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Longitude in Ptolemy’s Geography: Why Does His Map Look Elongated from West to East?

Shcheglov D. A.

It is well known that Ptolemy’s Geography used systematically exaggerated values for longitudes, which made his map look excessively stretched from West to East in comparison with modern maps. According to one recent hypothesis, the source of Ptolemy’s inaccurate longitudes lay in his adoption of a new value for the Earth’s circumference. Ptolemy chose to rely on an estimate provided by Posidonius instead of a smaller estimate by Eratosthenes, and as a result, all the distances converted from linear into angular units became exaggerated. The present article tests this hypothesis and finds that, on the one hand, it explains the stretching of the map only partially, and in some cases does not fit the data at all. On the other hand, the stretching of Ptolemy’s map can largely be explained by a systematic exaggeration of the distances between basic points.

Key words: ancient geography, Claudius Ptolemy, Eratosthenes, geographical longitude, Marcus Vipsanius Agrippa, Marinus of Tyre, stade as the unit of length

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Shcheglov D. A., Longitude in Ptolemy’s Geography: Why Does His Map Look Elongated from West to East?, Voprosy Istorii Estestvoznaniia i Tekhniki [Studies in History of Science and Technology], 2015, vol. 36, no. 2, p. 209–238

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