First things first, let’s know who was Eratosthenes, then we will proceed for the answer to this task.
Eratosthenes of Cyrene was a Greek mathematician, geographer and astronomer (he yearned to understand the complexities of the entire world so he was devoted to many areas). He lived between 276 BC – 194 BC and during that time he changed the way we view the world. He created geography and made some progress in science especially mathematics (he created a tool for discovering prime numbers). He is now known for being the first person to measure the Earth’s circumference. 
If you didn’t understand it completely, here is better explained. (Any doubt or suggestion be sure to leave it in the comment section)
References and further reading.
 – Eratosthenes – Wikipedia
 – (Project for school students) http://www.bibalex.org/psc/en/GetInvolved/Eratosthenes.aspx