Upon taking Constantinople in 1453, the Turks greatly affected European-Oriental trade.  New routes to the Orient began to be explored.  Christopher Columbus proposed, in various European courts, arriving to the East by sailing to the West.  His idea was founded on the belief that the Earth was round, formulated by the Greeks, but until then, never proven.  Basing his argument above all on the texts Tractatus de Imago Mundi by Pierre d'Ailly, Historia Rerum ubique Gestarum by Eneas Silvio Piccomini, Los Viajes de Marco Polo and probably the reports of Florentine mathematician and doctor Paolo dal Pozzo Toscanelli, who had written on the possibility of arriving to the Indies from the west, Columbus calculated that the circumference of the earth was 30,000 kilometers (10,070 kilometers less than the actual figure).  By his calculations, between the Canary islands and Cipango (Japan), there should have been 2,400 to 2,500 nautical miles, when in reality, the distance is four times greater.