Eratosthenes (276-194 B.C.) was a Greek mathematician and
astronomer who was a contemporary of Archimedes and Appolonius.
He wrote many books and was the director of the Library of
Alexandria. One of Eratosthenes' nicknames was "Pentathlos",
implying that he was a polymath. He wrote treatises on history,
theatre, ethics, and geography. However, he is remembered for
the two following discoveries: His accurate measurement of the
circumference of the earth, and his method for determining prime
numbers. While the idea that the Earth is round was not uncommon
among ancient Greek philosophers, Eratosthenes was able to
demonstrate this fact, and calculate the Earth's circumference
accurately. The ancients noticed that, in the city of Syene
(modern day Aswan), at noon on June 21, the sun is directly
overhead. Shadows of vertical columns and sticks disappeared. A
shaft of sunlight penetrated to the bottom of a well. This fact
aroused Eratosthenes' curiosity, and he did an experiment to
determine whether vertical stick