Columbus, Letter from the Fourth Voyage

Christopher Columbus 1451-1506

Original Source: The Voyages of Christopher Columbus, Being the Journals of his First and Third, and the Letters Concerning his First and Last Voyages, to Which is Added the Account of his Second Voyage Written by Andres Bernaldez. Now newly Translated and Edited, with an Introduction and Notes, by Cecil Jane. London: The Argonaut Press, 1930. (Copyright 2006. This text is freely available provided the text is distributed with the header information provided.)

"Ptolemy believed that he had well corrected Marinus, who is now found to have been very near the truth. Ptolemy places Catigara twelve lines from his west, which he fixed at two and one third degrees above Cape St. Vincent in Portugal. Marinus comprised the earth and its limits in fifteen lines. In Ethiopia Marinus draws beyond the equinoctial lines more than twenty—four degrees, and now that the Portuguese have sailed there, they find that this is true. Ptolemy says that the most southern land is the first place and that it does not lie more than fifteen and one—third degrees beyond. And the world is small. The dry land is six parts of it; the seventh only is covered with water. Experience has already shown this, and I have written it in other letters and with illustration from Holy Scripture concerning the situation of the earthly paradise, as Holy Church approves. I say that the world is not so great as the vulgar believe, and that a degree from the equinoctial line is fifty—six and two—thirds miles; easily this may be proved exactly. I leave this subject, inasmuch as it is not my intention to speak of this matter, but only to give an account of my voyage, hard and toilsome, although it is the most noble and profitable."

Columbus' Geographical Miscalculations

external image ColumbusWorld-1349721133712.jpg

Read Columbus' Geographical Miscalculations, by Douglas McCormick

" was widely known by the 15thCentury that the Earth is spherical. The question was, how big is the sphere? In 200 BCE, after all, Eratosthenes calculated the circumference of the earth to within one percent of its actual girth. He figured that one degree of latitude was equal to 59.5 nautical miles. In making his own calculation, however, Columbus preferred the values given by the medieval Persian geographer, Abu al Abbas Ahmad ibn Muhammad ibn Kathir al-Farghani (a.k.a. Alfraganus): one degree (at the equator) is equal to 56.67 miles. That was Columbus’s first error, which he compounded with a second: he assumed that the Persian was using the 4 856-foot Roman mile; in fact, Alfraganus meant the 7 091-foot Arabic mile...
"Taken together, the two miscalculations effectively reduced the planetary waistline to 16,305 nautical miles, down from the actual 21,600 or so, an error of 25 percent...
"Through a complicated chain of reasoning that mixed Ptolemy, Marinus of Tyre, and Marco Polo with some “corrections” of his own, Columbus calculated that he would find Japan at 85º west longitude (rather than 140° east)—moving it more than 8,000 miles closer to Cape St. Vincent.
All in all, he figured, the Indies were just 68 degrees west of the Canary Islands. Calculated travel distance: 3080 nautical miles. Actual distance from Tenerife to Jakarta: 7313 nautical miles. Margin of error: 58 percent..."

Map showing Columbus's view of distance between the Azores and Japan.
Map showing Columbus's view of distance between the Azores and Japan.

Look at Who was Right? a class activity from the Lawrence Hall of Science, at the University of California, for diagrams of Eratosthenes', Columbus, and the modern views of the Earth.