More than 2,200 years ago a librarian in Egypt measured the circumference of the entire planet using nothing but a stick, a shadow, and the distance between two cities, and landed within a few per cent of the right answer.

More than 2,200 years ago a librarian in Egypt measured the circumference of the entire planet using nothing but a stick, a shadow, and the distance between two cities, and landed within a few per cent of the right answer.

Around 240 BCE, Eratosthenes of Cyrene, the chief librarian at Alexandria, produced an estimate for the circumference of the Earth. He used the angle of a shadow cast by an upright rod, the report that the Sun stood directly overhead at the southern city of Syene on the summer solstice, and the estimated distance between the two cities. His answer was 250,000 stadia, later refined to 252,000. The figure has been repeated for centuries as one of …