Note 398-3 — THE FOURTH VOYAGE OF COLUMBUS

Ptolemy reckoned the length of the degree on the equator at 62 1/2 miles. The shorter measurement of 56 2/3 was the estimate adopted by the Arab astronomer Alfragan in the ninth century and known to Columbus through Cardinal d’Ailly’s Imago Mundi, the source of much if not most of his information on the geographical knowledge and opinions of former times. Cardinal d’Ailly’s source of information about Alfragan was Roger Bacon’s Opus Majus. Columbus was deeply impressed with Alfragan’s estimate of the length of the degree and annotated the passages in the Imago Mundi. Cf. Raccolta Colombiana, Parte I., tomo II., pp. 378, 407, and frequently. See this whole question in Vignaud, Toscanelli and Columbus, p. 79 et seqq.