**Eratosthenes** was a Greek mathematician who is famous for his work on prime numbers and for measuring the diameter of the earth.

- Eratosthenes also studied under the poet and scholar Callimachus who had also been born in Cyrene.
- Eratosthenes then spent some years studying in Athens.
- On the death of Callimachus in about 240 BC, Eratosthenes became the third librarian at Alexandria, in the library in a temple of the Muses called the Mouseion.
- Despite being a leading all-round scholar, Eratosthenes was considered to fall short of the highest rank.
- 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 and, although Platonicus is now lost, Theon of Smyrna tells us that Eratosthenes' work studied the basic definitions of geometry and arithmetic, as well as covering such topics as music.
- One rather surprising source of information concerning Eratosthenes is from a forged letter.
- Although the letter is a forgery, parts of it are taken from Eratosthenes' own writing.
- Other details of what Eratosthenes wrote in Platonicus are given by Theon of Smyrna.
- Thus may it be, and let any one who sees this offering say "This is the gift of Eratosthenes of Cyrene".
- Eratosthenes also worked on prime numbers.
- He is remembered for his prime number sieve, the 'Sieve of Eratosthenes' which, in modified form, is still an important tool in number theory research.
- Another book written by Eratosthenes was On means and, although it is now lost, it is mentioned by Pappus as one of the great books of geometry.
- In the field of geodesy, however, Eratosthenes will always be remembered for his measurements of the Earth.
- Eratosthenes made a surprisingly accurate measurement of the circumference of the Earth.
- Eratosthenes compared the noon shadow at midsummer between Syene (now Aswan on the Nile in Egypt) and Alexandria.
- It is certainly true that Eratosthenes obtained a good result, even a remarkable result if one takes 157.2 metres for the stadium as some have deduced from values given by Pliny.
- In it Rawlins argues convincingly that the only measurement which Eratosthenes made himself in his calculations was the zenith distance on the summer solstice at Alexandria, and that he obtained the value of 7°12'.
- Rawlins argues that this is in error by 16' while other data which Eratosthenes used, from unknown sources, was considerably more accurate.
- Eratosthenes also measured the distance to the sun as 804,000,000 stadia and the distance to the Moon as 780,000 stadia.
- Eratosthenes made many other major contributions to the progress of science.
- Eratosthenes made major contributions to geography.
- A study of the Nile had been made by many scholars before Eratosthenes and they had attempted to explain the rather strange behaviour of the river, but most like Thales were quite wrong in their explanations.
- Eratosthenes was the first to give what is essentially the correct answer when he suggested that heavy rains sometimes fell in regions near the source of the river and that these would explain the flooding lower down the river.
- Another contribution that Eratosthenes made to geography was his description of the region "Eudaimon Arabia", now the Yemen, as inhabited by four different races.
- The situation was somewhat more complicated than that proposed by Eratosthenes, but today the names for the races proposed by Eratosthenes, namely Minaeans, Sabaeans, Qatabanians, and Hadramites, are still used.
- Eratosthenes writings include the poem Hermes, inspired by astronomy, as well as literary works on the theatre and on ethics which was a favourite topic of the Greeks.
- Eratosthenes is said to have became blind in old age and it has been claimed that he committed suicide by starvation.

Born 276 BC, Cyrene, North Africa (now Shahhat, Libya). Died 194 BC, Alexandria, Egypt.

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African, Ancient Greek, Astronomy, Geography, Geometry, Origin Libya, Number Theory, Physics, Special Numbers And Numerals

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**O’Connor, John J; Robertson, Edmund F**: MacTutor History of Mathematics Archive