It is an element, which has no relation to reality, to which ancient astronomers resorted to explain the movements of the planets.

Until the time when Kepler discovered the three laws on planetary movements, in order to reconcile the data resulting from observations with geocentric or tolomeic theory, it was thought that each planet was animated by a double movement: one around the Earth in a large circle, called deferential, and one around a moving point on the deferer itself, called an epicycle.

From the combination of these two movements it was possible, even with some approximation, to explain the retrograde and stationary movements of the planets.

With the Keplerian discovery that planets make elliptical orbits, in which the Sun occupies one of the two foci, astronomy was able to get rid of these complex and artificial mechanisms.

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