(The "Colombus map" was drawn circa 1490 in the workshop of Bartolomeo and Christopher Colombus in Lisbon.) 
(Columbus' geographical conceptions.)Europe had long enjoyed a safe land passage to China and India— sources of valued goods such as silk, spices, and opiates— under the hegemony of the Mongol Empire. With the Fall of Constantinople to the Ottoman Turks in 1453, the land route to Asia became more difficult. The Columbus brothers had a different idea. By the 1480s, they had developed a plan to travel to the Indies, then construed roughly as all of south and east Asia, by sailing directly west across the "Ocean Sea”, for example the Atlantic.
Columbus, believed the (incorrect) calculations of Marinus of Tyre, putting the landmass at 225 degrees, leaving only 135 degrees of water. Moreover, Columbus believed that one degree represented a shorter distance on the earth's surface than was actually the case. Finally, he read maps as if the distances were calculated in Italian miles (1,238 meters). Accepting the length of a degree to be 56⅔ miles, from the writings of Alfraganus, he therefore calculated the circumference of the Earth as 25,255 kilometers at most, and the distance from the Canary Islands to Japan as 3,000 Italian miles (3,700 km, or 2,300 statute miles) Columbus did not realize Al-Farghani used the much longer Arabic mile (about 1,830 m).
(Handwritten notes by by Christopher Colombus on the latin edition of Marco Polo's Le livre des merveilles.)
