Eratosthenes' experiment. Illustration using satellite images of an Earth globe with an inset map of Egypt, part of a diagram showing the measurements and calculations made by the 2nd-century-BC Ancient Greek geographer Eratosthenes to determine the circumference of the Earth. In Egypt, Alexandria is due north of Syene (modern-day Aswan). It was here that Eratosthenes measured the angle of elevation 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, multiplied by 360 degrees and divided by the measured angle of solar elevation. His result (39,690 kilometres) was close to the modern value of 40,075 kilometres. Labelled version available.