How do you calculate earth's radius?

Answer 1

Eratosthenes (a greek) was the first person known to have accurately measured earth's radius. He did that about 250 BCE. The trick was to measure the length of the shadow cast by the library at Alexandria during the summer solstice, and knowing the distance between Alexandria and Cyene--a city on the Tropic of Cancer. A bit of trigonometry, and earth's radius falls right out.