(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?