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Eratosthenes of Cyrenemarker (Greek ; c. 276 BC – c. 195 BC) was a Greek mathematician, elegiac poet, athlete, geographer, and astronomer. He made several discoveries and inventions including a system of latitude and longitude. He was the first Greek to calculate the circumference of the earth (with remarkable accuracy), and the tilt of the earth's axis (also with remarkable accuracy); he may also have accurately calculated the distance from the earth to the sun and invented the leap day. 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.

According to the entry in the Suda ( 2898), his contemporaries nicknamed him (beta, the second letter of the Greek alphabet) because he supposedly proved himself to be the second best in the world in almost any field.


Eratosthenes was born in Cyrenemarker (in modern-day Libyamarker). He was the third chief librarian of the Great Library of Alexandriamarker, the center of science and learning in the ancient world, and died in the capital of Ptolemaic Egypt. He was never married.

Eratosthenes studied in Alexandria and claimed to have also studied for some years in Athensmarker. In 236 BC he was appointed by Ptolemy III Euergetes I as librarian of the Alexandrian librarymarker, succeeding the second librarian, Apollonius of Rhodes, in that post. He made several important contributions to mathematics and science, and was a good friend to Archimedes. Around 255 BC he invented the armillary sphere.In On the Circular Motions of the Celestial Bodies, Cleomedes credited him with having calculated the earth's circumference around 240 BC, using knowledge of the angle of elevation of the sun at noon on the summer solstice in Alexandria and in the Elephantinemarker Island near Syenemarker (now Aswanmarker, Egypt).

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 Eratosthenes criticised Aristotle for his blind chauvinism; he believed there was good and bad in every nation.

Eratosthenes' measurement of the earth's circumference

Measurements taken at Alexandria (A) and Syene (S)
Eratosthenes measured the circumference of the earth without leaving Egypt. Eratosthenes knew that on the summer solstice at local noon in the Ancient Egyptian city of Swenetmarker (known in Greek as Syene, and in the modern day as Aswan) on the Tropic of Cancermarker, 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 5000 stadia (about 500 geographical miles or 800 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 stadium 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 stadium" of about 157.5 m, his measurement turns out to be 39,690 km, an error of less than 1%.

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. Assuming enough difficulties in measurements, 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.

Other astronomical distances

Eusebius of Caesarea in his Preparatio Evangelica includes a brief chapter of three sentences on celestial distances ( Book XV, Chapter 53). He states simply that Eratosthenes found the distance to the sun to be " " (literally "of stadia myriads 400 and 80,000") and the distance to the moon to be 780,000 stadia. The expression for the distance to the sun has been translated either as 4,080,000 stadia (1903 translation by E. H. Gifford), or as 804,000,000 stadia (edition of Edouard 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 and Aristarchus of Samos had already found 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 meters, 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%. Scribal errors in copying the numbers, either of Eusebius' text or of the text that Eusebius read, are probably responsible.

The smaller of the foregoing two readings of Eusebius (4,080,000 stadia) turns out to be exactly 100 times the terrestrial radius (40,800 stadia) implicit in Eratosthenes' Nile map and given in the 1982 paper by Rawlins (p. 212) that analysed this map (see Further Reading). Greek scholars such as Archimedes and Posidonius normally expressed the sun's distance in powers of ten times the earth's radius. The Nile map – Eusebius consistency is developed in a 2008 Rawlins paper. The data would make Eratosthenes' universe the smallest known from the Hellenistic era's height, and made the sun smaller than the earth. His indefensible lunar distance would require the moon to go retrograde among the stars every day for observers in tropical or Mediterranean latitudes, and would predict that half moons occur roughly 10° from quadrature.

The Eusebius-confirmed 1982 paper's empirical Eratosthenes circumference (256,000 stadia instead of 250,000 or 252,000 as previously thought) is 19% too high. But the 2008 paper notes that the theory that atmospheric refraction exaggerated his measurement (a theory originally proposed in the 1982 paper, applied to either mountaintop dip or lighthouse visibility) is thus bolstered as the explanation of Eratosthenes' error. This is because accurately measuring the visibility distance of the Alexandria lighthouse (then world's tallest, built at Eratosthenes' location and during his time) and computing the earth's size from that should have given a result 20% high from refraction, very close to his actual 19% error. The 2008 paper wonders if the 40,800 stadia estimate originated with Sostratus of Cnidus (who designed the lighthouse), and offers a reconstructive speculation that the lighthouse was about 93 meters high, which is much below previous suppositions.

Prime numbers

Eratosthenes also proposed a simple algorithm for finding prime numbers. This algorithm is known in mathematics as the Sieve of Eratosthenes.


  • Περὶ τῆς ἀναμετρήσεως τῆς γῆς (On the Measurement of the Earth) (lost, summarized by Cleomedes)
  • Geographica (lost, criticized by Strabo)
  • Arsinoe (a memoir of queen Arsinoe; lost; quoted by Athenaeus in the Deipnosophistae)
  • A fragmentary collection of Hellenistic myths about the constellations, called Catasterismi (Katasterismoi), was attributed to Eratosthenes, perhaps to add to its credibility.

Named after Eratosthenes

See also


  1. The Suda states that he was born in the 126th Olympiad, (276–272 BC). Strabo (Geography, i.2.2), though, states that he was a "pupil" (γνωριμος) of Zeno of Citium (who died 262 BC), which would imply an earlier year-of-birth (c. 285 BC) since he is unlikely to have studied under him at the young age of 14. However, γνωριμος can also mean "acquaintance," and the year of Zeno's death is by no means definite. Cf. Eratosthenes entry in the Dictionary of Scientific Biography (1971)
  2. The Suda states he died at the age of 80, Censorinus (De die natali, 15) at the age of 81, and Pseudo-Lucian (Makrobioi, 27) at the age of 82.
  3. See also Asimov, Isaac. Asimov's Biographical Encyclopedia of Science and Technology, new revised edition. 1975. Entry #42, "Eratosthenes", Page 29. Pan Books Ltd, London. ISBN 0 330 24323 3. It was also asserted by Carl Sagan, 31 minutes into his Cosmos episode The Shores of the Cosmic Ocean [1]
  4. Oxford Reference (subscription required)
  5. * 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)).
  6. 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' 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' exact correctness are discussed by Rawlins in 1982b page 218 and in his Contributions and Distillate. See also, at [2], "The Shores of the Cosmic Ocean", chapter 1 of Cosmos: A Personal Voyage, a TV series by Carl Sagan, Ann Druyan and Steven Sotter (1978–1979), where a description of Eratosthenes' experiment is presented.
  7. 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 from the sun (the A. U.) was known less accurately than 16%.
  8. Mentioned by Hero of Alexandria in his Dioptra. See p. 272, vol. 2, Selections Illustrating the History of Greek Mathematics, tr. Ivor Thomas, London: William Heinemann Ltd.; Cambridge, MA: Harvard University Press, 1957.

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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