[hammock]

At what elevation are they measuring these arcs / circumference of the earth? At sea level? Feel like that needs to be explicit if we are trying to be technical. Because you can't "travel around the Earth at the equator" in "21,600 nautical miles, 24,857 miles or 40,003 kilometers" without going underneath a mountain or two.

[ajross]

They aren't. It was based on an early spherical model, but in the modern world a nautical mile is defined as exactly 1852 meters, period. The notion of the arc minute is historical context in the same way that a "mile" was originally a thousand (mille) paces.

[pletnes]

Possibly the geoid: https://en.wikipedia.org/wiki/Geoid?wprov=sfti1

It’s a surface describing constant (magnitude of) the acceleration of gravity.

[signa11]

i would guess that would be mean-sea-level...