Eratosthenes was born in Cyrene (present-day Libya) under Ptolemaic rule in c. 276 BCE. He became a renowned polymath, devising a mathematical sieve for determining prime numbers, and deducing with reasonable accuracy the circumference of the earth through latitudinal differences in shadow-length at noon. His mathematical bent influenced his Geography, which has not survived, and can only be inferred from citations, often critical, from successors like Hipparchus, Polybius and Strabo. His habitable world (oikoumene) is tripartite, divided into three continents Europe, Asia and Libya. Europe is subdivided by three promontories (the Mediterranean peninsulas). South Asia is delimited into four sealstones, geometrically shaped regions, the largest being the rhomboid India and Ariana. The oikoumene is encircled by a vast ocean, and bisected by a main parallel equating to roughly 36 degrees north, running from the Straits of Gibraltar to the coasts of the Eastern Sea. The habitable temperate zone is bounded by frigid and torrid zones inimical to life.
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