Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Our moon does not turn ?

  1. Feb 25, 2005 #1
    Our moon does not turn ??

    Not at all ??
    One would think that after a milenia of being bombarded by comet debris and asteroids that they would have nudged it a little..?
    couldnt we spin it ourselves .?
    I for one am tired of looking at the same view of the moon ,every full moon same picture.... :mad:
  2. jcsd
  3. Feb 25, 2005 #2


    User Avatar
    Science Advisor
    Homework Helper

    The moon does turn. It completes one rotation in about 28 days. Since it also completes one orbit in about 28 days, we see the same side.

    Given enough time and without outside interference, all the planets rotation periods (including Earth) would match their orbit period around the Sun and all of the planets' moons would do likewise. The Sun would be one such example of outside interference that would inhibit the Moon's rotation and orbit periods around the Earth from 'synching up'. Likewise, the Moon would be an example of outside interference that would inhibit the Earth from synching up it's rotation and orbit period around the Sun.
  4. Feb 25, 2005 #3


    User Avatar

    Staff: Mentor

    But why: tidal forces make the moon bulge at the center, which in turn causes a force imbalance that keeps it tidally locked with the earth.
  5. Feb 26, 2005 #4
    Is it possible that :

    1) moon got shot out of earth in the beggining of solar system and ever since it faces earth since it was slowly detached from earth without allowing it to rotate in any other sides ?

    2) moon has a dense core that is slightly off its centre, due to which only one side that is denser is facing the earths gravity ?
  6. Feb 26, 2005 #5


    User Avatar
    Science Advisor
    Homework Helper

    No, Russ is right about the reason for tidal lock (although your second reason is kind of right).

    Think about the tides of the ocean and you'll realize he's right. High tide occurs on the Moon facing side of the Earth and the opposite side from the Moon.

    Same thing happens with the planet's or moon's interior, but much, much, slower. Take Earth for example - any minisule deformity caused by the Sun's gravity at 0 degrees longitude when it's local noon over 0 degrees is approximately corrected when it's 6:00 local time over 0 degrees. However, unless the orbiting object is of a perfectly uniform distribution, one alignment of the planet is just slightly more stable gravitationally than any others, so the corrections are never really perfect.

    If the planet were rod shaped instead of spherical, the effect would be much more obvious. The long axis of any orbiting object tends to align itself with the object's radius.

    Your second answer is only kind of right because the Moon's going to rotate about it's center of mass, not off to the side of it. But your general concept is right.
  7. Feb 26, 2005 #6


    User Avatar
    Gold Member

    Following up on BobG's look at Earth's tides.

    Earth's tides follow the Moon around our planet. Or more accurately, the tides don't move while the Earth spins underneath them every 24 hours.

    What would happen to those tides if all the water in the oceans froze instantly? They would try to follow the Moon, but, being solid, they would have to drag the rest of the Earth with them. The drag would cause the Earth to spin spin down until the tidal bulges were aligned with the Moon.

    The Moon already is solid. It still has tides, but they're solid - inches not feet or yards. But it's enough. After billions of years, the Moon's rotation has slowed until any bulges are aligned with Earth.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook