epicycle

American  

[ep-uh-sahy-kuhl] / 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.

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epicycle British  

/ ˌɛpɪˈsaɪklɪk, ˈɛpɪˌsaɪkəl, -ˈsɪklɪk /

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

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epicycle Scientific  

/ ĕp′ĭ-sī′kəl /

1. 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.

2. See illustration at Ptolemaic system

3. A circle whose circumference rolls along the circumference of a fixed circle, thereby generating an epicycloid or a hypocycloid.

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Other Word Forms

• epicyclic adjective

Etymology

Origin of epicycle

1350–1400; Middle English < Middle French < Late Latin epicyclus < Greek epíkyklos. See epi-, cycle

Example Sentences

Ptolemy solved the problem of explaining the observed motions of planets by having each planet revolve in a small orbit called an epicycle.

From Textbooks • Oct. 13, 2016

Each planet orbits around a small circle called an epicycle.

From Textbooks • Oct. 13, 2016

Each epicycle orbits on a larger circle called the deferent.

From Textbooks • Oct. 13, 2016

According to the Ptolemaic system, it travelled on a large epicycle, which brought it sometimes closer to and sometimes further from the Earth.

From "The Invention of Science" by David Wootton

Different hypotheses might produce the same result: for example an eccentric circle will generate exactly the same movement as an epicycle upon a deferent.

From "The Invention of Science" by David Wootton