Epicycle
From LoveToKnow 1911
EPICYCLE (Gr. Esri, upon, and icbsXos, circle), in ancient astronomy, a small circle the centre of which describes a larger one.
Epicycle
Related Category: Astronomy: General
see Ptolemaic system.
More on Epicycle
Ptolemaic System - historically the most influential of the geocentric cosmological theories, i.e.
As a planet moves around on its epicycle, the center of the epicycle (called the ``deferent'') moves around the Earth. When its motion brings it inside the deferent circle, the planet undergoes retrograde motion.
Deferent and epicycle
The basic elements of Ptolemaic astronomy, showing a planet on an epicycle (A) with a deferent (C) and an equant (B).
epicycle A construct of the geocentric model of the solar system which was necessary to explain observed planetary motions. Each planet rides on a small epicycle whose center in turn rides on a larger circle (the deferent).
epicycle: a small circle whose centre moves around the circumference of a larger one. It was proposed as the shape of the orbit of some of the planets by Ptolemy.
Epicycle
An attempt to explain the retrograde loop in the earth centered universe, by attaching the planets to epicycles and having them revolve around that, and in turn around the earth.
Equant ...
epicycle: The small circle followed by a planet in the Ptolemaic theory. The center of the epicycle follows a larger circle (deferent) around Earth.
Epicycle - One of the circles upon which a planet moved according to the Ptolemaic (geocentric) model of the solar system. The center of the epicycle moved on a larger circle, called the deferent ...
Epicycle
(a) Circular orbit of a body round a point that is itself in a circular orbit round a parent body. Such a system was formulated to explain some planetary orbits in the Solar System before they were known to be elliptical. [A84] ...
EPICYCLE
Epicycles are circular orbits within orbits that were used to (incorrectly) describe the orbits of objects in the Ptolemaic system (about A.D. 150).
The epicycle and deferent are a way to explain the retrograde motions of the outer planets. The deferent is the large circle that is also eccentric (Earth not in the middle), and upon this large circle, the epicycle is locatd.
In some cases, epicycles were themselves placed on epicycles, as illustrated in the adjacent figure.
epicycle (From Stargazers to Starships Glossary - GSFC) A circle around a point which moves steadily around the celestial sphere.
Anomalies in a planet's motion were accounted for by the use of the epicycle, a circle centered on the circumference of a larger circle called the deferent.
Moreover, the system of movable eccentrics, and that of epicycles and deferents, accounted well for most of the irregularities observed in the motions of the Sun, the Moon, and the planets.
Perturbation theory has its roots in 17th century celestial mechanics, where the theory of epicycles was used to make small corrections to the predicted paths of planets.
The planets were thought by Ptolemy to orbit the Earth in deferent and epicycle motions.
This model generally led to reasonable predictions of planetary positions, but it has several unexplained coincidences: the centers of the epicycles for Mercury and Venus always line up with the position of the Sun, ...
He thought that each space body moved in a small circle (an epicycle) that was itself orbiting Earth. This explained why planets sometimes appeared to travel backward in the sky.
In Ptolemy's model, each planet is moved by two or more spheres (or strictly speaking, by thick equatorial slices of spheres): one sphere is the deferent, with a center offset somewhat from the Earth; the other sphere is an epicycle embedded in the ...
These circles upon circles were called "epicycles," and the larger circles were called "deferents." This is shown in the image to the right. This is a "three planet" system, with the Earth at the center, and two planets around it.
The deferent is the large circular orbit around which a planet was thought to orbit, in one or many epicycles. Epicycles are circular orbits within orbits that were used to (incorrectly) describe the orbits of objects in the Ptolemaic system (about A.
As more and more observations were made that did not fit Ptolemy's epicycles, new theoretical factors were added into the system, which became more and more complicated.
In the Ptolemaic universe, planet moves in a small circle called an epicycle, and the center of the epicycle moves along a larger circle around the Earth.
scientists could be quite ingenious--Eratosthenes in estimating the size of Earth, Hipparchus in locating the Sun's place in the sky by observing an eclipse of the Moon, Aristarchus in proposing his heliocentric theory, and even Ptolemy's epicycles ...
His theory, unfortunately, still had some serious defects, among them circular as opposed to elliptical orbits and epicycles, that made it no more precise in predicting ephemerides than the then current tables based on Ptolemy's model.
But the observational evidence of the time favoured the epicycle-based Ptolemaic system.
The passage of a celestial body or point across the ephemeris meridian. [S92]
Epicycle ...
He noted each star's position and brightness (he rated the brightness on a scale from 1 to 6, the brightest being 1). Hipparchus also devised the system of epicycles, ...
Small radial oscillations (epicycles) have an axial ratio 2:1 (tangential:radial extent - we'd have been in real trouble if Hipparchus had tried that for the planets instead of perfect circles). The epicyclic frequency is given by ...
 See also: Planet, Earth, Sun, Astronomy, Orbit
  
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