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The Question

(Submitted October 26, 1998)

Why is the rotation of Venus retrograde, or East to West - unlike nearby planets?

The Answer

We got two good answers to this question from our distribution of scientists. Here they are:

"The standard answer to this question and things like Neptune's tilt is that there was a large collision early in the planetary formation process. The models of planetary and solar system formation have the angular momentums of the planets and their orbits in the same direction as the initial angular momentum of the gas cloud. You need something like a collision to get anything else." - Eric Christian

"Of the nine planets, a bare majority (Earth, Mars, Jupiter, Saturn, and Neptune) rotate in a way we consider 'normal'. Mercury and Venus are slow, Venus, Uranus, and Pluto are retrograde, Uranus and Pluto are highly inclined. Mars' inclination varies chaotically over long (billion-year) time scales, so it is not always 'normal' either. It is only parochialism that makes us point and laugh at the zany antics of the other planets.

"How a planet rotates is related to how it was formed from the accretion of planetesimals. If more impacts occur on one side than the other, then it will tend to rotate accordingly. But the impacts are largely random. Tidal effects can also change the rotation." -David Palmer

Hope this helps to answer your question!

Maggie Masetti & Koji Mukai
for Ask an Astrophysicist

Questions on this topic are no longer responded to by the "Ask an Astrophysicist" service. See http://imagine.gsfc.nasa.gov/docs/ask_astro/ask_an_astronomer.html for help on other astronomy Q&A services.

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