Mathematician, Astronomer, Geographer, Writer, Poet, Musician, Scholar (c.246 BC – c.194 BC)

Eratosthenes was one of the greatest sages of ancient Greece. He was headmaster of the Library of Alexandria and the founder of geography as a science as we know it today. His most famous achievement was the measurement of the circumference of the Earth.

He was born in Cyrene, a Greek colony of North Africa. He was 11 years older than Archimedes, with whom he was good friend. Eratosthenes studied mathematics and astronomy in the Academy of Athens under his teachers Ariston and Arcesilaus. He then continued his studies in Alexandria under his teacher Callimachus, where he remained and worked for the rest of his life. He was one of the many Greek intellectuals who comprised the staff of the Library of Alexandria, the greatest spiritual center of humanity as the time, including Ctesibius, Hipparchus, Apollonius of Perga, Apollonius of Rhodes, Conon, Aristarchus, Heron and Philon of Byzantium. He served as the third headmaster of the Library of Alexandria.

Eratosthenes was a polymath; he was nicknamed “Pentathlos” because he excelled in numerous fields such as mathematics, astronomy, physics, geography and music. By far his most notable contribution in the sciences is the measurement of the circumference of the Earth, a feat that is recorded for the first time in ancient history. Knowing that at the river Syene (modern Aswan), 500 km away from Alexandria, during the summer solstice, the sun’s rays fall vertically at noon and that at the same date and time at Alexandria, the rays fall with an angle of 7,2 degrees, Eratosthenes calculated the distance between the river and Alexandria at about 820 km. By accepting that the sun rays are parallel to each other and that the difference in the geographic latitude between Syene and Alexandria is equivalent to the angle the sun rays form during that time, Eratosthenes, using a rod and its shadow calculated the equatorial length of the Earth at 41.000 km, with a negligible error of 1000 km, because he miscalculated the distance of Alexandria and Syene instead of 800 km.

Eratosthenes was a prolific writer. He wrote several books ranging from mathematics and astronomy to poetry and philosophy, most of which do not survive today. In his treatise Catasterism he compiles a catalogue of constellations and their respective stars, calculates the Earth’s polar diameter with great accuracy as well as the distance of the Earth and the Sun. One of his most famous contributions to mathematics is the Sieve of Eratosthenes, a method for finding prime numbers, of which Eratosthenes is the inventor. He also solved the Delian problem, the doubling of the cube in his treatise Mesolavos.

The scientific foundations of geography were laid by Eratosthenes. In his now lost treatise Geographica, he presents the history of geography, mathematical and physical geography and perigraphic (discriptional) geography, including oeconomic and ethnographic elements. Furthermore, he created a world map as well as a calendar called Chronological Table, which covered 1076 years starting from the Fall of Troy, featuring most significant scientific and historical events recorded at the time for each date, regarded as a groundbreaking undertaking in the history of sciences. In philosophy, Eratosthenes was concerned mostly with ethics, poetry inspired from astronomy and comedy plays.

Eratosthenes had the rare privilege of being recognized as a great scientific mind during his own time. He was praised for his wisdom by notable intellectuals of his time such as Archimedes and Ptolemy Euergetes. The fact that he calculated the Earth’s circumference using nothing but geometry, a sacred science to the Greeks, proves Eratosthenes’ wisdom and justifies his influence on the ancient world and the Western civilization.


  1. “Eratosthenes”. Helios New Encyclopaedic Dictionary. Passas, I. Athens: 1946. Print.
  2. Georgakopoulos, Konstantinos. Ancient Greek Scientists. Georgiades: Athens, 1996. Print.
  3. The Editors of Encyclopaedia Britannica. Eratosthenes. Encyclopaedia Britannica. Web. July 15, 2018.

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