Eratosthenes

Had he been born and raised farther north or south, the ancient astronomer Eratosthenes might never have come to think about the circumference of the Earth. It so happened that in his hometown of Syene (now Aswan), Egypt—which lies just north of the Tropic of Cancer—the Sun's rays were almost exactly perpendicular to the ground at noon on the summer solstice. One year in Alexandria, about 500 miles away, he noticed that the Sun's rays hit the ground at a deviation of about 7 degrees from the vertical on the same date and time. He believed, correctly, that the Sun was very distant and that its rays were essentially parallel when they hit the Earth. Therefore, he reasoned that the difference between the angles of incidence in Syene and Alexandria could only be due to curvature of the Earth's surface. Fairly basic geometry allowed him to use these figures to calculate the circumference of the Earth. Although one usually reads that his calculation was very close to that of modern scientists, we do not know this for certain; the units of length that he used (called stadia) were not fixed throughout the ancient world, and there is no record of precisely which length he had in mind. His measurement could have been anywhere from 0.5 to 17 percent off from modern measurements. Whatever the case, Eratosthenes's calculations were remarkably good, not only for being the very first known measurement of the Earth's circumference, but also for being made when not everybody even thought the Earth was round.