Eratosthenes was a Greek mathematician who lived in Egypt in the 3rd Century BC. He observed that in Syene, on the longest day of the year, columns cast no shadows when the sun was directly overhead. However, at the same time, the columns in Alexandria cast a significant shadow. This simple observation led Eratosthenes to conclude that the only way for both columns to cast shadows of differing lengths at the same time was if the surface of the earth was curved. A few simple geometric calculations later, he had calculated the circumference of the planet.
