How did Eratosthenes measure the earth's circumference?

The space age seemed to dawn upon us suddenly but its roots go back thousands of years. Many facts that our spacemen must know were gathered by scholars through the ages. The wise men of ancient Egypt and Babylon charted the heavens. The thoughtful Greeks figured the shape and almost the true size of the earth. This amazing feat was performed around 230 b.c. Eratosthenes estimated the circumference and diameter of our globe and he never traveled more than a few hundred miles from his native Greece. The language of science is math, and a knowledge of math was needed to solve such a problem. The star charting wise men of Babylon coped with the circle by dividing it into 360 equal degrees. The Greeks borrowed and used this idea. In the 5th century B.C. the brilliant Greek scholar Pythogoras invented a system of math. He or one of his students figured out that the earth is a sphere, though this truth later was lost for centuries. Eratosthenes of the 3rd century b.c. inherited all this ...  more