Eratosthenes of Cyrene lived approximately 275-195 BC. He was born in Cyrene which is now in Libya in North Africa. He studied with the scholar Lysanias of Cyrene and the philosopher Ariston of Chios who had studied under Zeno, the founder of the Stoic school of philosophy. Later, he studied under the poet and scholar Callimachus who had also been born in Cyrene. He then spent some years studying in Athens.
Although a brilliant scholar in so many fields, he was never considered the best in any one subject. This earned him the nickname “Beta,†for the second letter in the Greek alphabet.
Primarily a mathematician, a very important work of Eratosthenes was Platonicus which, among other topics, discussed the mathematics which are the basis of Plato's philosophy. Although this work is now lost, Theon of Smyrna wrote that Platonicus studied the basic definitions of geometry and arithmetic, as well as covering such topics as music.
Despite being lost to history, we know something of Eratosthenes’ work by the writings of others. Eutocius, in his commentary on Proposition 1 of Archimedes' Sphere and cylinder Book II, describes the way Eratosthenes solved the problem of duplicating the cube. Another later author, Theon of Smyrna, also wrote of the history of the problem of duplicating the cube. Eratosthenes later erected a column at Alexandria with an epigram inscribed on it relating to his own mechanical solution to the cube problem.
Another area in which Eratosthenes is remembered is his work with prime numbers. Although his original writings are lost, he is remembered for his 'Sieve of Eratosthenes' which, in modified form, is still an important tool in number theory research. It appears in the Introduction to arithmetic by Nicomedes. Meanwhile, another lost book written by Eratosthenes was On means. It is mentioned by Pappus as one of the great books of geometry.
Eratosthenes is probably best known for the surprisingly accurate measurement of the circumference of the Earth he made. Once again his treatise, On the measurement of the Earth, is now lost, however, some information about these calculations appear in works by other authors such as Cleomedes, Theon of Smyrna and Strabo. Comparing the noon shadow at midsummer between Syene (now Aswan on the Nile in Egypt) and Alexandria, assuming that the sun was so far away that its rays were essentially parallel, using the distance between Syene and Alexandria, he calculated the length of the circumference of the Earth as 250,000 stadia (about 46,250 km). The actual distance at the equator is 40,075km.
Eratosthenes’ other contributions to astronomy include measurements of the distances from the Earth to the moon and Earth to the sun. These were far less accurate than his Earth measurement. He also determined the obliquity of the Ecliptic, measured the tilt of the earth's axis with great accuracy obtaining the value of 23° 51' 15" and created a star map containing 675 stars. He also wrote a poem called “Hermes†which described the fundamentals of astronomy. Not content with just astronomy, he also suggested that a leap day be added to the calendar every fourth year and attempted to document the dates of literary and political events from the time of the siege of Troy.
Eratosthenes also made major contributions to the science of Geography and, in about 240 BC, became the third librarian at Alexandria, in the library in a temple of the Muses called the Mouseion. The library is said to have contained hundreds of thousands of papyrus and vellum scrolls.
Late in his life, he lost his sight. Filled with despair, that his vitality was gone, he starved himself to death at the age of 80.
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