Eratosthenes and Background Information

Imagine that you are living over 2000 years ago and you are convinced that the earth is round. How would you go about measuring it? One way would be to start walking in one direction and keep track of how far you go. If the earth is round as you believe, eventually you should return to where you started, however as we all know this was impossible at this time. Another method would be to drill a hole to the "other side" of the earth and measure the distance (diameter). Using this measurement, you could then determine how round the earth is (circumference) by multiplying it by Pi. Unfortunately, digging a hole through the center of the earth is just as out of the question as circumnavigating the globe by foot.

--> So, how did a Eratosthenes, the chief librarian in Alexandria, Egypt from over 2,200 years ago, calculate a remarkably accurate measurement of the circumference of earth?