Over 2,200 years ago, Eratosthenes, working at the Library of Alexandria, made a groundbreaking measurement. He learned that in Syene, Egypt, the sun was directly overhead at noon on the summer solstice, while in Alexandria, a tall obelisk cast a shadow at the same time. Eratosthenes realized that the difference in shadow angles between the two cities was due to Earth's curvature. Using this observation and the known distance between the cities, he estimated the circumference of the Earth.