The Ancient Geography of India/Appendix-C Correction of Error in Ptolemys Eastern Longitudes
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Appendix-C Correction of Error in Ptolemys Eastern Longitudes
[p.577]: Ptolemy's longitudes are so manifestly in excess of the truth that various methods of rectification have been suggested by different geographers.
That of M. Gossellin was to take five-sevenths of Ptolemy's measures, but his system was based upon the assumption that Ptolemy had made an erroneous estimate of the value of the degree both of the equinoctial and Rhodian diaphragms, as detailed by Eratosthenes. But for the geography of Asia, Ptolemy seems to have depended altogether upon the authority of Marinus, the Tyrian geographer, and of Titianus or Maes, a Macedonian merchant.
M. Grossellin's method was probably founded upon the average of Ptolemy's errors, deduced from the longitudinal excess of many well-known places. It is in fact an empirical correction of Ptolemy's errors, of the cause of which his theory offers nothing more than a mere guess. The true sources of Ptolemy's errors of longitude have been pointed out so clearly by Sir Henry Rawlinson that I cannot do better than repeat his explanation of them.1
1st. Upon a line drawn from Hierapolis on the Euphrates to the stone tower he converted road distance into measurement upon the map at a uniform reduction of 1 in 11½ instead of 1 in 8, or perhaps, which would be more accurate upon so long a line, of 1 in 7.
2nd. He computed an equatorial degree at 500 instead of 600 Olympic stadia, and thus upon the line of the Itinerary, which he assumed to be about the parallel of Rhodes, he allowed only 400 stadia to a degree, while the true measurement was 480.
3rd. In converting the schoeni of the Itinerary into Olympic stadia, he assumed their uniform identity with the
1 ' On the Site of the Atropatenian Ecbatana,' p. 122.
[p.578]: Persian parasang of 3-3/4 Roman miles, whereas Sir Henry believes the schcenm; to have been the natural measure of one hour employed by all caravans, both in ancient and modern times, to regulate their daily march, and to have averaged as nearly as possible a distance of 3 British miles.
The different corrections to be applied to Ptolemy's eastern longitudes on account of these three errors have been calculated by Sir Henry Rawlinson to amount to three-tenths, which is within one-seventieth part of the empirical correction used by M. Gossellin.
To show the accuracy of the correction here proposed, I need only refer to the difference of longitude between Taxila and Palibothra, which has been given at p. 9 of this work.