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Planetary rotation

  1. Apr 9, 2010 #1
    Why does the Sun and all of the planets except Mercury rotate in the same direction?
  2. jcsd
  3. Apr 9, 2010 #2
    Do you mean mean retograde orbital motion or retrograde rotation?

    Well if you mean rotation, that is the spinning movement of the planet on it's own axis, in our Solar System, all the planets, except for Venus and Uranus, rotate in the same direction as their orbital motion and Sun's rotation. Well if you accept the idea that all the planets and stars formed from the same molecular cloud, and due to angular momentum conservation it would be expected that their rotation and orbital movement would be in the same direction. Collisions with other objects is one possible explication for their different movements.

    Now if you mean orbital retrograde motion, actually, all of the planets orbit in the same direction as the Sun's rotation. But there is an effect, that if you were in the Mercury's surface, when Mercury is at perihelion, it's angular orbital velocity exceeds it's rotation velocity, so the Suns appears to move in the retrograde direction. Note that this an observational effect if you were at Mercury's surface.
    Last edited: Apr 9, 2010
  4. Apr 13, 2010 #3
    I have a point of curiosity that is related to this question. The Moon is face-locked to the earth because any rotation energy that would exceed one turn per orbit around the earth has been dissipated by tidal movements (right?). This is a result of its relatively close orbit for its size. The question is this: How much rotational energy would have to be added to the moon for it to turn around completely once per earth year? That is, in one year, we would see the Moon turn completely once, seeing the far side.
    Second, how long would it take for this added energy to be dissipated so that the Moon returned to its face-locked state? Thousands of years? Millions of years?
    Third, how would this energy dissipation be manifested? Entirely as heat, as the oceans and crust of the earth, moving tidally, converted it to molecular motion? Or would any of it be manifest as a change, however slight, of the rotation speed of the earth? Would it make any difference if the Moon's added rotation was prograde or retrograde? (Viewed from the earth, the moon would turn right-to-left or left-to-right.)
    This is, of course, not going to happen. I would assume any mass striking the moon hard enough and at the right angle to add this rotation would make quite a mess, and we would not be around long enough to enjoy the show. It's just an exercise for the curious mind, and I'm not learned enough to know how to begin.
  5. Apr 14, 2010 #4


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  6. Apr 14, 2010 #5


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  7. Nov 18, 2011 #6
    Maybe it just shows God has a sense of humour, and we humans are idiots.
  8. Nov 21, 2011 #7


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    Or maybe it is a result of complex celestial mechanics.
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