- Eratosthenes
**Eratosthenes of Cyrene**(Greek polytonic|Ἐρατοσθένης; 276 BC - 194 BC) was a Greekmathematician ,poet , athlete,geographer andastronomer . He made several discoveries and inventions including a system oflatitude andlongitude . He was the first person to calculate the circumference of the Earth (with remarkable accuracy), and the tilt of the earth's axis (again with remarkable accuracy); he may also have accurately calculated the distance from the earth to the sun and invented theleap day . [*http://www.wired.com/science/discoveries/news/2008/06/dayintech_0619*] He also created a map of the world based on the available geographical knowledge of the era. Eratosthenes was also the founder of scientific chronology; he endeavored to fix the dates of the chief literary and political events from the conquest of Troy.His contemporaries nicknamed him "beta" (the

Greek numeral "two") because he supposedly proved himself to be the second best in many fields.Fact|date=July 2008**Life**Eratosthenes was born in Cyrene (in modern-day

Libya ). He was the chief librarian of theGreat Library of Alexandria and died in the capital ofPtolemaic Egypt . He was never married. Eratosthenes studied in Alexandria and claimed to have also studied for some years inAthens . In 236 BC he was appointed by Ptolemy III Euergetes I as librarian of the Alexandrian library, succeeding the first librarian,Apollonius of Rhodes , in that post [*http://www.oxfordreference.com/views/ENTRY.html?entry=t111.e2478&srn=2&ssid=223186397#FIRSTHIT*] . He made several important contributions tomathematics andscience , and was a good friend toArchimedes . Around 255 BC he invented thearmillary sphere , which was widely used until the invention of theorrery in the 18th century.In 194 BC Eratosthenes became blind and, according to legends, a year later, he starved himself to death.

He is credited by

Cleomedes in "On the Circular Motions of the Celestial Bodies" with having calculated the Earth'scircumference around 240 BC, using knowledge of the angle of elevation of theSun at noon on the summer solstice in Alexandria and in theElephantine Island nearSyene (nowAswan , Egypt).Eratosthenes was the director of the great library of Alexandria, the Centre of science and learning in the ancient world. Aristotle had argued that humanity was divided into Greeks and everyone else whom he called barbarians and that the Greeks should keep themselves racially pure. He thought it was fitting for the Greeks to enslave other peoples. But Erathosthenes criticised Aristotle for his blind chauvinism, he believed there was good and bad in every nation. [

** p439 Vol. 1 William Woodthorpe Tarn "Alexander the Great". Vol. I, "Narrative"; Vol. II, "Sources and Studies0". Cambridge: Cambridge University Press, 1948. (New ed., 2002 (paperback, ISBN 0-521-53137-3)).*]**Eratosthenes' measurement of the Earth's circumference**Eratosthenes knew that on the

summer solstice at local noon in the Ancient Egyptian city ofSwenet (known in Greek as Syene) on theTropic of Cancer , the sun would appear at the zenith, directly overhead. He also knew, from measurement, that in his hometown of Alexandria, the angle of elevation of the Sun would be 1/50 of a full circle (7°12') south of the zenith at the same time. Assuming that Alexandria was due north of Syene he concluded that the distance from Alexandria to Syene must be 1/50 of the total circumference of the Earth. His estimated distance between the cities was 5000stadia (about 500 geographical miles or 950 km). He rounded the result to 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 frequently argued. The common Attic stadion was about 185 m, which would imply a circumference of 46,620 km, i.e. 16.3% too large. However, if we assume that Eratosthenes used the "Egyptian stadion" [*traianus.rediris.es/topo01/surveying.pdf*] of about 157.5 m, his measurement turns out to be 39,690 km, an error of less than 1%. [*There is a huge Eratosthenes-got-it-right literature based upon attacking the applicability of the standard 185m stadium to his experiment. Among advocates: F. Hultsch, "Griechische und Römische Metrologie", Berlin, 1882; E. Lehmann-Haupt, Stadion entry in "Paulys Real-Encyclopädie", Stuttgart, 1929; I. Fischer, "Q. Jl. R. astr. Soc. 16.2":152-167, 1975; Gulbekian (1987); Dutka (1993). The means employed include worrying various ratios of the stadium to the unstably defined "schoenus", or using a truncated passage from Pliny. (Gulbekian just computes the stadium from Eratosthenes's experiment instead of the reverse.) A disproportionality of literature exists because professional scholars of ancient science have generally regarded such speculation as special pleading and so have not bothered to write extensively on the issue. Skeptical works include E. Bunbury's classic "History of Ancient Geography", 1883; D. Dicks, "Geographical Fragments of Hipparchus", University of London, 1960; O. Neugebauer, "History of Ancient Mathematical Astronomy", Springer, 1975; J. Berggren and A. Jones, "Ptolemy's Geography", Princeton, 2000. Some difficulties with the several arguments for Eratosthenes's exact correctness are discussed by Rawlins in 1982b page 218 and in his [*]*http://www.dioi.org/cot.htm#fpfw Contributions*] and [*http://www.dioi.org/gad.htm#rcgn Distillate*] .Although Eratosthenes' method was well founded, the accuracy of his calculation was inherently limited. The accuracy of Eratosthenes' measurement would have been reduced by the fact that Syene is slightly north of the Tropic of Cancer, is not directly south of Alexandria, and the Sun appears as a disk located at a finite distance from the Earth instead of as a point source of light at an infinite distance. There are other sources of experimental error: the greatest limitation to Eratosthenes' method was that, in antiquity, overland distance measurements were not reliable, especially for travel along the non-linear Nile which was traveled primarily by boat. So the accuracy of Eratosthenes' size of the earth is surprising.

