epicycle

[ ep-uh-sahy-kuhl ]

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.

epicycle

/ 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

epicycle

/ 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.

Derived Forms

• epicyclic, adjective

Other Words From

• ep·i·cy·clic [ep-, uh, -, sahy, -klik, -, sik, -lik], adjective

Word History and Origins

Origin of epicycle¹

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

Word History and Origins

Origin of epicycle¹

C14: from Late Latin epicyclus, from Greek epikuklos; see epi- , cycle

Example Sentences

To account for the planets’ bizarre behavior, Ptolemy added epicycles to his planetary clockwork: little circles within circles could explain the backward, or retrograde, motion of the planets.

From Literature

The self-sealing dynamic can produce even more elaborate epicycles to resist falsification.

From Scientific American

First, that each planet, plus the Sun and Moon, had to be treated separately in the model, with its own offset from the Earth and its own epicycles.

From Literature

The problem for Kepler was that circles, eccentrics and epicycles were geometrical constructions; there was no evidence that any such gearing existed in the heavens.

From Literature

After all, the geocentric Ptolemaic theory of epicycles was mathematically appealing and its framework was broad enough to describe the motion of all planets on the sky.

From Scientific American