# Eratosthenes > Greek mathematician, geographer, poet, astronomer, librarian and music theorist **Wikidata**: [Q43182](https://www.wikidata.org/wiki/Q43182) **Wikipedia**: [English](https://en.wikipedia.org/wiki/Eratosthenes) **Source**: https://4ort.xyz/entity/eratosthenes ## Summary Eratosthenes was a Greek polymath who served as the chief librarian at the Library of Alexandria, one of the largest and most significant libraries of the ancient world. He is best known for calculating the Earth's circumference with remarkable accuracy, inventing the sieve of Eratosthenes algorithm for finding prime numbers, and creating the first reliable map of the known world. Living from approximately 276 BCE to 194 BCE, he made foundational contributions to mathematics, geography, astronomy, poetry, and music theory, earning recognition as one of the most versatile scholars of Hellenistic Greece. ## Biography - **Born**: Approximately 276 BCE in Cyrene (modern-day Shahhat, Libya) - **Nationality**: Greek (Cyrenaic) - **Education**: Studied under Aristocreon of Samos, Lysanias of Cyrene, and possibly Callimachus in Cyrene; later studied in Athens - **Known for**: Calculating Earth's circumference, inventing the sieve of Eratosthenes, compiling the first comprehensive map of the known world, serving as chief librarian at Alexandria - **Employer(s)**: Library of Alexandria (appointed by Ptolemy III Euergetes) - **Field(s)**: Mathematics, geography, astronomy, poetry, music theory, geometry, number theory ## Contributions 1. **Calculation of Earth's Circumference**: Eratosthenes measured the Earth's circumference by comparing the angle of the sun's rays at two different locations (Syene and Alexandria) at the same time, calculating a value of approximately 250,000 stadia (roughly 39,250-45,000 km depending on the length of the stadium used), remarkably close to the actual equatorial circumference of about 40,075 km. 2. **Sieve of Eratosthenes**: Invented the ancient algorithm for generating prime numbers, a method still used today in number theory and computer science for identifying prime numbers up to any given limit. 3. **Geography**: Produced the first reliable map of the known world, including parallels and meridians, and wrote the foundational treatise "Geographia" that established geography as a scientific discipline. 4. **Library of Alexandria**: Served as the third chief librarian, significantly expanding the library's collections and organizing its holdings systematically. 5. **Astronomy**: Measured the tilt of Earth's axis (obliquity of the ecliptic) with considerable accuracy and attempted to calculate the distances to the sun and moon. 6. **Poetry**: Wrote poetry including "Helios" (Sun), "Europa" (Europe), and "Antigone" (a tragedy about Antigone), demonstrating his literary capabilities beyond scientific pursuits. 7. **Music Theory**: Contributed to music theory through works on harmonics and the mathematical relationships in music. 8. **Chronology**: Attempted to establish a chronology of Egyptian kings and compiled a list of Olympic victors. ## FAQs ### What is Eratosthenes most famous for? Eratosthenes is most famous for being the first person to accurately calculate the Earth's circumference around 240 BCE. Using measurements of the sun's angle at noon in Syene (now Aswan) and Alexandria, he calculated the Earth's circumference to be approximately 250,000 stadia, an remarkably accurate estimate for his time. ### What is the sieve of Eratosthenes? The sieve of Eratosthenes is an ancient algorithm for finding all prime numbers up to a given limit. It works by iteratively marking as composite the multiples of each prime, starting from 2. This method remains fundamental in number theory and is still taught and used in computer science today. ### Where did Eratosthenes work? Eratosthenes worked primarily at the Library of Alexandria in Egypt, one of the largest and most important libraries of the ancient world. He was appointed as the third chief librarian by King Ptolemy III Euergetes around 240 BCE. ### What did Eratosthenes contribute to geography? Eratosthenes is considered the father of geography for his systematic approach to the field. He wrote "Geographia," created one of the first reliable maps of the known world using parallels and meridians, and established methodologies for measuring and describing the Earth that influenced geographers for centuries. ### Did Eratosthenes make any other contributions? Beyond mathematics and geography, Eratosthenes contributed to astronomy by measuring the Earth's axial tilt, wrote poetry including "Helios" and "Europa," worked on chronology by compiling lists of Egyptian kings and Olympic victors, and studied music