Eratosthenes first calculated Earth's circumference with remarkable accuracy in antiquity. Working in the third century BC, he compared the angle of the Sun's shadow at Syene and Alexandria on the summer solstice. From the difference, he estimated Earth's size using the distance between the two cities.
At Syene, near modern Aswan, the Sun was traditionally said to shine directly down a well at noon on the solstice. At Alexandria, Eratosthenes measured a shadow angle of about one-fiftieth of a circle. Multiplying the Alexandria–Syene distance by 50 produced an estimate close to the true meridional circumference.
The result depends on which ancient unit and distance estimate are reconstructed, so historians do not agree on one exact numerical value for his calculation. Nevertheless, the method was powerful and geometric, and it demonstrated that Earth's curvature could be measured without traveling around the planet. Eratosthenes also directed the Library of Alexandria and developed an early method for identifying prime numbers.