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Why do planets rotate?

  1. Dec 1, 2006 #1
    Hi.


    I cant seem to understand why a planet has to rotate about its own axis as it revolves about the sun. Why cant it just revolve around the sun without any rotation? What is the cause of rotation of a planet?

    Also, on what factors does the angle between the axis of rotation and the plane in which the planet moves depend? ( some planets roll along the plane of motion others spin like a top..why?)

    Regards.
     
  2. jcsd
  3. Dec 1, 2006 #2
    they don't have to. if tidal forces are strong they may lock one face, as with our moon, which requires the rotation to take as long as the orbit. sometimes the rotation may be in an integer ratio to the orbit as with mercury.

    i'm guessing that much of planets rotation comes from the angular momentum from the time of their formation
     
  4. Dec 1, 2006 #3

    HallsofIvy

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    It's not necessary for something to "cause" rotation. You are really asking "why is it that the rotation speed of the planets is not exactly 0?"
    It would be very surprising if the rotation speed of a planet were just exactly any pre-determined number!
     
  5. Dec 1, 2006 #4
    agreed, but zero rotation is a teeny weeny bit more 'special' than any other amount of rotation

    actually i don't know enough about mach's principle or frame dragging or gr in general to know if there is an absolute zero for angular momentum. but i'd be grateful for someone to enlighten me

    if i were alone in the universe and was spinning, would frame dragging spin the universe with me, rendering the spin not measurable?
     
    Last edited: Dec 1, 2006
  6. Dec 1, 2006 #5

    Garth

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    Good question, but a hypothetical situation.

    According to Mach's Principle the spin would not be measurable, according to GR it would be. Frame dragging in GR would cause a rotation of inertial compasses that is only a tiny amount of that of the whole spinning mass. Edit: However according to GR your rotation would be obvious from centrifugal and coriolis forces as you waved your arms about but in a fully Machian gravitational theory these inertial forces would disappear!

    Now, where can I find an empty universe to try this out.......

    Actually the Gravity Probe B satellite has already measured the Frame Dragging and Geodetic precessions of gyroscopes in Earth Polar orbit. The results should be published next April, so perhaps we do not need an empty universe to check this out! :smile:

    Garth
     
    Last edited: Dec 1, 2006
  7. Dec 1, 2006 #6

    russ_watters

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    In a system where the angular momentum is not precisely zero, things will rotate and revovle. The only way the solar system's angular momentum could be exactly zero is if it were exactly uniform in shape not rotating to begin with. And if that had happened, we wouldn't be here because there'd be no objects orbiting the sun! So the same angular momentum that led to the orbits of the planets led to their rotation.

    For your other question, they rotate in the same direction they orbit unless they have been purturbed to rotate in another direction.
     
  8. Dec 1, 2006 #7

    SpaceTiger

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    The angular momentum in the solar system (including both orbits and rotations) initially came from the molecular cloud that collapsed to form the sun and solar system. The molecular cloud, in turn, was probably torqued by other nearby clouds, as well as catastrophic events like supernovae. Remember that when an object collapses, its rotation speed increases in order to conserve angular momentum. Thus, the molecular clouds were only rotating very slowly prior to collapse, but it was enough to induce the rotation we see today.

    As for how the angular momentum was transferred to the rotation of planets, we're still not entirely sure. Certainly, there was some torquing in the young protoplanetary disk, perhaps induced by disk instabilities or tidal interactions between nearby planetesimals. In addition, collisions would have acted to randomize planetary rotations a little bit -- indeed, Venus and Uranus rotate retrograde.
     
  9. Dec 1, 2006 #8
    Alright. Thanks. So planets don't 'have to' rotate but due to reasons end up doing so.
    I don't know much of Mach's principle or Frame dragging so i didn't quite catch that but on the whole got my answer.

    Do planets experience reactive gyroscopic effects, meaning do their axis of spin end up precessing as they revolve?

    Ninad.
     
  10. Dec 1, 2006 #9

    SpaceTiger

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    That's right; in fact, we think that the universe exhibits no net rotation and that initially everything will start with little or no angular momentum. It is only through subsequent interactions (mostly through gravitational tidal forces) that macroscopic astronomical objects (e.g. galaxies, nebulae) begin to rotate.


    Yes, in fact, the tidal forces of the sun and moon induce just such precession on the earth.
     
  11. Dec 2, 2006 #10

    Chronos

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    Oh well, ST already explained it. Planets rotate [wrt their friendly neighborhood mother star] in abeyance to the laws of conservation of momentum. This is how stars form. The mother gas pocket rotates as it gravitationally collapses. This is called accretion. Planets are tiny swirls of turbulence in the accretion disk. Newtons principles are very much in play here. It is logical to assume the constituent mass of planet Earth [or any other planet] was already rotating when it formed. The old 'objects in motion tend to stay in motion until acted upon by an outside force' principle applies. Some planets, like Venus, were knocked out of their original axis inclination during the dark ages of the solar system.
     
    Last edited: Dec 2, 2006
  12. Dec 2, 2006 #11
    Any effect supernovae - or any significant body of matter - might have on a fetal Solar System would be negligible. The gravitational pull of a body many light-years away is negligible and the Moment (or torque) of such gravitational force would be just as negligible , especially if the molecular cloud is considered relatively uniform in density. Let me quickly mention that Supernovae are no more special than stars and other gigantic clumps of matter - they all provide varying degrees of a tremendous gravitational pull, but one which nearly vanishes as we get further and further away. Aside from an interesting example, they are no more special in this example than ANY other body of matter capable of exerting a similar gravitational pull.

