Reddit reviews The Life and Voyages of Christopher Columbus (Volume II)
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According to this link Columbus basically fucked up all the calculations, including using the wrong units and vastly overestimating how big Asia was.
>Washington Irving’s overly imaginative A History of the Life and Voyages of Christopher Columbus notwithstanding, it was widely known by the 15th Century 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. (This is, of course, the sort of confusion of units that sent the Mars Climate Orbiter into its terminal swan dive in September 1999.)
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>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.
And then there was the third error. “Not content with whittling down the degree by 25 percent,” Morison writes, “Columbus stretched out Asia eastward until Japan almost kissed the Azores.” 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.
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>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.