RETROGRADE MOTION

in astronomy, actual or apparent motion of a body in a direction opposite to that of the (direct) motions of most members of the solar system or of other astronomical systems with a preferred direction of motion. As viewed from a position in space north of the solar system (from some great distance above the Earth's North Pole), all the planets revolve counterclockwise around the Sun, and all but Venus, Uranus, and Pluto rotate counterclockwise on their own axes; these three, therefore, have retrograde motion. Of the more than 60 known satellites of the planets, only six display retrograde revolution. These include the four outermost moons of Jupiter; Phoebe, the outermost moon of Saturn; and Triton, the largest of Neptune's moons. The orbital planes of the satellites of Uranus and Pluto are tilted so greatly that the description of these bodies' motion as either retrograde or direct has little meaning. The revolutions around the Sun of all known asteroids are direct; of the known periodic comets, only Halley's Comet moves in a retrograde orbit. A separate sense of the term retrograde motion refers to the apparent brief reversal of the motion of a planet as seen from Earth; the effect depends upon the difference in orbital speeds of the planets.