theory through the mathematical relationships in harmonics. ## Why They Matter Eratosthenes fundamentally changed humanity's understanding of the Earth and laid the groundwork for modern geography and mathematics. His calculation of Earth's circumference was a landmark achievement in scientific history, demonstrating that ancient scholars could make precise measurements of planetary scale using simple geometric principles. His sieve algorithm remains a foundational concept in number theory and computer science, illustrating the enduring value of his mathematical innovations. As chief librarian at Alexandria, Eratosthenes helped preserve and organize vast amounts of knowledge, influencing the intellectual development of the ancient world. His systematic approach to geography—treating it as a quantitative science rather than merely descriptive—established it as an academic discipline that continues to evolve today. The map he created represented the first attempt to represent the known world using a grid system of latitude and longitude, a concept that persists in modern cartography. His interdisciplinary nature exemplifies the Hellenistic ideal of the polymath, demonstrating that excellence could be achieved across multiple fields. Without Eratosthenes's contributions, the development of scientific methodology, cartography, and our understanding of planetary science would have taken significantly different paths. ## Notable For - First accurate calculation of Earth's circumference - Inventor of the sieve of Eratosthenes algorithm for finding prime numbers - Third chief librarian of the Library of Alexandria - Father of geography as a scientific discipline - Creator of one of the first reliable maps of the known world - First to calculate the tilt of Earth's axis - Author of the foundational geographic treatise "Geographia" - Poet who wrote "Helios," "Europa," and "Antigone" - Pioneer in establishing chronologies of ancient civilizations - One of the most versatile scholars of ancient Greece ## Body ### Early Life and Education Eratosthenes was born in Cyrene (modern-day Shahhat, Libya) around 276 BCE. Cyrene was a prominent Greek colony in North Africa known for its intellectual culture. He received his early education in Cyrene under teachers including Aristocreon of Samos and Lysanias of Cyrene. He later continued his studies in Athens, where he was exposed to the major philosophical and scientific schools of the Hellenistic world. His Athenian education exposed him to the works of Plato, Aristotle, and the Stoic philosophers, shaping his interdisciplinary approach to knowledge. ### Career at the Library of Alexandria Eratosthenes was summoned to Alexandria by King Ptolemy III Euergetes to serve as the tutor to his son, Ptolemy IV Philopator. Shortly thereafter, he was appointed as the third chief librarian of the Library of Alexandria, one of the largest and most prestigious repositories of ancient knowledge. The library, founded by Ptolemy I Soter and expanded by his successors, contained hundreds of thousands of scrolls from across the Mediterranean world. As librarian, Eratosthenes was responsible for organizing, cataloging, and expanding the library's collections. He developed systematic methods for classifying texts and was known for his intellectual rigor. His position gave him access to vast resources and enabled him to conduct the research that would define his legacy. ### Mathematical Contributions Eratosthenes made several significant contributions to mathematics, with the sieve of Eratosthenes being his most famous. This algorithm, which identifies all prime numbers up to a given limit by iteratively marking the multiples of each prime, remains a fundamental method in number theory. The algorithm's efficiency and simplicity have ensured its continued use for over two millennia. In addition to the sieve, Eratosthenes worked on problems of measurement and proportion. He calculated the distances to the sun and moon, though his figures were less accurate than his Earth circumference calculation. He also studied perfect numbers and made contributions to the understanding of mathematical relationships. ### Geographic Achievements Eratosthenes is often called the "father of geography" for his systematic treatment of the subject. His three-volume work "Geographia" established geography as a scientific discipline and provided the first comprehensive description of the known world. He introduced the concept of parallels (latitude) and meridians (longitude) for mapping the Earth, creating a grid system that would influence cartography for centuries. His most famous geographic achievement was calculating the Earth's circumference. Noting that at noon