The error in longitude in Ptolemy’s geography

It is well known that all longitudes in Ptolemy’s Geography are cumulatively overestimated, so that his map is excessively stretched from west to east as compared with the modern map. In recent years, a number of scholars have suggested 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. As a result, all distances converted from linear units to angular became overestimated. This explanation has a necessary presupposition that the error in longitude on Ptolemy’s map grows linearly. This article argues that the error in longitude on Ptolemy’s map varies considerably depending on longitude, latitude and region. In particular, it grows most slowly in the Eastern Mediterranean, which is probably due to the fact that this region was the center of the ancient world. Therefore, the error in longitude on Ptolemy’s map cannot be explained by one universal reason, but only by a combination of different factors. Keywords: ancient geography, Claudius Ptolemy, geographical longitude.