The Accuracy of Ancient Cartography Reassessed: The Longitude Error in Ptolemy’s Map

This essay seeks to explain the most glaring error in Ptolemy’s geography: the greatly exaggerated longitudinal extent of the known world as shown on his map. The main focus is on a recent hypothesis that attributes all responsibility for this error to Ptolemy’s adoption of the wrong value for the c...

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Bibliographic Details
Published in:Isis 2016-12, Vol.107 (4), p.687-706
Main Author: Shcheglov, Dmitry A.
Format: Article
Language:English
Subjects:
Accuracy
Astronomy - history
Cartography
Dissent and Disputes - history
Earth (Planet)
Eratosthenes of Cyrene (276?-194? BC)
Geodesy
Geodetics
Geography
Geography - history
Geology - history
History of medicine and histology
History, Ancient
Humans
Hypotheses
Longitude
Mapping
Mathematical Concepts
Measurement
Scrutiny
Weights and Measures - history
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Summary:This essay seeks to explain the most glaring error in Ptolemy’s geography: the greatly exaggerated longitudinal extent of the known world as shown on his map. The main focus is on a recent hypothesis that attributes all responsibility for this error to Ptolemy’s adoption of the wrong value for the circumference of the Earth. This explanation has challenging implications for our understanding of ancient geography: it presupposes that before Ptolemy there had been a tradition of high-accuracy geodesy and cartography based on Eratosthenes’ measurement of the Earth. The essay argues that this hypothesis does not stand up to scrutiny. The story proves to be much more complex than can be accounted for by a single-factor explanation. A more careful analysis of the evidence allows us to assess the individual contribution to Ptolemy’s error made by each character in this story: Eratosthenes, Ptolemy, ancient surveyors, and others. As a result, a more balanced and well-founded assessment is offered: Ptolemy’s reputation is rehabilitated in part, and the delusion of highaccuracy ancient cartography is dispelled.
ISSN:0021-1753
1545-6994
DOI:10.1086/689763