epicycle
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.
Mathematics. a circle that rolls, externally or internally, without slipping, on another circle, generating an epicycloid or hypocycloid.
Origin of epicycle
1Other words from epicycle
- ep·i·cy·clic [ep-uh-sahy-klik, -sik-lik], /ˌɛp əˈsaɪ klɪk, -ˈsɪk lɪk/, adjective
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2024
How to use epicycle in a sentence
But even he retained epicycles and excentrics, and could not explain the unequal orbits of planetary motion.
Beacon Lights of History, Volume VI | John LordThey even supposed the moon to have one, perhaps two epicycles and we shall find this notion reflected in Chaucer.
Astronomical Lore in Chaucer | Florence M. GrimmAs new irregularities of motion of the sun, moon, and planetary bodies were pointed out, new epicycles were invented.
A History of Science, Volume 1(of 5) | Henry Smith WilliamsThe centres of the epicycles of Mars, Jupiter, and Saturn were supposed to be further away than the sun.
History of Astronomy | George ForbesThe planets move in a similar way on epicycles, but their deferents have no geometrical or physical relation to the sun.
History of Astronomy | George Forbes
British Dictionary definitions for epicycle
/ (ˈɛpɪˌsaɪkəl) /
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
a circle that rolls around the inside or outside of another circle, so generating an epicycloid or hypocycloid
Origin of epicycle
1Derived forms of epicycle
- epicyclic (ˌɛpɪˈsaɪklɪk, -ˈsɪklɪk) or epicyclical, adjective
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
Scientific definitions for epicycle
[ ĕp′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 Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
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