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Retrograde and prograde motion
Retrograde motion is motion in the direction opposite to the movement of something else and the contrary of direct or prograde motion. This ''motion'' can be the orbit of one body about another body or about some other point, or the rotation of a single body about its axis, or other phenomena such as precession or nutation of the axis. In reference to celestial ''systems'', retrograde motion ''usually'' means motion which is contrary to the rotation of the primary, that is, the object which forms the system's hub.
In the Solar System, all of the planets and most of the other objects that orbit the Sun, with the exception of many comets, do so in the "prograde" direction, i.e. the same sense as the rotation of the Sun. Also the rotations of most planets are prograde, with the exceptions of Venus and Uranus, which have retrograde rotations. Most satellites of planets revolve around their planets in the prograde sense. (In the case of the satellites of Uranus, this means they revolve in the same sense as Uranus's rotation, which is retrograde relative to the Sun.) There are some satellites which orbit in the retrograde sense, but these are generally small and distant from their planets, except for Neptune's satellite Triton, which is large and close. It is thought that these retrograde satellites, including Triton, are bodies which have been captured into orbit around their planets, having been formed elsewhere.
==Formation of celestial systems==
When a galaxy or a planetary system forms, its material takes the shape of a disk. Most of the material orbits and rotates in one direction. This uniformity of motion is due to the collapse of a gas cloud. The nature of the collapse is explained by the principle called conservation of angular momentum. In 2010 the discovery of several hot Jupiters with backward orbits called into question the theories about the formation of planetary systems.〔(Turning planetary theory upside down )〕 This can be explained by noting that stars and their planets do not form in isolation but in star clusters which contain molecular clouds and when a protoplanetary disk collides with or steals material from a cloud this can result in retrograde motion of a disk and the resulting planets.〔(Stars that steal give birth to backwards planets ), New Scientist, 23 August 2011〕〔(A natural formation scenario for misaligned and short-period eccentric extrasolar planets ), Ingo Thies, Pavel Kroupa, Simon P. Goodwin, Dimitris Stamatellos, Anthony P. Whitworth, 11 Jul 2011〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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