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 person known to calculate the Earth's circumference, and he also accurately calculated the Earth's axial tilt. He created the first world map to include parallels and meridians, based on the geographic knowledge of his time. Eratosthenes founded scientific chronology, using Egyptian and Persian records to estimate key events of the Trojan War, dating the fall of Troy to 1184 BC. In number theory, he devised the sieve of Eratosthenes, an efficient method for finding prime numbers.

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

Eratosthenes was born in 276 BC in Cyrene (in modern-day Libya), the son of Aglaos. Cyrene, founded by Greeks in the 7th century BC, was the capital of Cyrenaica, thanks to its coastal location, defensible position, abundant water, and fertile soil. After Alexander the Great took control in 332 BC and died in 323 BC, Cyrene was seized by Ptolemy I Soter, one of Alexander's generals, following a local civil war. Under Ptolemaic rule, Cyrene thrived economically, exporting horses and silphium, and was a prosperous center of Greek culture.

Historians disagree on Eratosthenes' background. Roller notes that the rarity of his and his father's names suggests humble origins, though Hellenistic society allowed upward mobility. Matthew, however, argues that his name, meaning "lovely strength," and his early education point to an aristocratic upbringing. Like any young Greek, he would have studied at the local gymnasium, learning physical skills, 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 the school's founder, Zeno of Citium, though their time together was brief because Zeno died soon after Eratosthenes arrived. Strabo also mentions the little-known Apelles of Chios as a teacher. 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 studied under Lysanias of Cyrene, a philologist and grammarian focused on Homer. The poet, scholar, and librarian Callimachus likely engaged with Eratosthenes in debates and scholarly discussions, though he was probably not a formal teacher.

Strabo criticized Eratosthenes for not showing enough respect to Zeno and for associating with so many different schools of thought. He 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 records the nickname Beta, because he was never the top expert in any field. Others called him Pentathlos (All-Rounded) for his diverse skills, though Pentathlos also referred to an athlete who competes in many events but comes in second in all. Strabo described Eratosthenes as a mathematician among geographers and a geographer among mathematicians.

Most of Eratosthenes' studies were in philosophy; mathematics was less prominent, and philology even less so. He could not formally study geography because it did not yet exist as a discipline, 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 from Alexander's companions—Androsthenes, Nearchos, Onesikratos, and Ptolemy I—who described their travels 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 Olympic game winners. Little else is known about this period. In 246 BC, Ptolemy III succeeded his father, Ptolemy II. Over the next twenty-five years, the Ptolemies...