Eratosthenes Measures the Earth

Around 240 BCE, Eratosthenes, the chief librarian of the Library of Alexandria, calculated the circumference of the Earth using an elegantly simple method. He learned that at noon on the summer solstice in Syene (modern Aswan), the sun was directly overhead, casting no shadow. In Alexandria, roughly 800 km north, a vertical stick cast a shadow at an angle of about 7.2 degrees. Using this angle and the distance between the two cities, he calculated the Earth's circumference to be approximately 39,375 km — remarkably close to the modern accepted value of 40,075 km. This achievement demonstrated the power of Greek mathematical reasoning applied to the physical world.