The Greek mathematician Eratosthenes (ca. 276–195 B. C.) measured the circumference of the earth from the following observations.
He noticed that on a certain day the sun shone directly down a deep well in Syene (modern Aswan). At the same time in Alexandria, 500 miles north (on the same meridian), the rays of the sun shone at an angle of 7.2° to the zenith.
Use this information to find the radius and circumference of the earth.