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Eratosthenes of Cyrene (; Greek: Ἐρατοσθένης, IPA: ; c. 276 BC^{[1]} – c. 195/194 BC^{[2]}) was a Greek mathematician, geographer, poet, astronomer, and music theorist. He was a man of learning, becoming the chief librarian at the Library of Alexandria. He invented the discipline of geography, including the terminology used today.^{[3]}
He is best known for being the first person to calculate the circumference of the Earth, which he did by applying a measuring system using stades, or the length of stadia during that time period. His calculation was remarkably accurate. He was also the first to calculate the tilt of the Earth's axis (again with remarkable accuracy). Additionally, he may have accurately calculated the distance from the Earth to the Sun and invented the leap day.^{[4]} He created the first map of the world incorporating parallels and meridians, based on the available geographical knowledge of the era.
Eratosthenes was the founder of scientific chronology; he endeavored to revise the dates of the chief literary and political events from the conquest of Troy. In number theory, he introduced the sieve of Eratosthenes, an efficient method of identifying prime numbers.
He was a figure of influence who declined to specialize in only one field. According to an entry^{[5]} in the Suda (a 10th-century reference), his critics scorned him, calling him Beta, from the second letter of the Greek alphabet, because he always came in second in all his endeavors.^{[6]} Nonetheless, his devotees nicknamed him Pentathlos, after the Olympians who were well rounded competitors, for he had proven himself to be knowledgeable in every area of learning. Eratosthenes yearned to understand the complexities of the entire world.^{[7]}
Seventeen hundred years after Eratosthenes' death, while Christopher Columbus studied what Eratosthenes had written about the size of the Earth, he chose to believe that the Earth's circumference was much smaller. Had Columbus set sail knowing that Eratosthenes' larger circumference value was more accurate, he would have known that the place where he made landfall was not Asia, but rather a New World.^{[8]}
The son of Aglaos, Eratosthenes was born in 276 BC, in Cyrene. Now part of modern-day Libya, Cyrene had been founded by the Greeks centuries earlier, and became the capital of Pentapolis (North Africa), a country of five cities: Cyrene, Arsinde, Berenice, Ptolemias, and Apollonia, Cyrenaica. Alexander the Great conquered Cyrene in 332 BC, and following his death in 323 BC its rule was given to one of his generals, Ptolemy I Soter, the founder of the Ptolemaic Kingdom. Under Ptolemaic rule the economy prospered, based largely on the export of horses and silphium, a plant used for rich seasoning and medicine.^{[3]} Cyrene became a place of cultivation, where knowledge blossomed. Like any young Greek, Eratosthenes would have studied in the local gymnasium, where he would have learned physical skills and social discourse as well as reading, writing, arithmetic, poetry, and music.^{[9]}
Eratosthenes went to Athens to further his studies. There he was taught Egyptian Thebes. Eratosthenes also wrote Olympic Victors, a chronology of the winners of the Olympic Games. It is not known when he wrote his works, but they highlighted his abilities.
These works and his great poetic abilities led the pharaoh Ptolemy III Euergetes to seek to place him as a librarian at the Library of Alexandria in the year 245 BC. Eratosthenes, then thirty years old, accepted Ptolemy's invitation and traveled to Alexandria, where he lived for the rest of his life. Within about five years he became Chief Librarian, a position that the poet Apollonius Rhodius had previously held. As head of the library Eratosthenes tutored the children of Ptolemy, including Ptolemy IV Philopator who became the fourth Ptolemaic pharaoh. He expanded the library's holdings: in Alexandria all books had to be surrendered for duplication. It was said that these were copied so accurately that it was impossible to tell if the library had returned the copy or the original. He sought to maintain the reputation of the Library of Alexandria against competition from the Pergamum. Eratosthenes created a whole section devoted to the examination of Homer, and acquired original works of great tragic dramas of Aeschylus, Sophocles and Euripides.^{[14]}
Eratosthenes made several important contributions to mathematics and science, and was a friend of 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 on Elephantine Island near Syene (now Aswan, Egypt).
