Around 240 BC Eratosthenes, who ran the library at Alexandria, was told that at noon on midsummer’s day the sun shone straight down a well at Syene, far up the Nile, and lit the water at the bottom. At the same hour in Alexandria an upright pole still cast a short shadow, leaning about a fiftieth of a full circle. If the sun’s rays arrive parallel, that lean is how far the Earth curves between the two towns, so the whole way round must be fifty times the distance between them: about 250,000 stadia. It may be the best idea anyone has ever had with a stick. What nobody can tell you is how good his answer was. No one is sure which stadion he meant, and the likely candidates differ by about a sixth, which is the difference between landing within two percent of the truth and missing it by fifteen. The method came down to us perfectly. It was the ruler that got lost.