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The Moon Motion Coincidence

  1. May 10, 2008 #1
    The moon rotates on its own axis with such exquisite precision that it turns exactly once in a time exactly equal to the time to complete its orbit around Earth. How did it get that way?

    More - if there are plausible or proven explanations, then will the Earth day sometime end up equal to the Earth year? Are there other moons in the solar system that have this relationship with the planets they orbit?

    I can accept if the Earth and Moon are seen as a double-planet system that had started out as a single lump that got split apart at some point and then settled together as they are now, then the rotational momentum of the pair orbiting each other might be much the same as the original, but I do not understand how this would make the synchrony inevitable, nor why it should apply only to Earth-Moon motion.

    PS: This started out in fun playing Pink Floyd's "Dark Side of the Moon", and ended up with "Why don't we get to see the dark side Daddy"? Umm... err gulp :|
  2. jcsd
  3. May 10, 2008 #2


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    It's no coincidence. You can Google "tidal locking" to find an explanation, or perhaps somebody else will explain it here. It's getting too late at night for me to think and write a clear explanation of the effect. :smile:
  4. May 10, 2008 #3
    yes, the orbital resonances are a result of earth-moon interactions. The moon used to be closer and rotated faster. Energy transfers due to tides the moon raises on the Earth and the deformation of the moon by the earth will cause the earth to always show the same face towards the moon, just like the moon always shows us the same face (meaning both the earth and the moon will have a rotational period of one month. But it will take a very long time for this to happen - I forget how much but I won't be around to see it and I doubt if any humans will see it either.

    As the moon recedes from the earth, it's orbital period lengthens, meaning the month gets longer. The moon's rotation will also lengthen so it will always show us the same face. It's not a coincidence - it was inevitable.

    What is a coincidence is that right now the moon's apparent diameter is so close to the sun's apparent diameter, resulting in solar eclipse effects like the diamond ring and Bailey's Beads. That's a coincidence!
  5. May 10, 2008 #4


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    It's not just the moon it happens with all bodies in orbit.
    The earth will eventually lock to facing the same side to the sun - it's just more efficent beteen the Earth and moon because of the close distance and the earth's equatorial bulge.
  6. May 10, 2008 #5


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    Not to mention a nearly full view of the corona.

    If there were such a thing as alien tourists, traveling to Earth to see one of our solar eclipses could be quite popular.
  7. May 11, 2008 #6
    Thanks folks for the links to tidal locking and gravitational field gradients.

    At first, failing to see anything tidal on the moon I was skeptical, but thinking it through.. the moon tug on moving masses on Earth also involves periodic and variable tugs on the moon. There was a time past when even the surface of both bodies was molten, and presumably moving about.

    Curiously - the top 100km molten, but not the interior?? according to this explanation.

    Finding at least some other solar system bodies that display this synchrony was the clincher. Very satisfying it is to be able to find the example of Pluto and Charon where both are locked facing each other. Finally, I found the example of using a weight attached to a artificial satellite by a long wire, to exploit the gravity gradient, and have the combination end up attitude stabilized with the satellite antennas and other kit pointing in the desired direction.
    Agreed! Quite aside from spectacular beautiful coincidences, there would be much else about the Earth that would make it stand out.
  8. May 11, 2008 #7


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    A planet doesn't need to be molten for tidal effects to deform it. These are very strong forces.
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