<P> The true circumference of the Earth is about 40,000 km (22,000 nautical miles; 25,000 miles), a figure first established approximately by Eratosthenes in the 2nd century BC, and the distance from the Canary Islands to Japan 19,600 km (10,600 nautical miles; 12,200 miles). No ship that was readily available in the 15th century could carry enough food and fresh water for such a journey . Most European sailors and navigators concluded, probably correctly, that sailors undertaking a westward voyage from Europe to Asia non-stop would die of thirst, scurvy or starvation long before reaching their destination . Spain, however, having just completed the expensive Reconquista, was desperate for a competitive edge over other European countries in trade with the East Indies . Columbus promised such an advantage . </P> <P> Europeans generally assumed that the aquatic expanse between Europe and Asia was uninterrupted . While hints of North America, Vinland, were already surfacing in Europe, historians agree that Columbus calculated too short a distance from the Canary Islands to Japan by the standards of his peers . </P> <P> There was a further element of key importance in the plans of Columbus, a closely held fact discovered by or otherwise learned by Columbus: the Trade Winds . A brisk westward wind from the east, commonly called an "easterly", propelled Santa María, La Niña, and La Pinta for five weeks from the Canary Islands off Africa . To return to Spain eastward against this prevailing wind would have required several months of an arduous sailing technique upwind, called beating, during which food and drinkable water would have been utterly exhausted . Columbus returned home by following prevailing winds northeastward from the southern zone of the North Atlantic to the middle latitudes of the North Atlantic, where prevailing winds are eastward (westerly) to the coastlines of Western Europe, where the winds curve southward towards the Iberian Peninsula . So he used the North Atlantic's great circular wind pattern, clockwise in direction, in both legs of his voyage . </P> <P> At 8: 00 in the morning of 3 August 1492, Columbus departed from Castilian Palos de la Frontera (on the river Saltes, at the confluence of the rivers Rio Tinto and Rio Odiel). Columbus and his crew embarked on a voyage to find a shorter route to India and the Orient with three medium - sized ships, the Niña (real name Santa Clara), the Pinta, and the Santa Maria . The ships were the property of Juan de la Cosa and the Pinzón brothers (Martín Alonso Pinzón and Vicente Yáñez Pinzón), but the monarchs forced the Palos de la Frontera inhabitants to contribute to the expedition as well . </P>

How long did it take explorers to cross the atlantic