Eratosthenes

Eratosthenes was a Greek scholar who estimated Earth’s circumference, developed mathematical geography, and directed the Library of Alexandria in the third century BCE.

Earth's circumference: The distance around Earth along a great circle. Eratosthenes estimated this quantity by comparing the Sun’s angles at Syene and Alexandria.
Shadow geometry: The use of shadow lengths and directions to infer angles, positions, or dimensions. A vertical object’s shadow in Alexandria revealed the Sun’s angular distance from the zenith.
Hellenistic period: The era following Alexander the Great’s conquests, marked by Greek-speaking kingdoms across the eastern Mediterranean and Asia. Eratosthenes worked in its scholarly centers during the third century BCE.
Geography: The study of places, environments, spatial relationships, and human activity on Earth. Eratosthenes helped establish geography as a mathematical and descriptive discipline.
Posidonius: A Greek Stoic philosopher and polymath who estimated Earth’s circumference in the first century BCE. His later estimate offered a competing ancient figure for Earth’s size.
Solar zenith angle: The angle between the Sun and the vertical direction at a location. The difference in this angle between two cities supplied the key measurement in his calculation.
Parallel lines: Lines in a plane that remain the same distance apart and never intersect. The Sun’s rays were treated as parallel when comparing their angles at the two cities.
Library of Alexandria: A major ancient institution for collecting and studying texts in Alexandria, Egypt. Eratosthenes served as its head and worked within its scholarly community.
Geographika: A lost multi-volume geographical work by Eratosthenes, known through later quotations and summaries. It presented his approach to mapping and describing the inhabited world.
Strabo: A Greek geographer and historian whose surviving work describes the ancient world. He preserved and discussed Eratosthenes’ geographical ideas.

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