The circumference of the earth is measured in this way: Suppose two astronomers, A and B, stationed on the same meridian, a certain distance apart, and with accurate instruments, should make careful observations on a certain star at the moment it crossed the meridian; and A should find the star 16 degrees south of the zenith, and B, who is exactly 415 miles south of A, should find it only 10 degrees south of the zenith; there would then be a difference of 6 degrees between the two places; and as they are 415 miles apart, one degree must be 1/6th of 415 or 69 1/6th miles. Now, if one degree, which is the 360th part of the earth's circumference, is 69 1/6 miles, the whole circumference must be 360 times 69 1/6 , or 24,900 miles.