Eratosthenes believed there was good and bad in every nation and criticized Aristotle for arguing that humanity was divided into Greeks and barbarians, and that the Greeks should keep themselves racially pure.^{[15]} As he aged he contracted ophthalmia, becoming blind around 195 BC. Losing the ability to read and to observe nature plagued and depressed him, leading him to voluntarily starve himself to death. He died in 194 BC at the age of 82 in his beloved Alexandria.^{[16]}
Eratosthenes calculated the circumference of the Earth without leaving Egypt. Eratosthenes knew that at local noon on the summer solstice in the Ancient Egyptian city of Swenet (known in ancient Greek as Syene, and now as Aswan) on the Tropic of Cancer, the Sun would appear at the zenith, directly overhead. He knew this because he had been told that the shadow of someone looking down a deep well in Syene would block the reflection of the Sun at noon off the water at the bottom of the well. Using a gnomon, he measured the Sun's angle of elevation at noon on the solstice in Alexandria, and found it to be 1/50th of a circle (7°12') south of the zenith. He may have used a compass to measure the angle of the shadow cast by the Sun.^{[17]} Assuming that the Earth was spherical (360°), and that Alexandria was due north of Syene, he concluded that the meridian arc distance from Alexandria to Syene must therefore be 1/50th of a circle's circumference, or 7°12'/360°.
His knowledge of the size of Egypt was founded on the work of many generations of surveying trips. Pharaonic bookkeepers gave a distance between Swenet and Alexandria of 5,000 stadia. This distance was corroborated by inquiring about the time that it took to travel from Syene to Alexandria by camel. He rounded the result to a final value of 700 stadia per degree, which implies a circumference of 252,000 stadia. Some claim Erathostenes used the Egyptian stade of 157.5 meters, which would imply a circumference of 39,690 km, an error of 1.6%, but the 185 meter Attic stade is the most commonly accepted value for the length of the stade used by Eratosthenes in his measurements of the Earth,^{[18]} which imply a circumference of 46,620 km, an error of 16.3%. It is unlikely, however, that Eratosthenes got an accurate measurement of the circumference of the Earth, given three errors in the assumptions he made:^{[17]}
If we repeat Eratosthenes' calculation with more accurate data, the result is 40,074 km, which is 66 km different (0.16%) from the currently accepted circumference of the Earth.^{[17]}
See History of geodesy
Eratosthenes continued from his knowledge about the Earth, his discoveries of its size and shape, and began to sketch it. In the Library of Alexandria he had access to various travel books, which contained various items of information and representations of the world that needed to be pieced together in some organized format.^{[19]} In his three-volume work Geographika, he described and mapped his entire known world, even dividing the Earth into five climate zones:^{[20]} two freezing zones around the pole, two temperate zones, and a zone encompassing the equator and the tropics.^{[21]} He had invented geography. He created terminology that is still used today. He placed grids of overlapping lines over the surface of the Earth. He used parallels and meridians to link together every place in the world. It was now possible to estimate one's distance from remote locations with this network over the surface of the Earth. In Geographika the names of over 400 cities and their locations were shown: this had never been achieved before.^{[3]} Unfortunately Geographika has been lost to history, but fragments of the work can be pieced together from other great historians like Pliny, Polybius, Strabo, and Marcianus.
Eratosthenes was described by the Suda Lexicon as a Πένταθλος (Pentathlos) which can be translated as "All-Rounder", for he was skilled in a variety of things: He was a true polymath. He was nicknamed Beta, because he was great at many things and tried to get his hands on every bit of information, but never achieved the highest rank in anything, so much so that Strabo accounts Eratosthenes as a mathematician among geographers, and a geographer among mathematicians.^{[24]}
Eratosthenes proposed a simple algorithm for finding prime numbers. This algorithm is known in mathematics as the Sieve of Eratosthenes.
In mathematics, the sieve of Eratosthenes (Greek: κόσκινον Ἐρατοσθένους), one of a number of prime number sieves, is a simple, ancient algorithm for finding all prime numbers up to any given limit. It does so by iteratively marking as composite, i.e., not prime, the multiples of each prime, starting with the multiples of 2. The multiples of a given prime are generated starting from that prime, as a sequence of numbers with the same difference, equal to that prime, between consecutive numbers. This is the sieve's key distinction from using trial division to sequentially test each candidate number for divisibility by each prime.
Eratosthenes was one of the most pre-eminent scholarly figures of his time, and produced works covering a vast area of knowledge before and during his time at the Library. He wrote on many topics — geography, mathematics, philosophy, chronology, literary criticism, grammar, poetry, and even old comedies. Unfortunately, there are only fragments left of his works after the Destruction of the Library of Alexandria.^{[29]}
Medicine, Statistics, Logic, Mathematics, Physics
Alexandria, Arabic language, Alexander the Great, Greek language, China
Turkey, Amasya, Authority control, Geography, Philosophy
Library of Congress, Diana, Princess of Wales, Latin, Oclc, Integrated Authority File
Ptolemy, Amber, Kent, Norway, Cornwall
Ptolemy, Archimedes, Almagest, Orbit, Pi
Earth, Human geography, Sociology, Economics, Geodesy
Odyssey, Thucydides, Homer, Iliad, Malta