Syene was the ancient Greek name for the Southern Egyptian town of Aswan. In ancient times, the most important part of the town was located on the southern tip of Elephantine Island. Eratosthenes, a contemporary of Archimedes, used Syene as the originating point (Alexandria was the terminal point)to make quite an accurate estimate of the circumference of the Earth. Aswan (أسوان Aswān) (24 05 N 32 56 E, population 200,000) is a city in the south of Egypt, the capital of the governorate of the same name. ... Elephantine Island, showing the nilometer (lower left) and the Aswan Museum. ...
At the same time, he observed the sun was not directly overhead at Alexandria; instead, it cast a shadow with the vertical equal to 1/50th of a circle (7° 12').
From these observations and "known" facts, Eratosthenes concluded that, since the angular deviation of the sun from the vertical at Alexandria was also the angle of the subtended arc, the linear distance between Alexandria and Syene was 1/50 of the circumference of the earth or 50 x 500 = 25,000 miles.
Actually, Syene is 37 miles to the north; (2) the true distance between Alexandria and Syene is 453 miles and not 500; (3) Syene lies 3° 30' east of the meridian of Alexandria; (4) the difference of latitude between Alexandria and Syene is 7° 5' rather than 7° 12' as Eratosthenes had concluded.
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