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| Eratosthenes studied at Alexandria and for some years in ]. In ] he was appointed by ] as librarian of the ]. He made several important contributions to ] and ], and was a good friend to ]. Around ] he invented the ], which was widely used until the invention of the ] in the ]. | Eratosthenes studied at Alexandria and for some years in ]. In ] he was appointed by ] as librarian of the ]. He made several important contributions to ] and ], and was a good friend to ]. Around ] he invented the ], which was widely used until the invention of the ] in the ]. | ||
| He is credited by ] in ''On the Circular Motions of the Celestial Bodies'' with having calculated the Earth's ] ca. ], using ] and knowledge of the angle of ] of the ] at noon in Alexandria and ] (now ], Egypt). | He is credited by ] in ''On the Circular Motions of the Celestial Bodies'' with having calculated the Earth's ] ca. ], using ] and knowledge of the angle of ] of the ] at noon in Alexandria and ] (now ], Egypt). See ] for a detailed description. Eratosthenes' method was used by ] about 150 years later. | ||
| Eratosthenes knew that on the ] at local ] on the ], the Sun would appear at the ], directly overhead — though Syene was in fact slightly north of the tropic. | |||
| He also knew, from measurement, that in his hometown of Alexandria, the angle of elevation of the Sun would be 1/50th of a circle (7.2°) south of the zenith at the same time. Assuming that Alexandria was due ] of Syene — Alexandria is in fact on a more westerly ] — he concluded that the distance from Alexandria to Syene must be 7/360 of the total circumference of the Earth. | |||
| The distance between the cities was known from caravan travellings to be about 5,000 ]. | |||
| He established a final value of 700 stadia per degree, which implies a circumference of 252,000 stadia. The exact size of the ''stadion'' he used is no longer known (the common Attic ''stadion'' was about 185 m), but it is generally believed that Eratosthenes' value corresponds to between 39,690 ] and 46,620 km. The circumference of the Earth around the poles is now measured at around 40,008 km. Eratosthenes' method was used by ] about 150 years later. | |||
| About ] Eratosthenes is thought to have coined or to have adopted the word '']'', the descriptive study of the Earth. | About ] Eratosthenes is thought to have coined or to have adopted the word '']'', the descriptive study of the Earth. | ||
Revision as of 06:35, 24 August 2005
Eratosthenes (Ερατοσθένης) (276 BC - 194 BC) was a Greek mathematician, geographer and astronomer. His contemporaries nicknamed him 'Beta' because he was the second best in the world in many subjects.
He was born in Cyrene (now in Libya), but worked and died in Ptolemaic Alexandria. He is noted for devising a system of latitude and longitude and computing the size of the Earth.
Eratosthenes studied at Alexandria and for some years in Athens. In 236 BC he was appointed by Ptolemy III Euergetes I as librarian of the Alexandrian library. He made several important contributions to mathematics and science, and was a good friend to Archimedes. Around 255 BC he invented the armillary sphere, which was widely used until the invention of the orrery in the 18th century.
He is credited by Cleomedes in On the Circular Motions of the Celestial Bodies with having calculated the Earth's circumference ca. 240 BC, using trigonometry and knowledge of the angle of elevation of the Sun at noon in Alexandria and Syene (now Aswan, Egypt). See History of Geodesy for a detailed description. Eratosthenes' method was used by Posidonius about 150 years later.
About 200 BC Eratosthenes is thought to have coined or to have adopted the word geography, the descriptive study of the Earth.
Eratosthenes' other contributions include:
- The Sieve of Eratosthenes as a way of finding prime numbers.
- The measurement of the Sun-Earth distance, now called the astronomical unit (804,000,000 stadia).
- The measurement of the distance to the Moon (780,000 stadia).
- The measurement of the inclination of the ecliptic with an angle error 7'.
- He compiled a star catalogue containing 675 stars, which was not preserved.
- A map of the Nile's route as far as Khartoum.
- A map of the entire known world, from the British Isles to Ceylon, and from the Caspian Sea to Ethiopia. Only Hipparchus, Strabo, and Ptolemy were able to make more accurate maps in the classical and post-classical world.
Eratosthenes was known under the name β, because he supposedly proved himself to be the second in the world in many fields. He was also reputedly known for his haughty character. In 195 BC he became blind and a year later he starved himself to death.
The fragmentary collection of Hellenistic sky-myths called Catasterismi (Katasterismoi) was given an attribution to Eratosthenes, a name to conjure with, to add to its credibility.
Named after Eratosthenes
- Eratosthenes crater on the Moon.
- Eratosthenian period in the lunar geologic timescale.