Note Back to Main Text — Chapter V · THE SEARCH FOR THE INDIES. WESTWARD OR SPANISH ROUTE.

The principal consideration, of course, was the distance to be traversed; and here Columbus was helped by an error which he shared with many geographers of his day. He somewhat underestimated the size of the earth, and at the same time greatly overestimated the length of Asia. The first astronomer to calculate, by scientific methods, the circumference of our planet at the equator was Ancient estimates of the size of the globe and the length of the Œcumene. Eratosthenes (B. C. 276-196), and he came—all things considered—fairly near the truth; he made it 25,200 geographical miles (of ten stadia), or about one seventh too great. The true figure is 21,600 geographical miles, equivalent to 24,899 English statute miles.[458] Curiously enough, Posidonius, in revising this calculation a century later, reduced the figure to 18,000 miles, or about one seventh too small. The circumference in the latitude of Gibraltar he estimated at 14,000 miles; the length of the Œcumene, or Inhabited World, he called 7,000; the distance across the Atlantic from the Spanish strand to the eastern shores of Asia was the other 7,000. The error of Posidonius was partially rectified by Ptolemy, who made the equatorial circumference 20,400 geographical miles, and the length of a degree 56.6 miles.[459] This estimate, in which the error was less than one sixteenth, prevailed until modern times. Ptolemy also supposed the Inhabited World to extend over about half the circumference of the temperate zone, but the other half he imagined as consisting largely of bad lands, quagmires, and land-locked seas, instead of a vast and open ocean.[460]