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This article attempts to shed some light on the origins of Ptolemy’s map of the world, which remains a mystery. The premise is that Ptolemy and other ancient geographers largely drew on the same or a similar pool of sources and common beliefs. Similarities between them can, therefore, give us a key to understanding the prehistory of Ptolemy’s map. Comparison of distances on his map with those given by other sources leads to the conclusion that a large area of the map, approximately from the Bosporus to the Indus, reproduced Eratosthenes’ geographical system, with linear distances converted to angular degrees according to Eratosthenes’ own scale. It is argued that this area represents a remnant of an earlier version of Ptolemy’s map. Analysis of latitude and longitude reveals notable differences between Ptolemy’s map and Eratosthenes’ ideas concerning the latitude of Babylon and the Alexandrian prime meridian, and the impact this seems to have had on the shape of neighbouring regions is noted. [ABSTRACT FROM AUTHOR]

Eratosthenes (circa 276 B.C.-194 B.C.) is considered a famous scientist of ancient Greece. He was a mathematician and geographer. Born in Cyrene, now Shahhat (Libya), he was appointed to teach the son of the Egyptian King Ptolemy III Euergetes. In 240 B.C., he became the third chief librarian the Great Library of Alexandria. Eratosthenes laid basics for mathematical geography. He was the first to calculate precisely in an original way the Earth meridian's length between Syene and Alexandria. For this purpose he used perpendicular projection of the sun rays during summer solstice (06.22) near the town Syene, now Aswan. His estimation of the length of the Earth's radius (6300 km) is close to present estimation (6371 km). He calculated that a year possesses 365.25 days. He also emphasized the significance of maps as the most important thing in geography. Eratosthenes was the first one to use the term "geographem" to describe the Earth. In this way he legitimized the term of geography. He also put into system geographical information from various sources in order to obtain a map of the world as precise as possible. [ABSTRACT FROM AUTHOR]