A New Perspective on Eratosthenes' Measurement of the Earth

Around 240 B.C., Eratosthenes made what is considered to be the most famous and accurate of the ancient measurements of the circumference of the Earth.1 It was accomplished by making presumably simultaneous measurements of the angles of the shadows cast by a vertical stick at Syene (today known as Aswan) and another at Alexandria, at noon on the day of the summer solstice (about June 21 every year). From these measurements, and knowing the distance from Syene to Alexandria along the assumed same meridian of longitude, Eratosthenes was able to provide a remarkably accurate estimate of the radius of the Earth.