Astronomy Education Historic Accomplishments History Mathematics People Science Written History

Eratosthenes: African and First Man To Measure The Circumference of Earth

More than 2000 years ago, a time when not even a calculator existed, let alone other technical devices that facilitate the research process, Eratosthenes calculated the spherical size of the Earth. He calculated it with considerable accuracy, obviously without the use of modern equipment but only by comparing the position of the Sun’s rays in two locations.


Lifetimes of selected prominent scholars

Born in 276 BC in Cyrene, the present-day city of Shahhat, Libya in North Africa, Eratosthenes grew to become a keen learner and a man of many talents. He pursued his studies in Athens then returned back to Cyrene where he excelled among his peers and stood out in scholarly endeavors. When he became well-known among his peers, the ruler of Egypt, Pharaoh Ptolemy III Euergetes asked him to come to tutor his son in the city of Alexandria, Egypt. In 236 BC, when the chief librarian of the eminent Library of Alexandria passed away, Eratosthenes (around 40 years old now) was appointed as Director of the Library of Alexandria, the most prominent intellectual institution in the ancient world. At the time, the library contained over half a million books in the form of scrolls. Eratosthenes expanded the library’s assets by creating accurate duplicates of all books to the extent that it was hard to tell the original from the copy. He wanted to maintain the reputation of the Library of Alexandria against competition from the other important ancient library of the time, the Library of Pergamum in Turkey. The library was a hub where scientists, philosophers and poets gathered to discuss their intellectual quests.

An illustration of scholars at the Library of Alexandria, Egypt

His many talents started to shine not only as a librarian but in other fields as well, for Eratosthenes made considerable contributions in mathematics, astronomy, history and poetry. This made his colleagues and friends at the library give him the nickname “Pentathlos”, which is an athlete who competes in five different events. The name was extremely fitting to a scholar who excelled in many different fields of knowledge. He however, used to call himself “philologos” which means the lover of learning. Eratosthenes made extensive writings and findings, most of which were lost but other scientists did report his work.

Eratosthenes created a mathematical method that is still used today which is known as the “Sieve of Eratosthenes”. The method involves an algorithmic system that he designed for finding prime numbers, which are whole numbers that are only divisible either by themselves or by the number one.

The term “geography” was used for the first time by Eratosthenes, which originated from geo (Greek for Earth) + graphy (field of study). He created a map of the known world and invented a system of latitude and longitude. Ptolemy reported that Eratosthenes was the first to calculate the tilt of the Earth’s axis with remarkable accuracy. He also calculated the distance between the Earth and the Moon, and the distance between the Earth and the Sun, but with less accuracy. He constructed an astral catalogue containing 675 stars. Moreover, he organized the dates of literary and political events starting from the siege of Troy (around 1194-1184 BC) to his own time, which allowed him to make a calendar with leap years by which he laid the foundation of chronology used in the world.

A reconstruction of Eratosthene’s map of the world 194 BC

Measuring the Earth’s Circumference

His most prominent and lasting achievement though was his calculation of the distance around the spherical Earth (the Earth’s circumference) with extreme accuracy. To compute the Earth’s circumference, he used his knowledge of the fact that the Earth is a spherical body in space along with simple geometry and trigonometry. At the time of Aristotle (384-322 BC), no one knew or calculated how big the Earth was, however, most scholars agreed that the Earth was a sphere based on three main observations;

  • When ships moved over the horizon, they seemed to disappear while their masts remained visible.
  • During the lunar eclipses, they witnessed the curved shadow of the Earth on the Moon.
  • They noticed that stars in the sky changed positions with a certain pattern.

Eratosthenes knew that the Egyptian city of Swenet (the present day Aswan) on the Nile River is located close to the Tropic of Cancer (about 23.5 degrees north), the northernmost latitude at which the sun at noon is directly overhead. He also heard about a famous well in Swenet, where one day each year, on the summer solstice, the sun’s rays would shine straight down into the well’s deep pit. The rays illuminated only the water at the bottom and not the sides of the well as it was the case on other days. This proved that the sun was directly overhead.

Eratosthenes worked in Alexandria, where he erected a pole and observed it on the summer solstice. He saw that it cast a shadow, indicating that the sun was slightly south. By recognizing the curvature of the Earth and the distance between Swenet and Alexandria, Eratosthenes was able through solving the equation to find that the Earth’s circumference is 250,000 stadia or 40,000 Km.

File:Eratosthenes measure of Earth circumference.svg

word-image According to the famous astronomer and mathematician Hipparchus, Eratosthenes was the inventor of the armillary sphere. The armillary sphere is an astronomical device that was used for 1800 years to determine the positions of celestial objects. Later, it was replaced with the invention of the telescope. The device is made of a central solid sphere representing the Earth. The sphere is surrounded by a broad tilted ring representing the sun’s annual path and known as the ecliptic. It is also the path on which lunar eclipses take place. The ecliptic holds the star constellations which make up the zodiac.

The armillary sphere

Eratosthenes produced many writings covering a range of different topics including geography, mathematics, chronology, philosophy, grammar, literary criticism, poetry and even wrote some old comedies. Alas, many of those were lost after the burning of the Library of Alexandria during Julius Caesar’s Civil War in 48 BC.

Eratosthenes’ documented writings include, Platonikos (discussing the mathematical foundation of Plato’s philosophies, Chronographies (a scientific depiction of important dates and events beginning with the Trojan War), Olympic Victors (a chronology of the Olympic games’ winners), Geographika, On The Measurement of The Earth, Arsinoe (a memoir of queen Arsinoe), Erigona (a poem describing the suicide of the Athenian Erigone, daughter of Icarius) and Hermes (a poem inspired by astronomy).

As an ethnologist, Eratosthenes had powerful views opposing other scholars of the time. Aristotle argued that all of mankind is divided into two groups, Greeks and Barbarians. Eratosthenes, who believed that every nation had its good and its bad, criticized Aristotle for that opinion. He said that it would be better to divide between people according to good and bad qualities rather than race and skin colour. For according to him, many Greeks were bad while at the same time, many Barbarians were refined.

In 194 BC, Eratosthenes the philologos died at the age of 82 in the city of Alexandria after dedicating his entire life to seeking knowledge and passing on that knowledge to all those around him and the generations that would come centuries after him.


  • Ridpath, Ian (2001). The Illustrated Encyclopedia of the Universe. New York, NY: Watson-Guptill. p. 31. ISBN 0-8230-2512-8.
  • Isaac. Asimov’s Biographical Encyclopedia of Science and Technology, new revised edition. 1975. Pan Books Ltd, London. ISBN 0-330-24323-3.
  • Selections Illustrating the History of Greek Mathematics, tr. Ivor Thomas, London: William Heinemann Ltd.; Cambridge, Massachusetts: Harvard University Press, 1957.
  • Dicks, D.R. “Eratosthenes,” in Complete Dictionary of Scientific Biography. New York: Charles Scribner’s Sons, 1971.

This site uses Akismet to reduce spam. Learn how your comment data is processed.