How big a stick would you need to measure the planet?   A thousand miles long? A million? How about three feet? That ought to be enough to do it, as an ancient Alexandrian man named Eratosthenes figured out.

Eratosthenes was a multi-talented man who lived in the third century B.C. He was puzzled one day to read that at noon on the 21st of June, pillars in the Egyptian town of Syrene cast no shadows. There is nothing so odd about this -- anything sticking vertically out of the ground will cast no shadow when the sun is standing straight overhead. What was odd, he thought, was that when he tried the experiment in Alexandria at the same time of day, a stick held vertically did cast a shadow.

How is this possible? If the earth is flat, all shadows should be the same. There would be no difference between noon in Syrene and noon in Alexandria. It's only if the earth is curved, Eratosthenes correctly guessed, that the shadows change lengths depending on how far north or south you are.

To visualize this, you can imagine a sheet of paper with two toothpicks stuck in it, several inches apart, and a lamp shining down from overhead. Neither toothpick casts a shadow. Bend the paper at one end, though, and the tilted toothpick begins to have one. The more you bend it, the longer the shadow.

In fact, once he had this insight into the shape of the earth, Eratosthenes was also able to correctly calculate its circumference. Can you figure out how he did it? Tune in next time for the answer.