As this father’s day coincides with the summer solstice, it is an appropriate time to recall the astonishingly accurate calculation of the circumference of the Earth that was made on this same day more than 22 centuries ago by one of the founding fathers of mathematics and scientific measurement.

Like other Greek astronomers and scholars at the time, Eratosthenes (276-195 B.C.), the head of the famous Library at Alexandria in Ptolemaic Egypt, assumed the Earth was round and revolved around the Sun. From his research, he discovered a fascinating fact: Every year, at noon time on this day (June 21) and no other, the Sun shone directly to the bottom of a deep well in the town of Syene (site of today’s Aswan Dam).