# Musing on Earth's gravity

(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?

• PS: By 'noticeable', I mean more than a few millimeters. Nov 25, 2016 at 1:01
• en.wikipedia.org/wiki/Barycenter Nov 25, 2016 at 1:36
• 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? Nov 25, 2016 at 14:06
• Information on (d): earthscience.stackexchange.com/questions/8917/… Nov 25, 2016 at 16:23
• What exactly is the "mid-point of the spin axis"? The axis is infinitely long, it doesn't have a mid-point. Nov 25, 2016 at 23:44