Eratosthenes' experiment. Diagram showing the measurements and calculations made in Egypt by the 2nd century BC Ancient Greek geographer Eratosthenes to determine the circumference of the Earth. Alexandria ('A') is due north of Syene ('S', modern-day Aswan). It was here that Eratosthenes measured the elevation (theta) of the Sun from the zenith on the summer solstice, when the Sun was directly overhead in Syene. The circumference of the Earth is the Syene-Alexandria distance (D), multiplied by 360 degrees and divided by theta. His result (39,690 kilometres) was close to the modern value of 40,075 kilometres.