Eratosthenes
Eratosthenes arrived in the world during 276 BC within the city of Cyrene. This settlement now exists as part of modern Libya and was founded by Greeks during the second half of the 7th century BCE. The location offered abundant water sources and rich soil that supported a prosperous economy. Local exports included horses and silphium which made the region a capital hub for Greek culture. His father Aglaos held humble origins though upward mobility existed in the Hellenistic world. Some scholars suggest his name meaning lovely strength implied noble upbringing. He studied physical skills alongside reading writing arithmetic poetry and music at the local gymnasium.
By the late 260s BCE Eratosthenes traveled to Athens to pursue advanced studies. Strabo records he learned Stoicism from Zeno of Citium who died shortly after his arrival. Apelles of Chios taught him while Aristo of Chios instructed him on Cynic philosophy. Bion of Borysthenes provided eclectic training before Arcesilaus of Pitane led the Platonic Academy. Lysanias of Cyrene served as a philologist teaching Homer to the young scholar. Callimachus likely crossed paths with him in debates though never formally taught him. He remained in Athens for twenty years studying and writing Platonikos about Plato's philosophy. His Chronographies focused on dates of the Trojan War while Olympic Victors listed game winners.
Ptolemy III succeeded his father Ptolemy II in 246 BCE marking a new era for the empire. The post of librarian became the most prestigious academic appointment during this period. Apollonius of Rhodes retired possibly influenced by Callimachus allowing Eratosthenes to replace him. Queen Berenike contributed favorably to his appointment alongside his roots in Cyrene. Archimedes sent material for comment praising contributions enthusiastically through letters. The Method of Mechanical Theorems was written specifically addressed to Eratosthenes. He composed works on geography philosophy rhetoric literary criticism grammar poetry and star lore. Later he served as advisor and companion to Arsinoe sister and wife of Ptolemy IV.
Eratosthenes described an arc measurement technique using shadows cast by vertical rods called gnomons. Two cities along the same meridian provided data points: Alexandria and Syene. On noon summer solstice shadows appeared in Alexandria but vanished completely in Syene. Rods cast no shadows while sun rays shone straight down into city-center wells. Cleomedes preserved details showing angles measured about 7.2 degrees representing one fiftieth of a full circle. Professional bematists reported distances between cities at roughly 5,000 stadia. Multiplying this figure by fifty yielded approximately 250,000 stadia total circumference. Modern measurements show equatorial circumference near 40,075 kilometers though ancient estimates varied significantly.
His three-volume work Geography described and mapped his entire known world with grids of overlapping lines. Parallels and meridians linked every place allowing distance estimation from remote locations. Over 400 cities had names recorded within these pages creating unprecedented feats of organization. Five climate zones divided Earth including two freezing poles temperate regions and equatorial tropics. Strabo noted arguments against Greek-Barbarian dichotomy emphasizing law-abiding qualities across peoples. Julius Caesar referenced Eratosthenes knowledge regarding Hercynian forest in De Bello Gallico. The second book contained calculations on Earth's circumference while third covered political geography using parallel lines to divide maps accurately.
An algorithm separated prime numbers from odd natural numbers less than or equal to given limits. This method iteratively marked multiples as composite starting with multiples of two. Though Hans-Joachim Waschkies claims he did not invent it he coined the term sieve. Hipparchus credited him with inventing the armillary sphere model featuring spherical frameworks of rings. These represented celestial longitude latitude ecliptic and other astronomically important features. He determined obliquity of ecliptic at 23 degrees 51 minutes 20 seconds improving upon earlier measurements. A calendar devised during Library tenure predicted 365 days annually plus leap years every fourth year.
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Common questions
When and where was Eratosthenes born?
Eratosthenes arrived in the world during 276 BC within the city of Cyrene. This settlement now exists as part of modern Libya and was founded by Greeks during the second half of the 7th century BCE.
Who taught Eratosthenes philosophy in Athens?
Strabo records he learned Stoicism from Zeno of Citium who died shortly after his arrival. Apelles of Chios taught him while Aristo of Chios instructed him on Cynic philosophy and Bion of Borysthenes provided eclectic training before Arcesilaus of Pitane led the Platonic Academy.
How did Eratosthenes calculate Earth's circumference?
Eratosthenes described an arc measurement technique using shadows cast by vertical rods called gnomons at Alexandria and Syene. Multiplying the distance between cities reported by professional bematists at roughly 5,000 stadia by fifty yielded approximately 250,000 stadia total circumference.
What works did Eratosthenes write about geography?
His three-volume work Geography described and mapped his entire known world with grids of overlapping lines. The second book contained calculations on Earth's circumference while third covered political geography using parallel lines to divide maps accurately.
Did Eratosthenes invent the sieve algorithm for prime numbers?
