"In making his own calculation, however, Christopher Columbus preferred the values given by the medieval Persian geographer, 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."