Europe had long enjoyed a safe passage to China and India— sources of valued goods such as silk, spices and opiates— under the hegemony of the Mongol Empire (the Pax Mongolica, or "Mongol peace"). With the Fall of Constantinople to the Muslims in 1453, the land route to Asia was no longer an easy route. Portuguese sailors took to traveling south around Africa to Asia. Columbus had a different idea. By the 1480s, he 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," i.e., the Atlantic.

Following Washington Irving's myth-filled 1828 biography of Columbus, it was commonly believed Columbus had difficulty obtaining support for his plan because Europeans thought the Earth was flat.[2] In fact, few people at the time of Columbus’s voyage, and virtually no sailors or navigators, believed this.[citation needed] Most agreed Earth was a sphere. This had been the general opinion of ancient Greek science, and continued as the standard opinion (for example of Bede in The Reckoning of Time) until Isidore of Seville misread the classical authors and stated the Earth was flat, inventing the T and O map concept. This view was very influential, but never wholly accepted. Knowledge of the Earth's spherical nature was not limited to scientists: for instance, Dante's Divine Comedy is based on a spherical Earth. Columbus put forth arguments based on the circumference of the sphere. Most scholars accepted Ptolemy's claim the terrestrial landmass (for Europeans of the time, comprising Eurasia and Africa) occupied 180 degrees of the terrestrial sphere, leaving 180 degrees of water.

Columbus, however, believed the calculations of Marinus of Tyre, putting the landmass at 225 degrees, leaving only 135 degrees of water. Moreover, Columbus believed one degree represented a shorter distance on the earth's surface than was commonly held. 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 (3700km, 2300 statute miles) Columbus did not realize Al-Farghani used the much longer Arabic mile (about 1830 meters).

Columbus' problem was, experts did not accept this estimate. The true circumference of the Earth is about 40000km (25000sm), a figure established by Eratosthenes in the second century BC,[3][4] and the distance from the Canary Islands to Japan 19600km (12200sm). No ship in the 15th century could carry enough food and fresh water for such a journey. Most European sailors and navigators concluded, correctly, sailors undertaking a westward voyage from Europe to Asia non-stop would die of starvation or thirst long before reaching their destination. Spain, however, only recently unified through the marriage of Ferdinand and Isabella, was desperate for a competitive edge over other European countries in trade with the East Indies. Columbus promised them one.

While Columbus' calculations were inaccurate concerning the circumference of the Earth and the distance from the Canary Islands to Japan, almost all Europeans were mistaken in thinking the aquatic expanse between Europe and Asia was uninterrupted. Although Columbus died believing he had opened up a direct nautical route to Asia, in fact he had established a nautical route between Europe and the Americas. The route to America, rather than to Japan, gave Spain a competitive edge in developing a mercantile empire.