    I argue the answer lies in statistics. The effect of collisions between increasingly larger particles is orders of magnitude greater than the gravitational torque resulting from large bodies such as supernovae. Additionally, the probability that such collisions will be off-center are far greater than a centric collision - after all, the center of mass of a clump of matter is a point, occupying an infinitesimal volume. Any collision in any line not passing through this point will cause a resulting Moment and thus a rotation.
    Every body in our Solar System appears scarred by such collisions, which have involved particles of increasingly larger dimensions. The Moon provides a great example. Mars another. Mercury, even Earth with all its environmental processes which tend to mask such happenings. The frequency of such collisions is great - was greater still in the early stages of the solar system - and the effects of such collisions of potentially great significance.

    Very interesting question and even better points. Some things you don't even think about anymore, but which when you try to answer you realize you really have no answer.
     
    Last edited: Dec 2, 2006
  13. Dec 2, 2006 #12

    SpaceTiger

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    The forces applied to a molecular cloud by a supernova would not be gravitational, but rather hydrodynamic. Molecular clouds, however, can be extremely massive, sometimes with millions of solar masses. Close encounters would certainly result in sizable gravitational torques and no, they're nowhere near uniform in density, the cores being considerably more dense than the outskirts.


    This theory fails to explain the fact that both the revolution and rotation of the majority of planets is aligned along the same axis. There certainly is randomness associated with subsequent collisions, but this must also conserve angular momentum. Clearly, the solar system has a sizable net angular momentum that can't be explained by collisions alone.
     
  14. Dec 7, 2006 #13

    Garth

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    Also remember that almost certainly the solar system did not form as an isolated system but part of a self-gravitating collapsing molecular cloud in which many systems were forming more or less simultaneously.

    Furthermore, some of the giant stars that formed in the system might have run through their fuel stock quickly and gone supernova while still in close proximity. Evidence that this might have happened close to the Sun is provided by the Allende meteorite
    The distance between neighbouring systems collapsing in Bok globules would be much closer than distances between star systems today.

    In fact they could form while interacting with each other.

    In such a case inter-system gravitational as well as hydrodynamic and indeed magnetohydrodynamic forces could have been significant.

    Such interactions might also explain our Solar System's angular momentum budget.

    Just a thought...

    Garth
     
    Last edited: Dec 7, 2006
  15. Dec 14, 2006 #14
    Tide locking?

    Can someone please explain tidal forces and the mechanism for two bodies to become tide locked?
     
    Last edited: Dec 14, 2006
  16. Dec 15, 2006 #15

    DaveC426913

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    No natural bodies are perfectly round; they have mountains and valleys. When a body such as the the Moon is rotating while in the influence of another body such as the Earth, the gravity acts upon these protrusions.

    When the Moon was once rotating faster than its revolution about the Earth, the mountains would be slightly stronger attracted to the Earth when they were aligned along the Earth-Moon axis. As the lunar mountains rotated away from that axis, they create drag - the Earth continues to pull at them. This drag has the effect of slowing the rotation slightly. After a long time, the Earth has sapped all angular momentum from the Moon until its longest axis (again. no natual object is perfectly round) is aligned with the Earth-Moon axis.

    This same force, working in the opposite direction (too slow rotation will cause mountains to be dragged faster, rather than slower), also ensures the Moon doesn't slow down further.

    Thus, the most stable position is the Moon is aligned with the Earth-Moon axis.

    BTW, thise effect is not limited to large bodies. If you leave satellites in orbit without attitude adjustment, they will eventuially get tidally locked too. In fact, they don't neeed to be rotating. A satellite placed in orbit with its long axis parallel to the Earth's surface will eventually be tugged into an orientation where its long axis is perpendicular to the Earth's surface.


    And a final note: this tiny force, when coming from a black hole, multiplied millions of times, is what kills you when falling into the black hole. Simple tides.
     
    Last edited: Dec 15, 2006
  17. Dec 15, 2006 #16

    SpaceTiger

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  18. Dec 15, 2006 #17
    To summarize, the moon (in this case) was slowed due to a difference in gravitational pull on surface features of the moon, and finally balanced out with the "high mountains" (the side with the most mass) facing the earth? Would a dropped yo-yo be a good analogy for the final phases of this action? A pendulum?

    I’m trying to put together an answer for a 9-year old, and all he sees is the same side of the moon facing the earth.

    Thank you
     
    Last edited: Dec 15, 2006
  19. Dec 15, 2006 #18

    DaveC426913

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    Well, not like a yoyo, no. A yo-yo picks up angular momentum that causes its rotation to continue after the end of the string is reached, winding it back up again.

    The Moon didn't slow down "too much" before speeding up again.

    Well, OK - think of it like a yo-yo that's operated by a novice. It unrolls, but does so reaaaaalllllly sloooowwwwwly, so that there's no follow through.


    Try this on your 9-year-old.


    Tell him to pretend he's on a merry-go-round.
    Tell him to imagine liying down on it so that his head and body are the same distance from the centre (i.e. he is parallel to the outer edge).
    Tell him he's tied with a strong rope so he can't fly off.
    Now spin the merry-go round up really fast.
    Will he be able to keep his head and body the same distance from the centre? No. What will happen is his feet will end up pointing straight off the merry-go-round while his head will point straight at the axis.
    If he tries to get back to his original position, he will have a tough time doing so. He will be in a stable orientation.

    The side of the Moon we always see is like his "feet". The side of the Moon we never see is like his "head".

    This is not an accurate analogy, and don't let him carry it too far, but it shows why bodies under external forces will align themselves along their long axes.
     
    Last edited: Dec 16, 2006
  20. Jan 16, 2007 #19
    angular momentum. if you throw a bunch of rocks together and make a planet, it wont be sitting still, itll be spinning and travelling
     
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