Eratosthenes

Eratosthenes of Cyrene (c. 276 BC – c. 195/194 BC) was an Ancient Greek polymath—a philosopher, scholar, mathematician, geographer, poet, astronomer, and music theorist. He rose to become the chief librarian of the Library of Alexandria. His work laid the groundwork for modern geography, and he coined the terms "geography" and "geographer." He is most famous for being the first known person to calculate the Earth's circumference, and he also made the first calculation of Earth's axial tilt, which proved remarkably accurate. He created the first global map projection using parallels and meridians based on the geographic knowledge of his time. He founded scientific chronology, using Egyptian and Persian records to date key events of the Trojan War, placing the fall of Troy in 1184 BC. In number theory, he devised the sieve of Eratosthenes, an efficient method for identifying prime and composite numbers.

His admirers called him Pentathlos, after the well-rounded Olympic competitors, because he excelled in every field of learning. However, according to the 10th-century Byzantine encyclopedia the Suda, some critics mocked him as Beta (Second, or Number 2), since he always came in second in his endeavors.

Eratosthenes was born in 276 BC in Cyrene (modern-day Libya), the son of Aglaos. Cyrene, founded by Greeks in the second half of the 7th century BC, became the capital of Cyrenaica due to its coastal location, defensible position, abundant water, and fertile soil. It came under Alexander the Great's rule in 332 BC, and after his death in 323 BC, following a local civil war, it was taken by Ptolemy I Soter, founder of the Ptolemaic Kingdom. Under Ptolemaic rule, Cyrene had a strong economy based on exporting horses and silphium, and it was known as a prosperous center of Greek culture.

The rarity of both Eratosthenes' and his father's names suggests humble origins, according to scholar Roller, though upward mobility was possible in the Hellenistic world. Matthew, however, argues that his name, meaning "lovely strength," and his early education imply noble birth and membership in Cyrene's aristocracy. Like any young Greek, he would have studied at the local gymnasium, learning physical skills along with reading, writing, arithmetic, poetry, and music.

In the late 260s BC, Eratosthenes went to Athens for further study. According to Strabo, he studied Stoicism under its founder, Zeno of Citium, though Zeno died soon after his arrival, so their interaction was limited. Strabo also mentions the little-known Apelles of Chios as one of his teachers. Eratosthenes is said to have studied under the Cynic Aristo of Chios and the eclectic Bion of Borysthenes, as well as Arcesilaus of Pitane, the new head of the Platonic Academy. His later mathematical work suggests he received mathematical training there. The Suda adds that he was a student of Lysanias of Cyrene, a philologist and grammarian specializing in Homer. The poet, scholar, and librarian Callimachus likely debated with Eratosthenes in scholarly circles, though he was probably never his formal teacher.

Strabo complained that Eratosthenes did not show enough respect to Zeno and criticized his association with diverse schools of thought. Strabo believed Eratosthenes was unwilling to commit to philosophy, learning just enough to appear a philosopher, and saw it as a distraction from his real work. Later authors may have agreed: the Suda notes he was called Beta because he was not seen as the top expert in any field. Others called him Pentathlos (All-Rounded) for his many skills, though that title also referred to an athlete who competes in many events but places second in all. Strabo described him as a mathematician among geographers and a geographer among mathematicians.

Most of Eratosthenes' studies focused on philosophy; mathematics was less prominent, and philology even less so. He could not formally study geography, as the discipline did not yet exist, but he was exposed to extensive geographic literature, including works by Homer (whom he considered the first geographer), Hecataeus of Miletus, Aeschylus, Herodotus, and others. Born forty years after Alexander the Great's death, he also encountered writings by Alexander's travel companions—Androsthenes, Nearchos, Onesikratos, Ptolemy I, and others—who recorded their journeys and whose conquests opened the way for Hellenistic explorers.

Eratosthenes stayed in Athens for about twenty years, studying and writing. During this time, he wrote *Platonikos*, exploring mathematics and music in Plato's philosophy, as well as the poetic works *Hermes* and *Erigone*. His *Chronographies* focused on key dates of the Trojan War, and his *Olympic Victors* listed winners of the Olympic games. Little else is known about this period. In 246 BC, Ptolemy III succeeded his father, Ptolemy II. Over the next twenty-five years, the Ptolemaic court...