on the summer solstice, the sun illuminated the bottom of a well in Syene (modern Aswan) while casting a shadow in Alexandria, he measured the angle of the shadow in Alexandria at approximately 7.2 degrees (or 1/50th of a circle). Knowing the distance between the two cities was about 5,000 stadia, he calculated the Earth's circumference as 250,000 stadia. Modern scholars estimate this to be accurate within 2-20% depending on the exact length of the stadium used. He also created a map of the known world that included Europe, Asia, and Africa, noting the locations of countries, mountains, rivers, and other geographic features. His work laid the foundation for all subsequent geographic research. ### Astronomical Work In astronomy, Eratosthenes made important measurements of celestial phenomena. He calculated the tilt of Earth's axis (the obliquity of the ecliptic) at approximately 23°51'20", remarkably close to the modern value of about 23.44°. This measurement was crucial for understanding seasonal variations and the Earth's position in the solar system. He also attempted to calculate the distances to the sun and moon, though these estimates were less accurate than his Earth measurements. His systematic approach to astronomical observation set standards for future researchers. ### Literary and Poetic Works Eratosthenes was also a poet of considerable talent. His poetic works included "Helios" (a poem about the sun), "Europa" (a poem about the continent of Europe), and "Antigone" (a tragedy based on the myth of Antigone). These works demonstrated his literary sophistication and contributed to the cultural life of Alexandria. His poetic output, while less famous than his scientific work, was respected in antiquity and contributed to his reputation as a true polymath capable of excelling in multiple fields. ### Music Theory Eratosthenes contributed to music theory through his studies of harmonics and the mathematical relationships underlying musical intervals. Ancient Greek music theory explored the connections between mathematics and music, and Eratosthenes worked on understanding these proportional relationships. His work in this area reflected the Pythagorean tradition of mathematical musicology. ### Chronology and Historical Work Eratosthenes attempted to establish systematic chronologies for ancient civilizations. He compiled lists of Egyptian kings and documented the winners of the Olympic Games, creating frameworks for understanding historical time. These chronological works, though partially lost, were used by later historians as reference points for ancient history. ### Legacy and Influence Eratosthenes's influence extended far beyond his lifetime. His geographic methods and concepts shaped the development of cartography throughout the Hellenistic period and into the Roman era. Ptolemy, the famous geographer and astronomer of the 2nd century CE, drew heavily on Eratosthenes's work despite some disagreements with his measurements. The sieve of Eratosthenes remained the primary method for finding prime numbers until the development of more modern algorithms, and it continues to be taught in mathematics courses worldwide. His approach to measuring the Earth inspired later scientists, including Ptolemy and, much later, the explorers and scientists of the Renaissance and Enlightenment periods who would refine and expand upon his methods. The lunar crater Eratosthenes and the Eratosthenes Seamount in the Eastern Mediterranean south of Cyprus are named in his honor, testifying to his lasting impact on science and geography. ### Death and Historical Records Eratosthenes died in Alexandria around 194-196 BCE. According to some accounts, he died by self-starvation after becoming blind in his old age, though this information comes from later sources. His death marked the end of a remarkable career that had established him as one of the greatest polymaths of the ancient world. Despite the loss of most of his writings, his contributions to mathematics, geography, astronomy, and other fields continue to be recognized as foundational to the development of science. ## References 1. Source 2. Integrated Authority File 3. Virtual International Authority File 4. Dictionary of African Biography 5. LIBRIS. 2009 6. Google Books 7. [Perseus Digital Library](https://catalog.perseus.tufts.edu/catalog/urn:cite:perseus:author.568) 8. International Standard Name Identifier 9. CiNii Research 10. A Short History of Astronomy 11. Q45271112 12. MacTutor History of Mathematics archive 13. Freebase Data Dumps. 2013 14. CONOR.SI 15. Treccani's Enciclopedia on line 16. Enciclopedia Treccani 17. Treccani Philosophy 18. Bibliography of the History of the Czech Lands 19. HMML Authority File