The physician, geographer, astrologer, and mathematician Paolo dal Pozzo Toscanelli (1397-1482) was born in Florence in 1397. He studied medicine at the University of Padua and was a close friend of Filippo Brunelleschi (1377-1446), possibly assisting him in the mathematical calculations for the construction of the dome of Santa Maria del Fiore in Florence. Almost all of his mathematical and astronomical works are lost, except for his observations on the comets that appeared between 1433 and 1472. Toscanelli's only surviving geographical study is his letter of 1474 to the canon of Lisbon Fernaõ Martins (15th cent.), which inspired Christopher Columbus's (1451-1506) plan. The letter, a copy of which was sent to Columbus with the attached map, proposed a shorter route to the Indies than the one taken by the Portuguese in their circumnavigation of Africa. Toscanelli claimed that the distance between Lisbon and Cipangu—now Japan—was barely 6,500 miles, or one-third of the estimated circumference of the Earth. Midway there was an island called Antilia: "And although I have spoken many times before of the short sea route that lies between here and the Indies, where the spices grow, which is, I believe, shorter than the one you pursue by way of Guinea [...] I have concluded that I can show this route more clearly and more convincingly by providing a map of the kind used by mariners. I accordingly submit such a map to His Majesty, devised by myself and drawn by my own hand, on which is laid out all the western part of the world [...] From the city of Lisbon in a line directly to the west, there are twenty-six spaces marked on the map, each representing two hundred and fifty miles, before arriving at the great and most noble city of Quisai […]." The geographic data available to Toscanelli, however, were inaccurate, perhaps on account of an error transmitted through the centuries concerning the length of the mile. The circumference of the Earth had been measured accurately by Eratosthenes and then by the Arabs, who confirmed its length of approximately 40,000 km. The length of the mile adopted by the Arabs, however, was greater than the value adopted in the Christian world. As a result, in the translation of these measurements, the Earth's circumference was reduced to about 30,000 km, closer to Ptolemy's estimate of roughly 33,000 km. Ptolemy's underestimate of the longitudinal length of the Earth eventually influenced Toscanelli's estimate of the westward sea route between Lisbon and the Indies. Toscanelli died in Florence on May 10, 1482.