[umanwizard]

20 million meters is indeed about the maximum distance between any two points on Earth.

[mcv]

Along the surface. Otherwise it's about 12 million meters.

[umanwizard]

True! I assumed they were using the distance on the surface here, not the path tunneling through the interior.

[JoachimS]

It also depends on the direction.

The earth is bulging at the equator (or getting flatened pole to pole) due to rotation. https://en.wikipedia.org/wiki/Equatorial_bulge