Abstract

The paper argues that the depiction of the Mediterranean coast of Africa in Ptolemy’s Geography was based on a source similar to the Stadiasmus of the Great Sea. Ptolemy’s and the Stadiasmus’ toponymy and distances between major points are mostly in good agreement. Ptolemy’s place names overlap with those of the Stadiasmus by 80%, and the total length of the coastline from Alexandria to Utica on Ptolemy’s map deviates from the Stadiasmus data by only 1% or 1.5%. A number of serious disagreements between Ptolemy’s map and the Stadiasmus regarding the length of particular coastal stretches can be explained by assuming that Ptolemy had to tailor the distance data derived from periploi to his other sources, especially, to the longitudes of the key reference points, such as Cape Phyces, Cyrena, Berenica, Aesporis, Thena, and Carthage. A notable stretching and the subsequent contraction of the coast between Alexandria and Cyrenaica, as are exhibited by Ptolemy’s map relative to the Stadiasmus’ data, can be explained by assuming that several points on this coast were tied to the position of Crete, which was moved to the west being pushed by the westward shift of Rhodes. A sharp contraction of the two coastal stretches of the Great Sirte, oriented along the north-south direction, can be explained by Ptolemy’s erroneously underestimated value for the circumference of the Earth. The analysis of this contraction, as well as of the east-west stretching of the coast between Alexandria and Cyrene in angular terms relative to the modern map, makes it possible to assess the magnitude of Ptolemy’s error and to determine the length of his stade. This analysis shows that Ptolemy’s value for the circumference of the Earth must have been underestimated by approximately 20–27%, and Ptolemy’s stade must have been approximately 175–185 m length. Comparison of the Stadiasmus distance data with the modern map shows that the average length of the stade was close to 179 m or to the “common” stade of 185 m for the stretch between Alexandria and Berenice.