Magnetic Compass.

Accurate navigation on the open ocean requires precise knowledge of a ship's direction of travel. Experienced sailors can make reasonably good estimations of travel direction based upon the position of the sun in the daytime sky and the North Star at night. This sort of informed guesswork, however, was often not sufficiently accurate to lead fifteenth-century ships on lengthy journeys across open water to a small point in the middle of the ocean such as the Azores. Moreover in cloudy conditions when the precise positions of the sun and North Star in the sky are indeterminable, even experienced mariners such as Christopher Columbus could find themselves helplessly lost if they depended exclusively on this sort of navigation. Missing one's target in the open ocean could be fatal; ships that became lost at sea often ran out...