In 240 BC Eratosthenes, mathematician, philosopher and director of the great Library of Alexandria, realized that he had the information necessary to calculate the size of the Earth. (1) At noon on the summer solstice, the sun fell fully into a well in the city of Syene, so it was directly overhead. (2) Syene was 5000 stadia (about 490 miles) away from Alexandria. (3) At noon on the summer soltice in Alexandria, he measured the shadow of a perpendicular pole in the ground (a gnomon) and determined that the angle of the sun was 7.2°. That was all he needed to compute the circumference of the earth and launch the science of geography so essential to navigating the world. How did he do it?