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How do the Planets and Sun get their initial rotation? Why do Venus and Mercury rotate so slowly compared to other planets and why does Venus rotate in a different direction to Mercury, Earth and Mars?

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In fact, many physicist way up in the 19th century were discussing whether God was needed in order to set all the planets in motion in their orbits. Like, an initial "kick" or something to set things going. Or, if you will, to "wind up" the whole Universe (Just like we think of the Universe much in computer terms today, they thought of it much in steam engine and wind-up toy terms). –  Thriveth Jan 22 at 18:17
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3 Answers

up vote 8 down vote accepted

Anglular momentum is conserved, so any tiny initial rotation that a the original ball of gas had becomes faster as the gas collapses down into a star and disk of planets.

Planets near the sun rotate slowly for the same reason that the moon always faces the same side to the Earth - tidal braking

Venus probably received a hit from a some lump of rock / proto-planet some time early in it's life which changed it's rotation. A similar event split the moon off from Earth

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To build on Martin Beckett's answer (especially because I am not sure how familiar you are with physics);

Currently we believe Stars form when objects known as Molecular Clouds (which are as one might guess, clouds of molecules in space, mostly comprised of hydrogen) collapse. It is important to note that these clouds are not 'static', they have some kind of motion, including some kind of 'average rotation', which is to say that overall the cloud is rotating (usually fairly slowly).

As was mentioned in Martin Beckett's answer, angular momentum is conserved; the typical example to give is to imagine a spinning figure skater, as she brings her arms in close to her body, she spins faster. If you don't believe this and have access to an office chair, it is easy to convince yourself (and possibly injure yourself too...). This holds true for the molecular cloud as well. As it collapses in on itself, it starts rotating faster and faster, forming a disc. The bulk of this coalesces into a big ball of hydrogen at the centre, which will eventually form a star. The matter in the disc slowly starts to clump together more and more to form the planets (it's a little more complicated than this, but if you're interested it's an easy topic to read up on). Similarly to how the overall cloud starts spinning faster and faster, the matter that forms these planets was spinning and maintains its spin as it clumps together into planets.

The previous post has covered your other questions.

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@martinb Thank you both for you reply. The related question given above, is very interesting with regards to spin, and how spin is developed. With regards to the rotation of Venus and Mercury, I thought it may well be to do with their position relative to the sun, but both planets are rotating at a far slower speed than the sun. Venus which is roughly the same size as the Earth, is rotating at 1/100th of the time it takes the Earth (and Mars) to rotate. Distance wise, proportionally, from the Sun centre, if the sun was having this effect, –  Clive Ballard Apr 1 '12 at 22:03
    
I would expect both the Earth and Mars to have different rotation rates. Both Venus and Mercury are obviously not locked to the suns rotation, otherwise they would be rotating at the same speed of the Sun, as per our Moon/Earth. As they are not locked, and in view of their small size relative to the massive variations of the Sun's surface, tidal effect would have no set pattern. It is then surprising to me, they are able to keep such a steady rotation, if it is in fact, tidal effect slowing their rotation. A personal view. Happy to be shot down, as trying to get it right in my mind. –  Clive Ballard Apr 1 '12 at 22:03
    
@CliveBallard - tidal friction is more complicated than just mass/distance, it's sensitive to non spherical-ness and any fluid on the surface. I'm sure the amount of tidal braking for mercury/venus-sun has been worked out but I don't know the details –  Martin Beckett Apr 4 '12 at 22:37
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If we leave out Earth for a moment, it appears that rocky planets slow down linearly on their rotation periods - the closer they become to the Sun. Earth is an anomaly probably because of the Moon which has speeded up its rotation up considerably. According to my graph of this linear relationship between 'day' length and distance from the Sun, an Earth without the Moon would have days which are 1,960 hours long. So the Sun must have some influence on the rotation periods of each planet.

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You may want to look up "tidal locking". –  dmckee May 15 '12 at 18:53
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