Note 463 — Chapter VI · THE FINDING OF STRANGE COASTS.

Footnote 463: Columbus was confirmed in this opinion by the book of the Arabian astronomer Alfragan, written about A. D. 950, a Latin translation of which appeared in 1447. There is a concise summary of it in Delambre, Histoire de l'astronomie du Moyen Âge, pp. 63-73. Columbus proceeded throughout on the assumption that the length of a degree at the equator is 56.6 geographical miles, instead of the correct figure 60. This would oblige him to reduce all Toscanelli's figures by about six per cent., to begin with. Upon this point we have the highest authority, that of Columbus himself, in an autograph marginal note in his copy of the Imago Mundi, where he expresses himself most explicitly: "Nota quod sepius navigando ex Ulixbona ad Austrum in Guineam, notavi cum diligentia viam, ut solitum naucleris et malineriis, et preteria accepi altitudinem solis cum quadrante et aliis instrumentis plures vices, et inveni concordare cum Alfragano, videlicet respondere quemlibet gradum milliariis 56-2/3. Quare ad hanc mensuram fidem adhibendam. Tunc igitur possumus dicere quod circuitus Terræ sub aræ equinoctiali est 20,400 milliariorum. Similiter que id invenit magister Josephus phisicus et astrologus et alii plures missi specialiter ad hoc per serenissimum regem Portugaliæ," etc.; anglicè, "Observe that in sailing often from Lisbon southward to Guinea, I carefully marked the course, according to the custom of skippers and mariners, and moreover I took the sun's altitude several times with a quadrant and other instruments, and in agreement with Alfragan I found that each degree [i. e. of longitude, measured on a great circle] answers to 56-2/3 miles. So that one may rely upon this measure. We may therefore say that the equatorial circumference of the earth is 20,400 miles. A similar result was obtained by Master Joseph, the physicist [or, perhaps, physician] and astronomer, and several others sent for this special purpose by the most gracious king of Portugal."—Master Joseph was physician to John II. of Portugal, and was associated with Martin Behaim in the invention of an improved astrolabe which greatly facilitated ocean navigation.—The exact agreement with Ptolemy's figures shows that by a mile Columbus meant a geographical mile, equivalent to ten Greek stadia.[Back to Main Text]