Eratosthenes' experiment was highly regarded at the time, and his estimate of the Earth’s size was accepted for hundreds of years afterwards. His method was used by

Posidonius about 150 years later.**The mysterious astronomical distances**Eusebius of Caesarea in his "Preparatio Evangelica" includes a brief chapter of three sentences on celestial distances ( [*http://www.tertullian.org/fathers/eusebius_pe_15_book15.htm Book XV*] , Chapter 53). He states simply that Eratosthenes found the distance to the sun to be "Polytonic|σταδίων μυριάδας τετρακοσίας και οκτωκισμυρίας" (literally "ofstadia myriads 400 and 80,000") and the distance to the moon to be 780,000stadia . The expression for the distance to the sun has been translated either as 4,080,000 stadia (1903 translation byE. H. Gifford ), or as 804,000,000 stadia (edition ofEdouard des Places , dated 1974-1991). The meaning depends on whether Eusebius meant 400 myriad plus 80,000 or "400 and 80,000" myriad.This testimony of Eusebius is dismissed by the scholarly

Dictionary of Scientific Biography . It is true that the distance Eusebius quotes for the moon is much too low (about 144,000 km) and Eratosthenes should have been able to do much better than this since he knew the size of the Earth andAristarchus of Samos had already foundFact|date=December 2007 the ratio of the Moon's distance to the size of the Earth. But if what Eusebius wrote was pure fiction, then it is difficult to explain the fact that, using the Greek, or Olympic, stadium of 185 metres, the figure of 804 million stadia that he quotes for the distance to the Sun comes to 149 million kilometres. The difference between this and the modern accepted value is less than 1%. [*Other than the distance to the moon, no celestial distance is unambiguously established as known in antiquity even to within a factor of two. As late as a century ago, the earth's distance to the sun (the A. U.) was known less accurately than 1%.*]**Works*** "On the Measurement of the Earth" (lost, summarized by

Cleomedes )

* "Geographica" (lost, criticized byStrabo )

* "Arsinoe" (a memoir of queen Arsinoe; lost; quoted byAthenaeus in the "Deipnosophistae ")

* A fragmentary collection ofHellenistic myths about theconstellation s, called "Catasterismi " ("Katasterismoi"), was attributed to Eratosthenes, perhaps to add to its credibility.**Named after Eratosthenes***

Sieve of Eratosthenes

* Eratosthenes crater on theMoon

*Eratosthenian period in thelunar geologic timescale

*Eratosthenes Seamount in the eastern Mediterranean Sea

* Jules Eratosthenes Brown (fictional character from the "Back to the Future " franchise)**ee also***

History of geodesy **Further reading*** Kathryn Lasky. "The Librarian Who Measured the Earth". New York: Little, Brown and Company, 1994. ISBN 0-316-51526-4. An illustrated biography for children focusing on the measurement of the earth. Kevin Hawkes, illustrator.

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* Pamias, Jordi, and Klaus Geus (trans., comm.), Eratosthenes. "Sternsagen (Catasterismi)." Griechisch / Deutsch. Bibliotheca Classicorum, 2. Oberhaid: Utopica, 2007. Pp. 258. EUR 29.95. ISBN 978-3-938083-05-5.**Notes****External links*** Bernhardy, Gottfried: "Eratosthenica" Berlin 1822 Reprinted Osnabruck 1968 (German text)

* [*http://www.wilbourhall.org/index.html#eratosthenes Bernhardy, Gottfried: "Eratosthenica"*] Berlin, 1822 (PDF) (Latin/Greek)

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* [*http://www.faust.fr.bw.schule.de/mhb/eratosiv.htm Eratosthenes' sieve in Javascript*]

* [*http://www.math.utah.edu/history/eratosthenes.html Eratosthenes' sieve as a simple algorithm*]

* [*http://www.math.utah.edu/~pa/Eratosthenes.html About Eratosthenes' methods, including a Java applet*]

* [*http://www.algonet.se/~sirius/eaae/aol/market/collabor/erathost How to measure the earth with Eratosthenes' method*]

* [*http://galileoandeinstein.physics.virginia.edu/lectures/gkastr1.html How the Greeks estimated the distances to the moon and sun*]

* [*http://www.pbs.org/wnet/hawking/cosmostar/html/cstars_eratho.html Eratosthenes on PBS.org*]

* [*http://www.phys-astro.sonoma.edu/observatory/eratosthenes/ Inter-collegiate project for measuring the earth with Eratosthenes' method*]

* [*http://astrosun2.astro.cornell.edu/academics/courses//astro201/eratosthenes.htm Measuring the earth with Eratosthenes' method*]

* [*http://aleph0.clarku.edu/~djoyce/mathhist/greece.html List of ancient Greek mathematecians and contemporaries of Eratosthenes*]

* [*http://www.newadvent.org/cathen/01303a.htm New Advent Encyclopedia article on the Library of Alexandria*]

* [*http://www.christopherhenden.com/previously/primenumbers/ Eratosthenes' sieve explored and visualised in Flash*]

* [*http://www.quitebasic.com/prj/math/eratosthenes/ Eratosthenes' sieve in classic BASIC all-web based interactive programming environment*]

* [*http://www.mapmonde.org/eratos/index.php?lang=en Following in the footsteps of Eratosthenes*] : project [*http://fr.wikipedia.org/wiki/La_main_%C3%A0_la_p%C3%A2te*] .Persondata

NAME=Eratosthenes

ALTERNATIVE NAMES=Eratosthenes of Cyrene; Ἐρατοσθένης

SHORT DESCRIPTION=Greekmathematician ,poet , athlete,geographer andastronomer

DATE OF BIRTH=276 BC

PLACE OF BIRTH=Cyrene

DATE OF DEATH=194 BC

PLACE OF DEATH=

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