Eratosthenes of Cyrene was a third century BC Greek mathematician, geographer and astronomer. He was head of the Library of Alexandria from 240 BC until his death: this was the most important library of the ancient world.

Eratosthenes was nicknamed him Beta, (the second letter of the Greek alphabet), because he was the second best in the world in almost any field. Eratosthenes was a friend of Archimedes, who also lived and worked in Alexandria. Archimedes was the greatest mathematician and inventor of the age.

The works Eratosthenes wrote are known to us only indirectly: the great Library was destroyed, and no copies survived.

Eratosthenes made several remarkable discoveries and inventions. He was the first person to calculate the circumference of the Earth with remarkable accuracy, and he invented a system of latitude and longitude. He calculated the angle of the earth's axis; he may also have accurately calculated the distance from the earth to the sun and invented the leap day. He created a map of the world based on the available geographical knowledge of the era.