I understood they designed it to be 1/10000000 of the distance from the North Pole to the Equator (through Paris). Then they had to calculate how far that was. They spent an awful lot of time and trouble measuring a large arc of the world by physical survey, but one of the guys made a small calculation error so it turned out later that the Earth is not exactly 40000km around.<p>I read a book about it a few years ago, but I can't remember the title.<p>Nautical miles are based on one minute of latitude, so they vary as the Earth is a bit squashed.