7.6 Longitude at sea

Until the end of the fifteenth century, sailors navigated with almost daily reference to land. In the Mediterranean it was difficult to go very far astray, and in western and northwest Europe navigation was coastal. Ships hugged the shore from Gibraltar to the Norway and the Baltic. The only exception to this rule was the trade between Scandinavia, Iceland, and occasionally Greenland. These routes were discovered (probably by accident) by the Vikings around 1000 CE. With the Portuguese voyages of discovery, in the fifteenth century, navigation became more difficult. For some time Portuguese sailors hugged the coast of Africa, as they carefully explored the contours of this continent. Both the winds and the currents there made sailing south difficult, however, and beginning with the voyages of Diaz (who rounded the Cape of Good Hope) in 1486, Columbus in 1492, and da Gama in 1498, Spanish and Portuguese sailors sailed the high seas for weeks on end without seeing land. How did they know where they were and whether they were on the right course?

Until the end of the fifteenth century, sailors navigated with almost daily reference to land. In the Mediterranean it was difficult to go very far astray, and in western and northwest Europe navigation was coastal. Ships hugged the shore from Gibraltar to the Norway and the Baltic. The only exception to this rule was the trade between Scandinavia, Iceland, and occasionally Greenland. These routes were discovered (probably by accident) by the Vikings around 1000 CE. With the Portuguese voyages of discovery, in the fifteenth century, navigation became more difficult. For some time Portuguese sailors hugged the coast of Africa, as they carefully explored the contours of this continent. Both the winds and the currents there made sailing south difficult, however, and beginning with the voyages of Diaz (who rounded the Cape of Good Hope) in 1486, Columbus in 1492, and da Gama in 1498, Spanish and Portuguese sailors sailed the high seas for weeks on end without seeing land. How did they know where they were and whether they were on the right course?

The only reference points on the high seas were the stars and Sun. Locations and courses now had to be spatial: a navigator needed to locate himself on a grid of imaginary lines of latitude and longitude.

The Portuguese pioneered the method of navigating by latitude. Ships had to be equipped with instruments (astrolabes, cross staffs) to measure the altitudes of stars or the Sun. It was not difficult to determine one's latitude to within about a degree by this method. Longitude was, however, a different matter. Observations of the Sun and stars were of no immediate help: in order to determine one's longitude with respect to, e.g., Lisbon, one had to find out the difference in local times between one's location and Lisbon. No easy method that was sufficiently accurate suggested itself. The magnitude of the problem is illustrated by the voyage of the Portuguese navigator Cabral who, on his way to the East Indies, swung west in the south Atlantic in order to pick up favorable winds and ran into the coast of Brazil. Further, the world maps prepared in the sixteenth century erred widely in the longitudes of places. The east-west length of the Mediterranean was in error by 19\x{00B0}--about 1100 miles! The longitudes of China and Japan were off by much larger margins. For nations engaged in trade with the East and West Indies, finding longitude at sea was a matter of national interest. Late in the sixteenth century the Spanish Crown instituted a large prize in the hope of a solution. This initiative was followed by the French, Dutch, and English governments in the seventeenth century.