In Einstein’s 1905 paper in which he describes special relativity (“On the Electrodynamics of Moving Bodies”), there is a mention of what would happen to a clock on the equator vs. a clock at one of the poles. The clock at the equator is moving, and so should run slower. It turns out this is wrong, and I was surprised to find that there is some discussion, especially involving those who are convinced relativity is wrong anyway, about how this could be. (OK, not really surprised, considering that crowd) It would be correct if the earth were a rigid sphere, but it’s not — the earth is oblate because it is rotating. And surely, the arguments go, Einstein was aware of this. Well, it doesn’t really matter (and don’t call me Shirley). The reason has to do with general relativity, and in 1905, Einstein hadn’t yet formulated the theory!

The earth’s geoid (basically, the surface at sea level, without disruptions like currents and tides) is an equipotential surface. The gravitational potential (gh) and the kinetic potential (1/2 v^2/c^2) are exactly the terms that go into the dilation calculations, and if you spin up a deformable planet, you will get an exact balance between the change in gravitational potential with the kinetic potential. The dilation terms have opposite signs, and cancel. So clocks anywhere on the geoid always tick at the same basic rate — no correction for your latitude is necessary, though the elevation above the geoid must still be taken into account.