Eratosthenes: Bio, Geographer, Astronomer & a Mathematician

Eratosthenes of Cyrene was one of the Greek polymaths: a mathematician, geographer, poet, astronomer, and music theorist. He was a learning man, who became the chief librarian of Alexandria’s library. His work is similar to what is now known as geography research, and he introduced some of the terminologies that are still used today.

The first individual to calculate the circumference of the Earth was Eratosthenes, and he even was the first to compute the tilt of the Earth’s axis too. 

He developed the World’s first global projection, which included parallels and meridians. Similarly, this astronomer attempted to revisit the dates of the main events of the mostly mythological Trojan War, dating from the Sack of Troy to 1183 BC, in chronology. 

He introduced the sieve of Eratosthenes, an effective method of identifying prime numbers, in number theory.

Quick Facts of Eratosthenes

Full Name

Eratosthenes of Cyrene

Nick Name


Birth Date

276 BC

Age of Death

82 years old



Birth Place


Father’s Name


Mother’s Name









Ancient Greek




Platonic Academy


Eratosthenes’ Geography, Catasterismi, Measuring the Earth


Father of Modern Geography


Astronomer, Inventor, Historian, Mathematician, and Geographer

Marital Status

Never Married



Net Worth


Early Life of Eratosthenes

In Cyrene, now part of modern-day Libya, in 276 BC, Eratosthenes was born. Cyrene was conquered in 332 BC by Alexander the Great. In 323 BC, Ptolemy I Soter donated Cyrene to his generals. 

Within the P-Ptolemaic Kingdom, Cyrene prospered. It was a place of cultivation that blossomed with knowledge. Exports of horses and silphium, a plant used for rich seasoning and medicine, have been the basis of the economy.

To further his studies, Eratosthenes went to Athens. He wrote Chronographies, a text which described dates of significance scientifically. His interest in Plato led him at an academic level to write his very first work, Platonikos. Under Callimachus, he investigated the art of poetry and wrote two Elegiacs.

This geographer published Olympic Victors, a chronology of the Olympic Games champions, as well. He was a man of many viewpoints, exploring the arts of Stoicism and Aristo of Chios. 

Eratosthenes teaching his student
Eratosthenes teaching his student

He died in Athens at 80 years of age.


In 245 BC, Eratosthenes was a librarian at the Alexandria Library, and after ten years of experiments, he invented the armillary sphere around 255 BC. Cleomedes credited him in 240 BC with the calculation of the circumference of the Earth. 

The children of Ptolemy III Euergetes were tutored by him. He made significant contributions to mathematics and science and was Archimedes’ friend. He died around the year 255 BC in Alexandria. 

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He is credited with inventing the sphere of armillaria. He was a great poet and writer as well. Eratosthenes wrote several poetry works, including Euripides and Aeschylus’ works.

Eratosthenes believed that there was both good and bad in every nation, which then criticized Aristotle for arguing that humanity gets divided into Greeks and barbarians, leading him to argue that the Greeks should keep themselves pure racially. 

When he was old, he contracted ophthalmia, becoming blind around 195 BC—losing the ability to read and observe nature plagued and depressed him, causing him to starve himself to death voluntarily. He died in Alexandria in 194 BC, at 82.

Career of Eratosthenes

Eratosthenes measured the meridian to have a range of 252,000 stadia. Among the Greek results, the calculation of the diameter of the Earth is the most common. In a book that has not been saved, the process was described.

The system of Eratosthenes for measuring the diameter of the Planet has been lost. To popularize the discovery, Cleomedes identified a condensed edition. Taking Earth as spherical, the distance between Alexandria and Syene, 250,000 stadiums, will be fifty times that of Earth.

Since one stadium in Egypt is equivalent to 157.5 meters, 39,375 km is the result, which is 1.4 per cent less than the actual total, 40,076 km. The diameter of the Planet is 1/50th of a circle’s circumference.

Statue of  Eratosthenes
Statue of Eratosthenes

The system of Eratosthenes was probably more complex, as Cleomedes himself said. The methodology was based on survey trips undertaken by expert dentists. Historians assume that a modern length unit based on the meridian length, which corresponds to 252,000 stadia, was introduced by Eratosthenes.

