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Tidal locking radius

  1. Mar 17, 2006 #1
    How does one compute the tidal locking radius of say a planet on a putative moon its area?

    Is there a formula?
    rieman zeta
  2. jcsd
  3. Mar 17, 2006 #2


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    There's a formula for how fast tidal lock occurs, see for instance


    The very rough justification for this formula goes like this:

    tidal height is proportional to (M/a)^3
    stored tidal energy is proportioanl to tidal height squared
    some fraction of the stored tidal energy gets disapated every cycle. A cycle occurs every time the planet rotates.

    This gives a M^2/a^6 dependence on the tidal braking torque, or an a^6/M^2 dependence on the "time constant". This addresses only the dependence on mass and distance, but those are the main variables of interest.

    'a' here is the semi-major axis of the orbit of the body being locked around the more massive body, what you would be calling (I think) the "tidal locking radius" of the more massive body.

    Note that you have to specify the time allowed for the lock occured - theoretically, anything will lock up given enough time.

    For more details, see the quoted textbook source. I really don't know much more than what I've quoted (plus the comments I've added about the a^6 dependency) - specifically I don't have much insight into the numerical values of Q and k2 (though Brian Davis probably does, I don't think he's on this board).
    Last edited: Mar 17, 2006
  4. Mar 17, 2006 #3

    Although I am light years ahead of where I was before your reply, I would invite others to continue to edify me.

    Riemann Zeta
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