This goes into detail about Columbus’ multiple miscalculatons:
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.)
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.
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.
SO a combination of error, willful blindness, and pig-headedness. Plus, maybe he heard that the Basques (or Bristolians) had seen land over there, assumed it was evidence of the other end of Asia.
As for the Basques (and Bristol and the Irish). The Basques may have preferred not to talk about their secret fishing hole, like modern anglers, but it’s pretty hard to hide the fact that ships would come back laden with plenty of cod. I’m sure it was a shared secret among the hundreds of Basque fishermen, local merchants, pub owners, etc. This:
If you wonder why they did not want to share, I recommend Farley Mowat’s book Sea of Slaughter describing the Gulf of St. Lawrence and surrounding lands, and the description of the earliest explorers - schools of cod so thick you could practically walk in the water; cod 10 feet long was a common occurrence. Anyone fishing there knew that spreading the word would (a) lead to overcrowded conditions as every fisherman in Europe headed there and (b) consequently severely depress the price so nobody could make a living fishing. As it was, they had a secret motherlode. Like the article says, they could claim it was from Iceland, but anyone who knew things knew Icelanders were not letting others fish in the area if they could help it.
There is evidence in Labrador of Basque fish camps where pottery remains indicate they put ashore to dry and salt fish, fairly close to contemporaneous with Columbus.
As for China and Japan and Indonesia (Spice Islands). They were well described in Polo’s memoirs. Plus, as was beaten to death in another thread, China and the Mediterranean world have known about each other since the Roman times at least. (IIRC they even exchanged ambassadors with Rome). The silk road, which also traded spices, was a lucrative business.
Daniel Boorstin in The Explorers describes the evolution of modern exploration. The kingdoms along the silk road each took their tax on the cargoes, and jealously prevented westerners from travelling to see the source of the goods. When Genghis Khan conquered Asia, from China to the Mediterranean, he created an empire where he allowed free travel. For about 200 years, until the empire fell apart, travel was unhindered and people like Polo and has father and uncle could travel to China and back without being harassed. Then it all closed up again, but now Europeans knew where all these spices were coming from, with tales of how shiploads of precious spices could be gotten for a pittance if only one could sail there. Think of it as that era’s equivalent of the search for the map of the lost gold mine.
the Portuguese monopolized the “around Africa” method, so Spain was willing to give westward a try against the advice of the scholars who knew it was a mistake.
