The Round Earth

Reader A asks: How did we discover that the Earth is round?

Reader B: Probably fishermen who saw the top of masts coming first when approaching other nearby ships.

Me: An ancient Greek mathematician (Pythagoras? Euclid?) asked a friend who lived a few hundred miles south of him, a known distance, to drive a stake in the ground on a certain day and measure the angle the sun’s shadow made when the sun was at its zenith. He used the difference between that angle and the one he measured at home (Athens) to compute the Earth’s circumference to within a few percent of what we now know it to be.