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Tidal bulge on the moon, and its facing etc

  1. Mar 16, 2013 #1
    The fact the moon always faces us the same way fascinates me. Apparently the explanation is that our tidal bulge, and its tidal bulge "sync", which sorta makes sense.

    I wanted to know a bit more about this, if you guys dont mind helping a layman understand it all.

    Do we have evidence of a tidal bulge on the moon? (obviously we have evidence of tidal bulge of the oceans!)

    There are craters all over the side that faces us (apparently moreso than the "darkside), so presumably its very old, and hasn't always been synced, or was maybe synced in the opposite direction?

    Do we know how long its been synced this way?

    How common is this feature of a satellite? Are we aware of any planets that do this with their sun? Or any other moons that do this? Is it statistically rare?

    Sorry if my questions seem dumb, this stuff seems hard to google without getting either simple explainations, or a whole bunch of wacko stuff. Appreciate the help, cheers!
  2. jcsd
  3. Mar 16, 2013 #2


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    The planet Mercurey is close to that
    tidal forces from the sun have slowed Mercury's axial rotation to 59 days. It has an orbital period of only 88 days. So quite close

    I couldnt find what an earlier axial rotation period was. This with expertise in orbital mechanics would be able to say.

    but it could hint at that maybe at one time, in the dim distant past, its orbital rotation period may have equalled its axial rotation period and that would have been like the Earth-Moon scenario

  4. Mar 17, 2013 #3
    I always thought the darkside had a lot more craters than the side which faces The Earth?! I do also have to say, the Moon having a rotational period the same as the orbital period has long been considered a fascinating coincidence, somehow aligned to many different causes of reasoning. It is however, relatively common... Phobos, Deimos, Io, Europa and Ganymede all have the same length of day and year (I checked a total of 10 moons, of which, 9 have this same characteristic... is this correct or just badly researched?).

    I am however, also fascinated as to this reasoning; it seems to be a facet of observed moons being closer to a rule than an exception but why?! One would think the same might apply to planets orbiting a star; is it just that our planets are not yet old enough to have resolved to their 'prefered' natural state?
  5. Mar 17, 2013 #4


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    In most cases, planets will eventually become tidally locked with their parent star, although it may take a very long time.
  6. Mar 18, 2013 #5

    D H

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    That's not quite right. The Moon does have a tidal bulge, but this isn't the driving factor for the Moon's orientation. The key factor is that the Moon's center of mass is offset from it's center of figure by about 2 km. The crust on the far side of the Moon is considerably thicker than is the crust on the near side. This asymmetric mass distribution means that a restoring torque arises whenever the Moon's orientation deviates from the stable configuration.

    For a long, long time, since shortly after the Moon formed about 4.5 billion years ago. The Moon formed at about 1/10 of it's current distance from the Earth, which means tidal forces were about 1000 times stronger than they are now. Tidal locking was quick.

    Anticipating the follow-on question, "Why does the Moon have this asymmetric mass distribution?", -- The answer is we don't know. There are lots of conjectures out there, but as far as I know, none of them has been confirmed.
  7. Mar 18, 2013 #6

    jim mcnamara

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  8. Mar 18, 2013 #7
    Actually my follow-on question would be, if this is due to the moons mass, and something that has apparently been around for ages, why is the near earth side very covered in impact craters..(Not that there has to be an explaination, just if anyone knows)

    Thanks for the useful and informative answers there guys! :)
    Last edited: Mar 18, 2013
  9. Mar 19, 2013 #8


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    There's no reason why the near side shouldnt have lots of craters, but the far side is still much more cratered than the near side

    actually, what I find more interesting is the presence of all the Mare's on the near side which are almost non-existant on the far side ( a couple of small ones )



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    Last edited: Mar 19, 2013
  10. Apr 16, 2013 #9

    Wes Tausend

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    General readers,

    Offhand, shouldn't the near side of the moon be somewhat sheltered from debris impacts by Earth and it's gravitational shadow/field? Plus is it also possible that slightly more objects are directed to impact the backside by a more directional combination of earth/moon inline gravities?


    There is a short, decent explanation of Tidal Locking on Wikipedia. I once kidded my young grandson that we should "spin" the moon with rocket engines so we didn't have to look at the same side all the time with a telescope. But I found that once the rocket engines were shut off, the moon would eventually return to a lock condition. The wiki lock exlanation proved too complicated for a young child, but is ok for a young adult.

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