The Greek geographer and librarian at the Great Library of Alexandria, Eratosthenes (c.273—c.192 B.C.E.), was aware that the sun reached the bottom of a well in Egypt only once a year, on the first day of summer. The well was near Aswan and the Tropic of Cancer (where the sun is directly overhead at noon on the summer solstice). Eratosthenes estimated the distance between the well and Alexandria based on the length of time it took camel caravans to travel between the two places. He measured the angle of the sun’s shadow in Alexandria at the same time as the well was lit by the sun, and then used a mathematic formula to determine that the circumference of the Earth was 25,000 miles (about 40,000 kilometers)—amazingly close to the actual figure!