Eratosthenes

(284 - 192 BC)

Eratosthenes was a Greek astronomer who lived between 276-197 BC. He made the first accurate calculation of the Earth's circumference, using only basic geometry.

Eratosthenes learned that once a year a particular well in southern Egypt was lit all the way to the bottom. On that day, he measured the shadow cast by a vertical post in Alexandria, hundreds of kilometers away, and found the curvature of Earth between the two cities. He then extrapolated to find the distance over a curvature of 360 degrees. He determined that the polar diameter was about 7,850 miles, when it is in fact 7,900 miles! That's very close considering he didn't have a computer, or satellites or a calculator to help him!

Eratosthenes also kept a table of star locations and contributed to the field of mathematics. He devised a prime number sieve, which could list all the prime numbers smaller than any given number.

**The preceding information is provided by the University Corporation for Atmospheric Research's Windows to the Universe.**