(Jon of the Clue Crew points to an astronomical diagram on the monitor.)  In the 200s B.C. Eratosthenes calculated Earth's girth using the difference in the angle of the Sun's rays between Aswan & this Egyptian city
He was also the first to calculate the tilt of the Earth's 
axis (again with ... in Alexandria and on Elephantine Island near Syene (modern 
Aswan, Egypt).