Eratosthenes

Eratothenes of Cyrene lived from approximately 276 BCE to 195 BCE.  He was a Greek mathematician and astronomer of significant renown.Eratosthenes is best known for his surprisingly accurate determination of the Earth's circumference.  Using his knowledge that the Egyptian town of Syene (now modern day Aswan) was approximately on the Tropic of Cancer, he reasoned correctly that the Sun would be shining directly overhead (zenially) on the first day of Summer (the Summer Solstice, approximately June 21st).By knowing the distance to the Egyptian town of Alexandria (more or less due north of Syene), and measuring the angle made by the Sun's noontime shadow cast there by a vertical stick (gnomon), he was able to estimate the "entire walk around distance", or circumference of the Earth, from North Pole to South Pole and back again, of the Earth.  By some accounts, his measurement was within one percent accuracy.