Eratosthenes measured the circumference of Earth around 240 BC based on the fact that the Sun appears in the zenith at noon on the summer solstice in Syene modern Aswan At the same time in Alexandria the angle of the shadow of a post was slightly more than 7° you can vary this parameter in this Demonstration which is roughly 150 of the circumference 7360 ≈ 00194444; the distance between the cities is 5000 stadia an ancient Greek unit about 185 meters or about 925 km The calculation gives 92502 ≈ 46250 km a value very near todays value of 40000 km with an error of 16; an amazing feat for the epoch in which he lived