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Eratosthenes described and mapped his whole known universe in his three-volume book, Geography. The World has been split into five temperature zones, two cold zones at the poles, two temperate zones, and the equator and tropical zones.

He put over the Earth’s surface grids with alternating lines and used parallels and meridians to link every World’s position together. Sadly, his geography has been lost to history, but other great historians, including Pliny, Polybius, Strabo, and Marcianus, may piece together fragments of the work.

Father of Modern Geography

He also has been called the “Father of Modern Geography” and still used today. His rejection of Homer’s topography irritated those who felt that the universe portrayed in the Odyssey was legitimate.

He thought of the World as an immovable globe, a shifting location on its surface. Also, he speculated that the Mediterranean was once a large lake that filled the countries that surrounded it, and when a passage opened up, it was only connected to the ocean to the west.

Eratosthenes was a polymath who was proficient in a lot of things. Since he was brilliant at many things and wanted to get his hands on any piece of knowledge, he was known as Beta. Strabo identifies him among geographers as a mathematician and a geographer among mathematicians.

Similarly, in his dissertation on the heavens’ divisiveness, Eusebius of Caesarea wrote about Eratosthenes’ measurements of the path to the Sun. The sense of the term ‘myriads’ depends on whether it means a multitude of 400 plus 80,000 or a myriad of ‘400 and 80,000.’ And Eratosthenes measured the diameter of the Sun. 

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He also calculated that there are 365 days in a year and that there will be 366 days every fourth year. Er was still very proud of the solution he had made to double the Cube.

Similarly, he devoted his approach to King Ptolemy, with a letter and an epigram showing a bronze blueprint. Archimedes, recognizing his passion for learning and mathematics, devoted his book The Process to him.

Experiments and Works

Eratosthenes was one of the most influential intellectual figures of his day, and before and after his time at the university, he created works covering a wide field of expertise. Geography, algebra, history, chronology, literary criticism, grammar, art, and even old comedies he wrote on many subjects. 

Sadly, after the destruction of the Library of Alexandria, there are only his books’ fragments.

Some Facts of Eratosthenes

  • Eratosthenes was born in 276 BC in the city of Cyrene, Libya.
  • He learned physical skills and social interactions as well as writing, reading, poetry, music, and arithmetic.
  • He was the head of the Library of Alexandria from 240 BC until his death.
  • According to the Suda, his contemporaries called him Beta because he was the second best person in any field.
  • He published Olympic Victors, a chronology of Olympic Games champions.
  • Eratosthenes lived in Alexandria for the rest of his life after accepting the invitation of Ptolemy.
  • In around 255 BC, he invented the armillary sphere.

Networth of Eratosthenes

There is no relevant data on how much money or property Eratosthenes had. All we know is, he was brilliant, and the best we can guess is he was rich at that time as he was one of the respected men in the community and a great scholar.

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Eratosthenes Quotes

  • Eratosthenes declares that it is no longer necessary to inquire as to the cause of the overflow of the Nile since we know definitely that men have come to the sources of the Nile and have observed the rains there.
  • He is a mathematician among geographers, and yet a geographer among mathematicians, and consequently on both sides, he offers his opponents occasions for contradiction.
  • The method of producing these numbers is called a sieve by Eratosthenes since we take the odd numbers mingled and indiscriminate and we separate out of them by this method of production, as if by some instrument or sieve, the prime and incomposite numbers by themselves, and the secondary and composite numbers by themselves, and we find separately those that are mixed.


  1. Who was Eratosthenes and what did he do?

Alexandria, Greek scientific writer, astronomer, and poet, who made the first measurement of the size of Earth for which any details are known.

2. Why is Eratosthenes called the father of geography?

Eratosthenes was called “The Father Of Geography,” since he was very knowledgeable about the earth. He invented a system of latitude and longitude and he also might have calculated the distance from the earth to the sun and invented the leap day.

3. What was Eratosthenes greatest achievement?

Eratosthenes tried to accurately measure the circumference of the Earth, but is was Eratosthenes that came through. 

4. How did Eratosthenes measure the size of the Earth quizlet?

He used geometry and ratios to determine the size of the Earth, by basing his calculations on observations of the sun.

5. How did Eratosthenes know what time it was?

Eratosthenes actually measured the length of the shadow of a tall building in Alexandria when the Sun reached its highest point in the sky on the Summer Solstice.

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