Eratosthenes of Cyrene was known to his contemporaries as a pentathlete (*pentathlos* in Greek), for his extraordinary ability to excel in various disciplines. And this scholar from Ancient Greece stood out in mathematics and geography, but also in astronomy, history and even poetry. He was, furthermore, the third librarian of the eminent library of Alexandria, a position which he held until his death.

**Join us on this journey through the life of Eratosthenes of Cyrene ** the first to calculate the circumference of the earth and establish part of the nomenclature that is still used in geography today.

## Brief biography of Eratosthenes of Cyrene, the great Greek mathematician and geographer

As is often the case, what we know about Eratosthenes of Cyrene (including part of his work) we know mostly from third parties. It is known that he was born in Cyrene, a town in North Africa located in present-day Libya, and that in the geographer’s time it was an important Greek commercial center. Little is known about his family; We know only the name of his father, Aglaos.

However, a more or less wealthy lineage is suspected (possibly dedicated to trade), since around 250 BC we find a young Eratosthenes studying in Athens, at Plato’s Academy.

### In the footsteps of knowledge

The Platonic academy had been impregnated, thanks to the director of the time, Arcesilaus (315 – 240 BC), with an important skeptical element. **Arcesilaus, philosopher born in Pitane, entered the Academy during the time of Polemon and Crates ** When, years later, he agreed to direct it, he decided to qualify the dogmatism imposed by the previous directors and redirect the teachings towards a current more linked to skepticism.

The objective was to teach students to think and reach their own conclusions, an essential fact to become a good philosopher. In this environment the young Eratosthenes was educated, an environment that would immerse him forever in a constant search for knowledge.

And it is that **Eratosthenes was interested in any field of knowledge ** His theories and calculations on mathematics and geography are well-known, but he also cultivated poetry and studied the movement of the stars, in addition to writing several works on history. Because of his incredible retention capacity, his multifaceted intelligence and his deductive abilities, his contemporaries called him the *pentathlos*that is, the pentathlete, “the one who excels in everything.”

### Third director of the great library of Alexandria

Around the year 236 BC, attracted by his fame, Ptolemy III called him to Alexandria with the mission of taking charge of his spectacular library, in addition to acting as tutor to his children. Eratosthenes heads there, amazed by the possibilities offered by the impressive Egyptian city, in those years a benchmark of culture and knowledge.

In Alexandria, the pentathlete becomes the third director of the library, a position that he will not abandon until his death, much discussed by historians (as is also common in ancient Greek characters). According to some, Eratosthenes became blind and, desperate because this prevented him from continuing his research, he committed suicide at the age of eighty. However, others claim that he lived to be eighty-two.

Be that as it may, it is clear that **In Alexandria the wise man was able to display all his knowledge and his thirst for knowledge ** It is very likely that much of the work we know of him was created during his years in Egypt; specifically, the three volumes on geography that he wrote, in which, by the way, he established the term to refer to the science that studies the dimensions and characteristics of the earth.

### The first to calculate the earth’s circumference

Eratosthenes has gone down in history for many things; among them, for **the formula to establish prime numbers, known as the “sieve of Eratosthenes” ** The famous philosopher also counts among his creations what is considered the first complete functional world map, with meridians and parallels.

But probably what he is best known for is his calculation of the circumference of the earth, which is surprising for its accuracy. Armed only with a gnomon (measuring stick) and his wits, Eratosthenes measured the shadow cast in Alexandria at noon on the summer solstice, which he concluded measured 7.5 degrees. Then, in Syene (present-day Aswan), he realized that, **on the same day at the same time, the stick in question did not cast any shadow ** From this he drew two conclusions: first, that indeed and as was already suspected, the earth is spherical, and not flat; and second, that with a simple rule of three he could calculate its exact circumference.

This rule of three contemplated the following: if from Syene to Alexandria there was a distance equivalent to our 800 kilometers, and to this corresponded the 7.5 degrees of the shadow, the total circumference would correspond to the total kilometers of the earth. In this way, the mathematician **He stated that the earth’s circumference measured 40,000 kilometers ** If we take into account that current calculations show 40,075 kilometers, we realize that Eratosthenes’ result was practically exact.

Eratosthenes met other great figures of his time: he was a disciple of Ariston of Chios, a Stoic philosopher, and corresponded with Archimedes himself, who was eleven years older than him. His passion for knowledge was broad and deep and he never gave up the pursuit of knowledge. In his long existence he also had time to invent the mesolabio, a device considered the first prototype of a “calculator”. It is said that, aware of the importance of his invention, Eratosthenes sent it as an offering to a temple, with an explanatory text about the device. A text in verse form, by the way.