Eratosthenes of Cyrene deduced the circumference of the Earth by noticing the difference in the length a shadow cast in wells positioned at Syene (modern day Aswan) and Alexandria in Egypt. He measured the length a shadow cast at both places and comparing the differences, was able to deduce the angle at which those two sticks would meet if they were extended all the way to the center of the Earth. By describing the curve between the points, and knowing the distance between the two cities, he calculated the circumference of the Earth to be about 39,690km. The actual measurement (now figured out by satellite technology) is a little over 40,000km. His margin of error was less than two percent.
Carl Sagan first introduced me to the story of Eratosthenes in his series “Cosmos” and I immediately fell in love with this fine example of human ingenuity. I pay homage to him in this page, passing on this tale of simple tools and observation to deduce great things here.