More than 2,000 years ago Eratosthenes compared the position of the Sun’s rays in two locations to calculate the spherical size of the Earth with reasonable accuracy.
He was a native of Cyrene, North Africa As a young man, he traveled to Athens to pursue his studies. His teachers included the scholar Lysanias of Cyrene and the philosopher Ariston of Chios who had studied under Zeno, the founder of the Stoic school of philosophy.
He returned to Cyrene and made such a name for himself in scholarly endeavors that the Greek ruler of Egypt brought him to Alexandria to tutor his son.
When the chief librarian of the famous Library of Alexandria died in 236 BCE, Eratosthenes was appointed to the prominent position around the age of 40. The library is said to have contained hundreds of thousands of papyrus and vellum scrolls.
Eratosthenes remained in Alexandria for the rest of his life, and it was there he made his calculations regarding the circumference of the earth and wrote his best-known works, including a three-volume set on geography credited with coining that term.
Eratosthenes tried to fix the dates of literary and political events since the siege of Troy. His writings included a poem inspired by astronomy, as well as works on the theatre and on ethics.
One of the important works of Eratosthenes was Platonicus which dealt with the mathematics which underlie Plato's philosophy. This work was heavily used by Theon of Smyrna when he wrote Expositio rerum mathematicarum.
Eratosthenes’ only surviving work is Catasterisms, a book about the constellations, which gives a description and story for each constellation, as well as a count of the number of stars contained in it.
Eratosthenes of Cyrene - Greek astronomer, geographer and mathematician
