Note 9 — Chapter X · Globes and Globe Makers of the Early Seventeenth Century. The Dutch Scientific Masters and Their Preëminent Leadership

Wagner, H. Lehrbuch der Geographie. Leipzig, 1903. pp. 78-81; Frisius, G. De principiis astronomiae et cosmographiae. Antwerp, 1530. Chap. titled “De novo modo inveniendi longitudinem”; Ptolemaeus. Geographia. Chap. 4. Ptolemy here refers to an eclipse of the moon, in the year 331 B.C., which was observed in Arbela the fifth hour, in Carthage the second hour. He therefore noted a difference in time of three hours between the two places, and he therefore concluded the difference in longitude to be 43 degrees. Since the actual difference in longitude is but 34 degrees his error was of considerable magnitude, which found expression in his maps, and in the maps of those who followed him, as the greatest of geographical teachers, well into the seventeenth century. The method of determining longitude by means of the observation of the eclipses of the moon remained practically the only method until the end of the fifteenth century. Attention may here be called to work of Cassini and of other astronomers of his period. See II, 141.