Earth's circumference
Earth's circumference is a measure of Earth's figure, approximately an oblate spheroid with an equatorial bulge (or planetary flattening) due to Earth's rotation. The equatorial circumference equals 40,075.017 km (24,901.461 mi) and corresponds to the perimeter of a circle, simply 2π times Earth's equatorial radius. The polar circumference equals 40,007.863 km (24,859.734 mi) and corresponds to the perimeter of an ellipse, a complicated formula involving Earth's polar radius and Earth's flattening. The difference between the two values is 67.154 km (41.728 mi) or about 0.17%. The first known scientific measurement and calculation was done by Eratosthenes, by comparing altitudes of the mid-day sun at two places a known north–south distance apart. He achieved a great degree of precision in his computation. The Earth's shape deviates from spherical by flattening, but by only about 0.3%. Measurement of Earth's circumference has been important to navigation since ancient times. In modern times...
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