Who Was Eratosthenes? The Polymath Who First Calculated Earth's Circumference
Eratosthenes of Cyrene was a Greek polymath and Alexandria's chief librarian, best remembered as the first known person to calculate Earth's circumference.
Eratosthenes of Cyrene was an Ancient Greek polymath known as a philosopher, scholar, mathematician, geographer, poet, astronomer, and music theorist. He is best remembered as the first known person to calculate the Earth's circumference. He eventually became the chief librarian at the Library of Alexandria. This article follows his life from Cyrene to Athens to Alexandria, looks at the nicknames his contemporaries gave him, and ends with a practice task for describing a historical career in English.
The short answer: a scholar of many fields
The word polymath fits him because the list of his roles is so long. The source calls him a philosopher, scholar, mathematician, geographer, poet, astronomer, and music theorist. Few people are described with so many titles at once, and the rest of his story shows how those titles connect to specific work.
Two of his achievements stand out in the source. The first is the calculation of the Earth's circumference, the achievement for which he is best remembered. The second is that he was the first to calculate Earth's axial tilt. Both are measurements of the planet itself, which helps explain why geography became so central to his reputation.
Early life in Cyrene
He was born in 276 BC in Cyrene, the son of Aglaos. The context of his hometown matters. Cyrene came under the rule of Alexander the Great in 332 BC. After Alexander's death it came under Ptolemaic rule, and it was known for exporting horses and silphium.
So Eratosthenes was born into a city whose political world had already been reshaped by Alexander's conquests and by the rule of the Ptolemies who followed. The source does not tell us more about his childhood, so we should not guess at it.
Athens: about twenty years of study
In the late 260s BC, Eratosthenes went to Athens to further his studies. He studied there for about twenty years, and during that time he wrote works including Platonikos and Chronographies.
The titles already hint at a range of interests. Platonikos and Chronographies are different kinds of work, and the second connects to one of the dating efforts the source mentions below.
The call to Alexandria
He was summoned from Athens to Alexandria to replace the librarian Apollonius of Rhodes during the reign of Ptolemy III. Ptolemy III had succeeded his father Ptolemy II in 246 BC. He eventually became the chief librarian at the Library of Alexandria.
It is worth noticing the verb summoned. He did not simply drift into the post: he was called from Athens to fill a specific vacancy left by another scholar.
A link with Archimedes
Archimedes was closely affiliated with him and sent him material for comment. That detail shows that Eratosthenes was regarded as a reader whose comments were worth seeking.
What he is known for
Geography
He is coined as the originator of the terms geography and geographer. He also created the first global projection of the world incorporating parallels and meridians. Strabo said Eratosthenes was a mathematician among geographers and a geographer among mathematicians.
Mathematics
He introduced the sieve of Eratosthenes, a method of identifying prime numbers and composite numbers.
Chronology
He dated the sack of Troy to 1184 BC using Egyptian and Persian records. This shows that his interest in measurement extended from the shape of the Earth to the dating of past events.
Two nicknames, two opinions
His devotees nicknamed him Pentathlos, meaning all-rounded, for his knowledge in every area of learning. Some critics, however, called him Beta (Second, or Number 2) because he always came in second in his endeavors, according to the Suda.
| Nickname | Meaning | Given by |
|---|---|---|
| Pentathlos | All-rounded | His devotees |
| Beta | Second, or Number 2 | Some critics, according to the Suda |
The two names look at the same breadth from opposite sides. One praises the range of his knowledge; the other is attributed to critics who saw him as always second. The source records both, and a careful reader can hold both without choosing for it.
The last years
As he aged his eyesight failed, and he voluntarily starved himself to death at about age 80 in Alexandria around 196 BCE. The source gives no further explanation, so it is best to report the fact plainly.
Timeline
- 332 BC: Cyrene came under the rule of Alexander the Great.
- 276 BC: Eratosthenes was born in Cyrene.
- Late 260s BC: He went to Athens to further his studies.
- 246 BC: Ptolemy III succeeded his father Ptolemy II.
- c. 196 BC: He died in Alexandria at about age 80.
Practice: describe a career in the past tense
Here is a practice exercise built from the facts above. The model answer is my own example sentence, not a statement by Eratosthenes or anyone else.
Task: Write three English sentences about Eratosthenes in the simple past, using one fact about his birthplace, one about his move, and one about his work.
Model answer:
Short commentary: Each sentence uses a past-tense verb: was born, was summoned, became. The middle one is passive, because the focus is on what happened to him rather than on who did it. Try rewriting it in the active voice, for example by naming Ptolemy III's court as the subject only if you can support it from your own source.
More ideas to try
- Write a one-paragraph summary of his contributions to geography and mathematics, using only the source.
- List the nicknames Pentathlos and Beta and explain in your own words why each was given.
- Make a set of timeline cards from his birth in Cyrene to his death in Alexandria.
Using a language app for this task
The Riretriever language app teaches English, Spanish, Chinese, and Japanese through courses built around historical figures and classic literature. It builds a level-by-level course from figures the learner chooses. The app says its characters are reproduced by AI in conversations and that these are not their actual statements. I cannot say whether Eratosthenes is included. If you choose figures to study, you could try keeping your own timeline cards beside any course, and remember that AI conversations are not real quotations.