. a small circle the center of which moves around in the circumference of a larger circle: used in Ptolemaic astronomy to account for observed periodic irregularities in planetary motions.
2.
Mathematics. a circle that rolls, externally or internally, without slipping, on another circle, generating an epicycloid or hypocycloid.
Origin of epicycle
1350-1400
1350-1400; Middle English < Middle French < Late Latin epicyclus < Greek epíkyklos. See epi-, cycle
Related forms
epicyclic
[ep-uh-sahy-klik, -sik-lik] /ˌɛp əˈsaɪ klɪk, -ˈsɪk lɪk/ (Show IPA),
Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2018.
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Examples from the Web for epicycle
Historical Examples
• The small circle carried by a bigger one was called an epicycle.

Oliver Lodge
• The eighth sphere had neither deferent nor epicycle but to it were attached the fixed stars.

Florence M. Grimm
• To help out of this difficulty, the worst possible mechanical scheme was invented, that known as the epicycle.

David Todd
• For the epicycle is a sphere which changes place in the circumference of the large sphere.

Isaac Husik
• epicycle, ep′i-sī-kl, n. a circle having its centre on the circumference of a greater circle on which it moves.

• The planet is supposed to move equably in the epicycle, and to be carried by the Sun unequably in the proportion of the distances.

John Elliot Drinkwater Bethune
• The circle Dd, described by D, the centre of the epicycle, was called the deferent.

John Elliot Drinkwater Bethune
• An epicycle in addition to the hypocycle was introduced into Mercury's orbit.

John Elliot Drinkwater Bethune
• In this epicycle he was not supposed to revolve, but to librate, or move up and down in its diameter.

John Elliot Drinkwater Bethune
• epicycle, a conception of the ancient astronomy used to explain the irregular, and at times retrograde, motions of the planets.

Various
British Dictionary definitions for epicycle

epicycle

/ˈɛpɪˌsaɪkəl/
noun
1.
(astronomy) (in the Ptolemaic system) a small circle, around which a planet was thought to revolve, whose centre describes a larger circle (the deferent) centred on the earth
2.
a circle that rolls around the inside or outside of another circle, so generating an epicycloid or hypocycloid
Derived Forms
Word Origin
C14: from Late Latin epicyclus, from Greek epikuklos; see epi-, cycle
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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Word Origin and History for epicycle
n.

late 14c., from Latin epicyclus, from Greek epikyklos, from epi (see epi-) + kyklos (see cycle (n.)).

Online Etymology Dictionary, © 2010 Douglas Harper
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epicycle in Science
 epicycle   (ěp'ĭ-sī'kəl)    In Ptolemaic cosmology, a small circle representing a temporary adjustment to the position of a planet as it orbits the Earth. The five known planets, along with the Sun and Moon, were conceived as moving through the sky in large circular paths with the Earth at their center. As a planet moved along its path, it occasionally departed from its regular motion to follow a much smaller circle centered on the orbital path itself. These smaller circles, or epicycles, were necessary to reconcile the observed motions of the planets with a geocentric model of the universe. The epicycles of the inferior planets Mercury and Venus were fixed to the orbit of the Sun and explained why those planets were never observed far from it in the sky. The epicycles of the superior planets Mars, Jupiter, and Saturn explained why those bodies were sometimes observed to move backward in their orbits, a phenomenon known as retrograde motion and explained in a heliocentric model by the differing orbital velocities of the Earth and the planet being observed. See illustration at Ptolemaic system.A circle whose circumference rolls along the circumference of a fixed circle, thereby generating an epicycloid or a hypocycloid.
The American Heritage® Science Dictionary