After proving that the earth is round, third century B.C. thinker Eratosthenes had the information he needed to measure the earth's circumference.

If the earth's surface was curved, then it had to be shaped much like a ball, meaning a little geometry would lead Eratosthenes to an accurate estimate.

He imagined his two sticks, one in the northern town of Syrene, and one in the southern town of Alexandria, as lines extending downward until they meet at the center of the earth.

Drawing more lines coming straight down to represent sunlight, Eratosthenes moved one stick along the outside of the circle until the shadow it would cast matched the shadow his stick actually made. Now he knew how much of the circle extended between Syrene and Alexandria, about seven degrees.

Needing to know the actual distance from Alexandria to Syrene, Eratosthenes paid camel caravan drivers to go from one town to the other, who reported a distance of 500 miles.

Assuming 500 miles equals 7 degrees of the circle, the whole circle should be 25,000 miles.

Eratosthenes' answer, deduced with nothing but two sticks and some camels, came within a few percent of the actual circumference.