Eratosthenes calculated the circumference of the Earth using measurements from two cities in Egypt - Syene and Alexandria. He knew that on the summer solstice, the sun was directly overhead at Syene at noon. In Alexandria, 500 miles north, he measured the sun's shadow to be 7.5 degrees. Using this angle and the distance between the cities, he calculated the circumference of the Earth to be approximately 25,000 miles, with an error of only 4%.