Conversation so far:
What was Eratosthenes greatest achievement?
Eratosthenes is best known for being the first person to calculate the circumference of the Earth, which he did by comparing angles of the mid-day Sun at two places.
Did Eratosthenes think the sun revolves around the Earth?
No
Did Eratosthenes map the moon?
No
Did Eratosthenes teach other Greek Philosophers?
As head of the library, Eratosthenes tutored the children of Ptolemy, including Ptolemy IV Philopator who became the fourth Ptolemaic pharaoh.
Is there anything else interesting about the Library of Alexandria?
All books had to be surrendered for duplication at the Library of Alexandria. Eratosthenes copied them so accurately it was impossible to differentiate the copy from the original.

Question: What else had Eratosthenes discovered?
Self-contained rewrite: What else had Eratosthenes discovered?