How big is the earth, exactly? O.K., it’s very big—but how would you go about measuring it? This problem vexed ancient people for a long time. You can’t really sit it on the bathroom scale, or go around the world with a tape measure (particularly if you weren’t really sure what was on the other side). But Archimedes' clever pal Eratosthenes, librarian and mathematician, figured out a way to do it, using just simple tools, a deep well, the sun, and delightful math—and his answer was almost exactly right! This is his story, an inspiration to librarians and mathematicians, and mathematical librarians, and wells, everywhere.