Eratosthenes (c. 276–c. 194 BC)

Greek geographer and mathematician whose map of the ancient world was the first to contain lines of latitude and longitude, and who calculated the Earth's circumference with an error of about 10%. His mathematical achievements include a method for duplicating the cube, and for finding prime numbers (Eratosthenes' sieve).

No work of Eratosthenes survives complete. The most important that remains is on geography – a word that he virtually coined as the title of his three-volume study of the Earth (as much as he knew of it) and its measurement.

Eratosthenes was born in Cyrene (now Shahhat, Libya) and educated in Athens. At the age of 30 he was invited by Ptolemy III Euergetes to become tutor to his son and to work in the library of the museum at Alexandria. In 240 BC he became the chief librarian.

Eratosthenes divided the Earth into five zones: two frigid zones around each pole; two temperate zones; and a torrid zone comprising the two areas from the Equator to each tropic.

In Chronography and Olympic Victors, many of the dates he set for events (for example, the fall of Troy) have been accepted by later historians. He also wrote many books on literary criticism in a series entitled On the Old Comedy.