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Why do we only see one side of the moon?

  1. Jul 5, 2003 #1
    My friend who is taking an Astronomy course said that the Moon and the Earth rotate at the exact same speed because the Earth and Moon were both hit by something at the same time... or something to that effect. It didn't really make logical sense to me.

    Does anyone know why we only see one side of the moon?
     
  2. jcsd
  3. Jul 5, 2003 #2
    Greetings!

    I think he has something wrong there as well. The moon and the earth do not rotate at the same speed. Rather, the moon rotates at the same speed that it revolves around the earth. This is because it is tidal locked in the earth's gravity.

    This site has some good info for you:
    http://starryskies.com/The_sky/events/lunar-2003/eclipse9.html
     
  4. Jul 6, 2003 #3

    Phobos

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    Welcome to Physics Forums, Azrioch! :smile:

    Like Brad_Ad23 said, the Moon and Earth don't rotate (spin on their axis) at the same speed.

    Visual aid...Take a quarter and a penny (or whatever 2 Italian coins that have faces). Lay them flat and move the penny around the quarter (orbit) such that the face on the penny is always facing the quarter. See that the orbital period of the penny (once around the quarter) matches the penny's period of rotation (one spin on its own axis). That's essentially the earth-moon situation.

    As explained by the links provided, this is due to gravity which pulls the Earth and Moon into a slightly elongated shape (like how the moon rises the ocean tides on the Earth...this works on rock too just to a much lesser extent). As that elongated shape rotates forward (with the rotation of the planet/moon), there is a slight braking action as the gravity of the other pulls back on that raised portion. The Moon's rotation has been slowed to a point where it's rotation speed matches it's orbital period. So it maintains the same face toward the Earth. But the moon still rotates with respect to the sun (as you see from the quarter/penny example).

    more helpful links...
    http://itss.raytheon.com/cafe/qadir/q866.html
    http://www.seds.org/billa/tnp/luna.html
     
  5. Jul 7, 2003 #4
    If the same side of the moon is always facing earth then it is not rotating around it's own axis at all. According to GR, the moon is following a "straight path" around the Earth.
     
  6. Jul 7, 2003 #5

    chroot

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    You're an idiot.

    - Warren
     
  7. Jul 7, 2003 #6

    selfAdjoint

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    If you face a lampost and walk around it, always facing toward it, you will find that the landscape behind it rotates the same way it would if you stood still and turned around. In fact you will successively face every point of the compass. To put it shortly, you will have turned around. And that's what the moon does vis-a-vis the earth.
     
  8. Jul 8, 2003 #7
     
  9. Jul 8, 2003 #8

    russ_watters

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    So unnecessary.[zz)]
     
  10. Jul 8, 2003 #9
    Hi Warren,
    How are you doing, I mean really?

    You seem like such a nice guy and your comments demonstrate your absolute brilliance in deconstructing the errors you find in all of these silly questions!!! ((who needs questions anyway when one has all the answers?... oops!))

    Keep up the good work and maybe some day you will win the Nobel Prize for insightful criticism!!

    :smile:
     
    Last edited by a moderator: Jul 8, 2003
  11. Jul 8, 2003 #10

    chroot

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    Hell, I'd be happy with the Nobel in physics, personally.

    Some questions beg answers; some statements beg intelligent counterpoint. Others... simply aren't worth it.

    - Warren
     
  12. Jul 8, 2003 #11
    And so why make the effort to belittle someone? Oh yes, it is to state your superiority for everyone to see. What else could it be?
     
  13. Jul 8, 2003 #12

    chroot

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    Why yes, you're correct. I am, in fact, better than you. Good of you to see it. :)

    - Warren
     
  14. Jul 8, 2003 #13
    so your ego escapes into the open...
     
  15. Jul 8, 2003 #14

    chroot

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    Was it ever contained???

    - Warren
     
  16. Jul 8, 2003 #15

    ok so it has taken over your rationality...which apparently is not hard to do.
     
    Last edited by a moderator: Jul 8, 2003
  17. Jul 8, 2003 #16
    To respond to SelfAdjoint,

    In a very weak sense you are correct. Orbits are merely the paths of least action in a warped spacetime, however, it does not necessitate an object in orbit not rotate. See for example: The earth rotates as it revolves around the sun.

    It is merely that at certain distances, gravity tidal locks objects into a rotational period equal to the revolutionary period.
     
  18. Jul 8, 2003 #17
    i read also that the moon is moving away from the earth.. something like an inch a year.. or half inch..

    would take some time to break free i guess
     
  19. Jul 8, 2003 #18

    chroot

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    Yep, the Earth is slowing its rotation, becoming tidally locked to the Moon. To conserve angular momentum, the Moon's orbit increases a little.

    - Warren
     
  20. Jul 8, 2003 #19
    how come planets that are in the suns orbit don't drift towards the sun slowly?
     
  21. Jul 8, 2003 #20

    chroot

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    Er, why would they?

    - Warren
     
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