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 andcircumference of the earth.