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Except for ice, acetic acid, bismuth and gallium and a few other things materials generally shrink when they cool and solidify, so I'm pretty sure Earth has as well.

It probably wouldn't be measurable over a period of years, but models of the Earth's current and historic rates of heat flow can probably be used to estimate a rate of change of Earth's average size and possibly oblateness.

Right now WGS84 uses 6378137.0 meters for Earth's equatorial radius and a flattening at the poles of about 1/298.257222.

How fast might those change over any given million years?

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    $\begingroup$ Perhaps relevant question: earthscience.stackexchange.com/q/18534/15419 $\endgroup$ – BMF Mar 1 at 5:01
  • $\begingroup$ @BMFForMonica Thanks! That's close but I don't think it's a duplicate. It's possible that an answer can be written here based on estimating the slopes shown in Figure 2 of the link in an answer there if we assume that most of the change is due to the mantle. But I'm not sure if those assumptions are correct. $\endgroup$ – uhoh Mar 1 at 5:06
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    $\begingroup$ keep in mind the earth is slowly gaining mass due to meteors. Its a small effect but on the scale involved likely bigger than thermal changes. $\endgroup$ – John Mar 2 at 2:55
  • $\begingroup$ @John good point! ;-) $\endgroup$ – uhoh Mar 2 at 2:57
  • $\begingroup$ Do you factor the water into the size? Earth may be losing mass overall due to loss of hydrogen and some helium, in greater amounts than the added mass by meteors, that said, meteors contribute to the surface. The loss of water (by loss of hydrogen) is more tricky. Are you defining size by sea level or by average rock altitude, which would be over a mile lower given average sea level depth of about 2.3 miles). earthscience.stackexchange.com/questions/16688/… $\endgroup$ – userLTK Mar 5 at 14:16

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