I've just calculated that the circumference of the Earth is about 40,000 km. I calculated this by running Google Earth, and used the measure tool to get the distance from the North Pole to the South Pole. Then I doubled it. It was pretty close to 40,000 km so I evened it out a bit to get this nice number (you can do that; I've learned that from a course about semi-conductors. Yes, that course sucks). I figure this was probably the way they first came up with an approximate for the Earth's circumference.