It was around 250 BCE when Eratosthenes was a librarian in the famous library of Alexandria He discovered in his reading that there was a yearly festival in the town of Syene (modern Aswan) on June 20. On that date, and only on that date, it was possible to look down into a well and see the sun! This remarkable phenomenon was possible because on June 20 the sun was direcly overhead and its light rays would go straight down the well and reflect back to the observer.
Eratothenes also observed that on that same date in Alexandria an obelisk cast a distinct shadow. Therefore, the sun was not directly overhead in Alexandria on June 20. One can understand that if the earth is a sphere. In fact, from the angle of the suns rays from the vertical on June 20 and the distance from Alexandria to Syene he was able to deduce the circumference of the earth. The following figure shows how:
A question for you
If the circumference of the earth is 240000 stadii and the meter is defined as one ten-millionth of the distance from the Pole to the Equator then how long was the ancient stadium?