Enthusiasts of the history of science are familiar with the ingenious method used by the ancient Greek scientist Eratosthenes to measure the Earth’s circumference.  Briefly described, he measured the angle of a shadow cast by a stake driven into the ground at midday in the Egyptian city of Alexandria.  He compared this to the fact that, at the same time, a stake located far to the south at Cyene, Egypt cast no shadow at all.  Knowing from geometry that alternate interior angels are equal, he was able to calculate the number of degrees on the Earth’s spherical surface between Cyene and Alexandria, and from this, to produce a number for the sphere’s circumference.