Eratosthenes is considered the first scientist in the history of the world who managed to measure with excellent precision the circumference of our earth at a time when people thought that the world was flat instead of being round.

Personal information:

  • When was he born:  276 BC
  • Where he was born:  Cyrene, Libya
  • When he died:  194 BC
  • Where he died:  Alexandria, Egypt

Who was Eratosthenes?

Eratosthenes was an important Greek mathematician and astronomer , who for many decades was director of the Alexandrian library and was recognized as one of the most important minds of his time.

Biography of Eratosthenes:

Eratosthenes of Cyrene was born in 276 BC in Cyrene , North Africa, and died in 194 BC in Alexandria , Egypt. He was a disciple of the Greek poet Callimachus of Cyrene and the grammarian Lisanias , director of the Library of Alexandria and friend of Archimedes . His enemies called him the ” Beta “, because according to what they thought, he was second best in everything. Those who knew and appreciated him, however, called him the ” Alpha ” because they knew he was the first in almost everything.

Eratosthenes ended his days dying of voluntary starvation at age 80, as he was extremely depressed for having lost his sight.

Contributions and inventions:

One of the main contributions of Eratosthenes to science and astronomy was his important work regarding the measurement of the earth . Eratosthenes, studying the papyri in the library of Alexandria, found a report in which it was said that when the sun’s rays fell on a rod during the noon of the summer solstice , no shadow was produced .

He correctly assumed that, if the Sun was located at a great distance, its rays when reaching the earth should reach it in parallel, assuming that the earth was flat as it was considered at that time and therefore there should be differences between the shadows cast by objects at the same time on the same day, regardless of location. However, when it was shown that this did happen, he deduced that the earth was not flat and using the known distance between the two cities and the measured angle of the shadows, he calculated the circumference of the earth at approximately 250 stadia, about 40,000 kilometers, an estimate. which was almost close to reality.

Other of his contributions was that he managed to calculate the distance to the Sun in 804,000,000 stages and the distance to the Moon in 780,000 stages. He managed to measure the inclination of the ecliptic at 23º 51 ′ 15 ″ quite well and with great precision.

Another of his important astronomical works was a catalog of about 675 stars . He also created one of the most advanced calendars for his time and a chronological history of the world since the beginning of the Trojan War.

He conducted research in geography drawing maps of the known world, vast stretches of the River Nile, and described the Eudaimon region , which we now know as Yemen in Arabia. He also invented the sieve of Eratosthenes .

Eratosthenes sieve:

The sieve consists of a method that allows us to find all the prime numbers smaller than a given natural number ” N “. This algorithm could be summarized as follows:

  • It should start with the number 2 , we highlight the number 2 as a prime, but we cross out all multiples of 2 (that is, we cross out 4, 6, 8, etc.).
  • Then we continue with the next number that is not crossed out in the table, the number 3 , we highlight the number 3 as a prime and we cross out all the multiples of 3 (that is, we cross out 6, 9, 12, etc.).
  • The next number that is not crossed out in the table is number 5 , so we must highlight it as a prime and cross out all multiples of 5 (that is, we cross out 10, 15, 20, etc.).
  • This process must be repeated until we reach the number N, having previously crossed out all the multiples of the prime numbers found.

Land measurement:

To achieve measurements of the earth he invented and used a trigonometric method , different notions of latitude and longitude that had already been introduced in the area of ​​science, apparently by Dicearco , for which he well deserves the title of father of geodesy .


His importance to the modern world is not based solely on the fact that he made significant progress in the area of geometry ; but he owes the ” sieve of Eratosthenes “ , a system to determine prime numbers was one of the most important geographers and he drew a map of the River Nile to Khartum, in which he showed its two Ethiopian tributaries, and giving the first explanation Satisfactory on why the Nile River was flooding. In addition, he drew up a calendar with leap years , drew up a small stellar catalog , was a poet, and wrote treatises on Ethics.

Works of Eratosthenes:

Among his most important works we can mention the following:

  • Erigone’s works : it was the best known and it was an astronomical and geographical poem that deals with the shape of the Earth , its temperature, the different climates and the constellations .
  • Treatises of moral philosophy and other philosophical works. All his works were closely linked to mathematics .
  • The Chronography : in this work he manages to collect the dates of the most important literary and political events that had occurred so far; It is believed that The Olympics, cited by Diogenes Laercio and Athenaeus, were part of the Chronography.
  • Treatise on the ancient Attic comedy : of it are Arjitectonic and Skenographic fragments in which he spoke of the decoration, costumes, declamation and plot of works by Aristophanes and Cratino among others.
  • He studied the work of Homer and wrote a biography on the life of the poet that has not survived to this day.


There are no recognized phrases of Eratosthenes, but there are two phrases written by Carl Sagan that reflect his essence:

  • “Eratosthenes had no other tools than sticks, eyes, feet and head and a great desire to experiment with these tools correctly deduced the circumference of the earth with enormous precision and a minimum error percentage.
  • “Eratosthenes criticized Aristotle for his blind chauvinism , he believed that in all countries there were good and bad things.”

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