(a) What, if anything, is the difference between Earths geographic center, i.e. mid-point of the spin axis, and the gravitational center?

(b) What is the average distance between the Earth's center of gravity, and the Earth-Moon system's center of gravity?

(c) The recent 'super-moon' is a reminder that the moon's distance has an orbital wobble. So, by how much does this vary the distance between the Earth vs Earth-Moon gravitational centres, and

(d) Is the moon's varying distance from the Earth sufficient to create any noticeable effect upon sea/ocean tidal amplitudes?

  • $\begingroup$ PS: By 'noticeable', I mean more than a few millimeters. $\endgroup$ Commented Nov 25, 2016 at 1:01
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    $\begingroup$ en.wikipedia.org/wiki/Barycenter $\endgroup$
    – Spencer
    Commented Nov 25, 2016 at 1:36
  • $\begingroup$ I think there's more than one question here. (c) and (d) are easy to relate and fit together (and FWIW, I'm pretty sure the answer to (d) is "yes"), but I think the others need asking separately. Might I suggest seperating them out? $\endgroup$ Commented Nov 25, 2016 at 14:06
  • $\begingroup$ Information on (d): earthscience.stackexchange.com/questions/8917/… $\endgroup$
    – arkaia
    Commented Nov 25, 2016 at 16:23
  • $\begingroup$ What exactly is the "mid-point of the spin axis"? The axis is infinitely long, it doesn't have a mid-point. $\endgroup$ Commented Nov 25, 2016 at 23:44

1 Answer 1


For (a) through (c), consider the differences between the geoid and the reference ellipsoid, in addition to the barycentre.

For (d): A slightly higher perigean spring tide than usual, meaning in some geographical locations perhaps an amplitude difference of inches rather than millimetres.


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