The Earth is oblate because it spins and is not a rigid body. Since its spin axis changes relative to its crust, then the meridian of maximum diameter changes too, right?

I understand that polar motion is very small (just a few meters), but the tidal bulge is larger (over 20 kilometers), so I wonder if the side effects of polar motion are more dramatic than I had initially thought.

To be clear: I am talking here about polar motion, not precession or nutation.

  • $\begingroup$ Reading the wiki article, I wonder whether the definition of North (usually averaged over a year) doesn't follow the polar wander. en.m.wikipedia.org/wiki/Polar_motion. As tides play a large role, it matters whether it is spring tide or not (this sun-moon relative alignment). Hard to guesstimate 😀 $\endgroup$ Sep 25, 2022 at 9:45
  • $\begingroup$ 20km is more than I have read before, I've heard of crustal tides in the order of 30cm. $\endgroup$
    – James K
    Sep 25, 2022 at 10:43
  • 1
    $\begingroup$ The way they are modeled in the Explanatory Supplement to the Astronomical Almanac, the answer is no. The algorithm for computing an observer's geocentric position does not account for polar motion. The observer's lat, lon and altitude may change, but this is modeled as the observer moving, but the reference ellipsoid doesn't change because of it. Put another way: of course it does, but to simplify things, different effects are classified into different models and accounted for separately. $\endgroup$ Sep 25, 2022 at 15:29


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