While thumbing through a book one day, he ran across a passage that read, on the longest day of the year, the sun cast no shadow in Syene, a city to the south of Alexandria. As he read on, he found that the sun's reflection could also be seen in a deep well. "Well!" he thought to himself, "everyone knows the earth is flat! How then, could there be no shadow in Syene when there is a definite shadow in Alexandria?"
A graphics file , showing a flat earth will be here
Erastothenes hired a friend of his to walk from Alexandria to Syene to mark the distance.
His friend found the distance between the two places to be about 500 miles. With this information and his knowledge of geometry, Erastothenes determined the circumference of the earth to be approximately 25,000 miles. And we all know what has a circumference? Yes!! He proved that the earth was round. Erastosthenes had changed the world forever with his discovery, for this led many seafaring explorers to set sail in search of new and exciting trade routes. And we all know what happened after that.