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HOW TO MEASURE THE EARTH WITH A STICK

        Have you ever heard of the Greek mathematician and astronomer Eratosthenes? His name is probably best known among astronomers. Why do they think so highly of him?           Eratosthenes was born about 276 B.C.E. and received some of his education in Athens, Greece. He spent a good part of his life, however, in Alexandria, Egypt, which at that time was under Greek rule. In about 200 B.C.E., Eratosthenes set out to determine the dimensions of the earth by using a simple stick. ‘Impossible!’ you may say. How did he do it?         In the city of Syene [now called Aswan], Eratosthenes observed that at noon on the first day of summer, the sun was directly overhead. He knew this because there was no shadow cast when the sunlight reached the bottom of deep wells. However, at noon on the same day in the city of Alexandria, which was located 5,000 stadia to the north of Syene, a shadow could be observed. That gave Eratosthenes an idea.       Eratosthenes set up a gnomon, a simp