I remember that the metre was originally defined as one 40,000,000th of the earth's circumference. The diameter is therefore 40,000 kilometres. U= 40 000 km D= U/pi D= 40 000 / 3,14159 D=12 732 km - and that's about right. However, the earth is not a sphere, so it depends on where the measurement is taken. Source: Two hundred and fifty things architects should know, Michael Sorkin