Aswan

The famous cartographer Eratosthenes had heard that in Aswan, then still called Syene, you could see the bottom of a deep vertical well only on 21 June at 12am. Eratosthenes concluded that the sun stood vertically over the place at that time. He then decided in Alexandria to measure the shadow of a gnomon, a stick perpendicular to the surface at the same time. He calculated that the sun deviated 7 degrees, 14 minutes from the vertical. This is 1/50 of 360 degrees, or 1/50 of the circumference of the earth. He decided that the circumference of the earth had to be 50 times the distance from Alexandria to Syene. He knew that it took a camel 50 days at 100 furlongs per day. Consequently, he concluded that the circumference of the earth was 250,000 furlongs. Depending on whether you take the Egyptian or Greek distance for furlongs, respectively 157 m and 185m, you arrive at 39,400 km or 46 250 kilometers. Not bad when you know the circumference of the earth as we know it today is about 40 000 km.

Dromos