Eratosthenes was a Greek mathematician, astronomer, and geographer who took the first known exact measurement of the Earth’s circumference. He accomplished this by measuring the angles of the sun’s shadows thrown in two separate Egyptian cities: Alexandria and Syene (modern-day Aswan) on the summer solstice. Using the distance between the two towns and the difference in shadow angles, he computed the Earth’s circumference with amazing accuracy, arriving at a result near to the true measurement.