Eratosthenes of Cyrene was a natural philosopher and poet who is best known for his development of modern cartographical principles. He is best known for correctly calculating the circumference of the Earth and the tilt of its axis, and for making the first world map with latitude lines. His abilities were so great in so many different fields that some of his contemporaries nicknamed him Pentathalos, as a champion of multiple skills.
Eratosthenes was born in Cyrene (in modern Libya) sometime between 285 and 276 BC. The exact year is not known. According to the 10th-century Suda, a Byzantine encyclopedia, he was born in the 126th Olympiad (276-273 BC). His later move to Alexandria can be accurately dated to either 245 or 246 BC, and is believed to have taken place when Eratosthenes was in his mid-40s.
Eratosthenes' father was Aglaus. He is mentioned nowhere else in Cyrenaean records, so his family was probably not prominent.
Eratosthenes is known to have studied under the philosopher Ariston of Chios, who was among the students of Zeno. His other teachers included the grammarian Lysanias and the poet-scholar Callimachus, who had scholastic links with the library at Alexandria. At the time, Alexandria was considered to be the Mediterranean center of learning.
Strabos records that Eratosthenes also traveled to Athens and studied with Zeno, who died in roughly 261 BC. This is unlikely with the Olympiad birthdate, because that would make Eratosthenes extremely young to study with a renowned philosopher. However, the word "gnorimo" can also be translated as "acquaintance."
When Ptolemy III Euergetes came to power in Egypt in 246 BC, he invited Eratosthenes to come to Alexandria to be the tutor of his son Philopator. In 236 BC, Eratosthenes was appointed the third librarian of the library of Alexandria, succeeding Apollonius of Rhodes. He also became good friends with Archimedes.
The Suda records that Eratosthenes died at the age of 80. Since that time, nearly all of Eratosthenes' written work has been lost. His achievements survive only through the writings of others.
Eratosthenes is most widely known for having calculated the circumference and axial tilt of the Earth by using only his detailed surveying knowledge of Egypt, his own location at Alexandria, and a well near modern-day Aswan. He used Egyptian stadions for his units of length rather than the longer Attic stadion, which resulted in a final error of less than 2%.
His method was based on geometry. Eratosthenes knew that the summer solstice Sun did cast a shadow in Alexandria, and he knew how far south of the zenith the solstice noonday Sun came. Because Aswan is very close to the Tropic of Cancer, the Sun shone so close to straight down a local well at noon during the summer solstice that it was believed to cast no shadow at all. The error can be accounted for by the very small difference and by the fact that the Earth is not a perfect sphere.
Calculating the size of the Earth was a natural extension of Eratosthenes' survey and cartographic work. He is credited with making the earliest world map to include parallels and meridians. Along with much of the rest of Eratosthenes' original work, it has been lost, but other accounts and copies of it make it possible to reconstruct it.
Eratosthenes is also credited with accurately calculating the distance from the Earth to the Sun. As part of his attempt to standardize chronology, he may have invented the leap day. He also tried to put dates to past events all the way back to the conquest of Troy. This was the first scientific chronology.
The Sieve of Eratosthenes was an early algorithm for finding prime numbers. It checks numbers for divisibility by previously known prime numbers. Leonhard Euler used a version of this sieve in one of his own proofs, roughly 2,000 years later.
All of Eratosthenes' poetry has been lost. Only 2 poems are known by name: "Hermes" and "Erigone." However, contemporaries mention "Erigone's" verse as being flawless.
According to the Suda, Eratosthenes was nicknamed "bemata" ("platforms") because he was second in the world in nearly every known branch of learning. However, this is believed to be a scribal mistake for the word "beta," the second letter in the Greek alphabet. Although he was a towering figure of his time, his time was also full of towering figures.