Watch the full episode with Drew Brophy here: https://www.youtube.com/watch?v=nweuPcZvMOA
In ancient Syene, sunlight hit the bottom of a deep vertical well only at high noon on the summer solstice, meaning the sun was directly overhead. Five hundred miles north in Alexandria on the same day, there was a shadow at 7.2 degrees. Divide 360 by 7.2 and you get 50, multiply that by the 500-mile distance and you arrive at 25,000 miles. One elegant geometric relationship and the circumference of the Earth was solved thousands of years ago.