An algorithm separated prime numbers from odd natural numbers less than or equal to given limits though Hans-Joachim Waschkies claims he did not invent it. He coined the term sieve and Hipparchus credited him with inventing the armillary sphere model featuring spherical frameworks of rings.
All sources
50 references cited across the entry
- 1bookEratosthenes' GeographyEratosthenes — Princeton University Press — 2010
- 2bookThe Handbook of GeoscienceFrank R. Spellman et al. — Scarecrow Press — 2013
- 3bookA History of Science: The Beginnings of ScienceHenry Smith Williams et al. — Harper & Brothers — 1904
- 4bookSuidae lexiconIn aedibus B. G. Teubneri — 1967
- 5bookAsimov's Biographical Encyclopedia of Science and TechnologyIsaac Asimov — Pan Books Ltd — 1975
- 6bookCosmosCarl Sagan — Random House Publishing Group — 2013
- 7bookJews and Greeks in ancient CyreneShim'on Applebaum — Brill — 1986
- 8bookEratosthenes and the measurement of the Earth's circumference (c. 230 bc)Christopher Anthony Matthew — Oxford University Press — 2023
- 9journalAncient Greek Athleticism1896
- 10journalDouris' Cup and the Stages of Schooling in Classical AthensA. D. Booth — 1985
- 11encyclopediaEratosthenesCharles Scribner's Sons — 1971
- 12bookThe method of Archimedes, recently discovered by Heiberg; a supplement to the Works of Archimedes, 1897Archimedes et al. — Cambridge University Press — 1912
- 14bookGeodesyW. Torge et al. — De Gruyter — 2012
- 16webHow WGS 84 defines EarthSigurd Humerfelt — 26 October 2010
- 17bookThe Enigma of ReasonHugo Mercier — Harvard University Press — 2017
- 18bookPreparation for the GospelEusebius of Caesarea — Clarendon Press — 1903
- 19bookLa Préparation ÉvangéliqueEusebius of Caesarea — Éditions du Cerf — 1980
- 20journalEratosthenes, Hipparchus, and the Obliquity of the EclipticAlexander Jones — 2002-02-01
- 21bookA History of AstronomyWalter William Bryant — 1907
- 22bookCallimachusJohn Ferguson — 1980
- 23bookThe History of the TelescopeHenry C. King — Courier Corporation — 2003
- 24bookAnaximander in Context: New Studies in the Origins of Greek PhilosophyDirk L. Couprie et al. — SUNY Press — 2003
- 26encyclopediaEratosthenes2011
- 27bookInvention of Racism in Classical AntiquityBenjamin Isaac — Princeton University Press — 2013
- 28bookDer Platonismus in der AntikeStuttgart-Bad Cannstatt — 1987
- 29journalEratosthenes as Platonist and PoetFriedrich Solmsen — 1942
- 30bookMathematics useful for understanding PlatoTheon of Smyrna — Wizards Bookshelf — 1979
- 31journalEratosthenes as Platonist and PoetFriedrich Solmsen — American Philological Association — 1942
- 32bookAgainst those in the disciplinesSextus Empiricus — Oxford University Press — 2018
- 33webThe Eratosthenes' mesolabio (mean-taker)2023-09-23
- 34bookArchimedes opera omniaEutocius
- 35bookAnfänge der Arithmetik im Alten Orient und bei den GriechenHans-Joachim Waschkies — 1989
- 36bookEratosthenes von Kyrene. Studien zur hellenistischen Kultur- und WissenschaftgeschichteKlaus Geus — Munich — 2002
- 37bookOn the Track of the Books: Scribes, Libraries and Textual TransmissionSara Panteri — De Gruyter — 2019-06-17
- 38bookThe divisions of the tetrachord: = Peri tōn toy tetrachordoy katatomōn = Sectiones tetrachordi ; a prolegomenon to the construction of musical scalesJohn H. Chalmers et al. — Frog Peak Music — 1993
- 39bookGrundriss der Geschichte der Philosophie. Die Philosophie der AntikeHans Krämer — Basel — 2004
- 40bookKleine SchriftenFriedrich Solmsen — 1968
- 41bookA history of Greek mathematicsThomas Little Heath — Clarendon Press — 1921
- 43bookThe Cambridge History of Science: Volume 1: Ancient ScienceCambridge University Press — 2018
- 44bookIntroduction to Geography: People, Places & EnvironmentCarl Dahlman et al. — Pearson — 2014
- 45encyclopediaDictionary of World Biography: The Ancient WorldJames T. Chambers — 1998
- 47webAsk an Astronomer
- 48bookThe Forgotten Revolution: How Science Was Born in 300 BC and Why It Had to Be RebornLucio Russo — Springer — 2004
- 49magazineGreek Scholar's Work Shows Usefulness of Measurement2012-06-19
- 50magazinePlato as 'Architect of Science'Leonid Zhumud — 1998