The Error in Longitude in Ptolemy’sGeographyRevisited

Abstract

It is well known that all longitudes in Ptolemy’s Geography are cumulatively overestimated, so that his map is excessively stretched out from west to east in comparison with the modern map. In recent years, a hypothesis have been advanced that this stretching can be explained as a result of the change in the value of the Earth’s circumference from a larger one proposed by Eratosthenes to a lesser one by Posidonius. This explanation has two necessary presuppositions: (1) that Ptolemy’s map is stretched out by a factor of ∼1·4 which coincides with the ratio between Eratosthenes’ and Posidonius’ values, and (2) that Ptolemy’s error in longitude grows almost linearly. This article argues that the situation is more complex and nuanced. In fact, the error in longitude on Ptolemy’s map (and the stretching factor of the map, accordingly) varies considerably depending on longitude, latitude, and region. In particular, the error grows most slowly in the Eastern Mediterranean, which is probably due to the fact that this region was the centre of the ancient world. Therefore, Ptolemy’s error in longitude cannot be explained by one universal cause, but only by a combination of different factors.

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