Eratosthenes, in full Eratosthenes of Cyrene, (born c. 276 bce, Cyrene, Libya—died c. 194 bce, Alexandria, Egypt) was a Greek scientific writer, astronomer, and poet, who made the first measurement of the size of Earth for which any details are known.
At Syene (now Aswān), some 800 km (500 miles) southeast of Alexandria in Egypt, the Sun’s rays fall vertically at noon at the summer solstice. Eratosthenes noted that at Alexandria, at the same date…