Measuring the Earth (Geodesy)

Around 240 BCE, the Alexandrian polymath Eratosthenes realized he could measure the entire earth without leaving Egypt. He knew that at noon on the summer solstice the sun stood directly overhead at Syene (modern Aswan), casting no shadow, while at Alexandria to the north a vertical pole still cast one. By measuring that shadow's angle—about 1/50th of a circle—and the distance between the two cities, he scaled up to a circumference of roughly 252,000 stadia, strikingly close to the true value. It was one of antiquity's most elegant feats: pure geometry turning sunlight and shadow into the size of the world.