A poet, scientist and historian, Eratosthenes of Cyrene (275–195 B.C.E.) was a Renaissance Man 1,500 years before the Renaissance began. One of his remarkable feats was to measure the circumference of the earth.

As no tape measure existed that was long enough, Eratosthenes turned to geometry. He knew the earth was round—as everyone did in those days. He knew that south of Alexandria at Aswan (then Syene) the sun’s rays fall vertically to the ground (that is, an upright pole will cast no shadow) at noon on the summer solstice. He also knew that the distance from Aswan to Alexandria is about 500 miles (Eratosthenes used a standard of measurement called a “stadium,” which is slightly more than one-tenth of a mile).

He needed only one more piece of information. At high noon on the summer solstice in Alexandria, Eratosthenes set up a vertical pole and measured the angle of the sun’s shadow cast by the pole: about 7 degrees. So he divided 7 into 360 (the number of degrees in a circle) and multiplied the result by 500 (the distance in miles between Aswan and Alexandria). Now he had the answer: about 25,700 miles. But because Aswan is slightly east of Alexandria, and because ancient measurements of distance were inexact, Eratosthenes calculated slightly high. The earth’s actual circumference is about 25,000 miles.