Eratosthenes (astronomer)

Eratosthenes was an ancient Greek astronomer and mathematician known for calculating the Earth's circumference with remarkable accuracy around 240 BCE. He did this by measuring the angle of the Sun’s rays at two different locations at the same time—Syene and Alexandria—where he knew the Sun was directly overhead at noon in Syene. By measuring the shadow and the distance between the two cities, he used geometry to estimate the Earth's size. His work demonstrated the Earth is round and provided one of the earliest known scientific measurements of its shape and size.