It's very interesting that the definition of a metre the length of a 2-second pendulum would be so close to the definition of a metre as 10 millionth of the meridian along a quadrant. So I asked myself whether this coincidence is unique to the earth or if it would happen in other planets.

It turns out that what is unique about the earth is its density. Any planet, whether big or small, will have the same close relationship between the two definitions of metre, as long as it has the same density as the Earth. In a big planet, the metre would be longer, but the two definitions of metre would still agree if the planet's density is right.

I would provide a full formal derivation, but that might be off-topic :-)

Daniel.

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