SIZE OF THE EARTH (Eratosthenes 200 BC)
Eratosthenes knew that at the summer solstice the sun shone directly into a well at Syene at noon. At the same time, in Alexandria, Egypt, approximately 787 km due north of Syene (now Aswan), the angle of inclination of the sun's rays was about 7.2°. With these measurements he computed the diameter and circumference of the earth.
He wrote the proportion 7.2 / 360 = 787 / X
so X = 39350 km is the circumference
and the earth's radius is 2 p R = 39350
( remember that p = 3.14159 ), or R = 6263 km
NOT BAD - TODAY WE MEASURE 6371 km