Retrograde and direct motion

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Artist's conception of a protoplanetary disk

Direct motion is the motion of a planetary body in a direction similar to that of other bodies within its system, and is sometimes called prograde motion.

Retrograde motion is motion in the opposite direction. In the case of celestial bodies such as the retrograde Carme group of moons, such motion may be real, defined by the natural rotation or orbit of the body.

Every so often a planet seems to change direction and go the other way for a short period of time (in relation to the fixed stars). It makes a "retrograde loop" or squiggle in its track before carrying on as normal. This apparent retrograde motion, however, is only seen because the Earth is moving. It is only seen when outer planets are near opposition.

Prograde motion in planetary systems[change | change source]

Prograde or direct motion is usually the case. That is because both the star and its planets formed out of the same disk of gas. By definition the disk rotates all in the sane direction because gravity links all the objects which are formed by the shrinking of the primordial disk of gas.

All eight planets in the Solar System orbit the Sun in the direction of the Sun's rotation, which is counterclockwise when viewed from above the Sun's north pole. Six of the planets also rotate about their axis in this same direction.

The exceptions – the planets with retrograde rotation – are Venus and Uranus. Venus's axial tilt is 177°, which means it is rotating almost exactly in the opposite direction to its orbit.[1]

Uranus has an axial tilt of 97.77°, so its axis of rotation is approximately parallel with the plane of the Solar System. The reason for Uranus's unusual axial tilt is not known with certainty. During the formation of the Solar System, it is thought an Earth-sized protoplanet collided with Uranus, causing the skewed orientation.[2] But see also Grand tack hypothesis.

The orange moon is orbiting in the opposite direction to its primary

References[change | change source]

  1. McBride, Neil; Bland, Philip A.; Gilmour, Iain 2004. An Introduction to the Solar System. Cambridge University Press. p. 248. ISBN 978-0-521-54620-1.
  2. Bergstralh, Jay T.; Miner, Ellis; Matthews, Mildred (1991). Uranus. pp. 485–86. ISBN 978-0-8165-1208-9.