retrograde motion


retrograde motion

In astronomy, the actual or apparent motion of a body in a direction opposite to that of the predominant (direct or prograde) motions of similar bodies. Observationally and historically, retrograde motion refers to the apparent reversal of the planets' motion through the stars for several months in each synodic period. This required a complex explanation in Earth-centred models of the universe (see Ptolemy) but was naturally explained in heliocentric models (see Copernican system) by the apparent motion as Earth passed by a planet in its orbit. It is now known that nearly all bodies in the solar system revolve and rotate in the same counterclockwise direction as viewed from a position in space above Earth's North Pole. This common direction probably arose during the formation of the solar nebula. The relatively few objects with clockwise motions (e.g., the rotation of Venus, Uranus, and Pluto) are also described as retrograde.

This entry comes from Encyclopædia Britannica Concise.
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