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What would happen if the moon moved closer?

Discussion in 'Science & Technology' started by Harbringer, May 31, 2010.

  1. Harbringer

    Harbringer Your A One Flower Garden

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    What would be the effects of the moon moving closer to the earth, like say 100 miles, or one thousand miles?
     
  2. warpus

    warpus In pork I trust

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    The first effect I can think of is the tides would become a lot stronger. This would affect a whole bunch of stuff.
     
  3. Sterf

    Sterf Warlord

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    A 100 or even a thousand miles wouldn't make much of a difference I'd say. The lunar cycle would be shorter, tides stronger (as would gravity as a whole).

    As it is it's moving away from us at about an 4 centimeters a year.
     
  4. Hakim

    Hakim Parasocial

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    If it was closer to earth and had the same speed, it would (eventually) crash into earth because it wouldn't have the higher speed required to balance earth gravitation at the lower orbit.
     
  5. Olleus

    Olleus Deity

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    Conservation of momentum and energy means that if someone pushed the moon closer to earth, the length of the day would lengthen.
     
  6. uppi

    uppi Deity

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    No. It would just go on an elliptical trajectory. Conservation of angular momentum means that to crash the moon into the earth you need to bring them very close (or destroy almost all angular momentum) so that the trajectory of the moon intersects the earth.

    Despite some popular opinions, that it would only require small disturbances to send a moon or planet into a death spiral, this is actually impossible without massive energy transfer.
     
  7. Perfection

    Perfection The Great Head.

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    Well even if you have a large disturbance you won't get a death spiral.
     
  8. Hakim

    Hakim Parasocial

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    :blush: Ouch. Thanks for correcting me though.
     
  9. Leifmk

    Leifmk Deity

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    The closest you can get is if you make the orbit eccentric enough that it encounters significant (on a relevant timescale) drag from the primary's upper atmosphere during the closest approach. Then the orbiting body will gradually lose momentum and energy on every orbit and eventually experience terminal aerobraking/lithobraking.

    Still won't be a spiral trajectory, mind you. And yes, for something orbiting far out (like the Moon around the Earth, or even something in geosynchronous orbit) the energy required for such a change is rather a lot per unit mass; not so much for something that's already in a low orbit (like the ISS).
     
  10. Gigaz

    Gigaz civoholic

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    That would not work. The moon is has much more weight than the earths athmosphere. The moon would break apart because of the inhomogenous gravitation long before it lost any measurable part of its momentum.
     
  11. Olleus

    Olleus Deity

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    A quick calculation shows that if one edge of the moon was touching the outer atmosphere (NASA says that 120km high up) then there would be a net force of 4 Nkg-1 between the two sides of the moon. I guess that would probably be enough to break it up, but Im not sure. Thats only a difference of 2 x10^-6 N kg-1 for every meter you travel, which feels tiny.
    Perhaps someone with more knowledge of statics could answer this?
     
  12. Perfection

    Perfection The Great Head.

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    As Gigaz notes, it really doesn't matter, if the Moon was that close, tidal forces would rip it to shreds anyways.
     
  13. Leifmk

    Leifmk Deity

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    By this point in the thread we were clearly no longer talking specifically about the moon orbiting the earth but about any orbiting body about a planet, or whatever.

    Case in point: Low-orbiting satellites actually do experience very slow aerobraking, and will not remain stable in the long run.
     

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