The position of the Sun and the brighter stars relative to the North Star long provided the key bearings for navigation at sea. In addition to observing the heavenly bodies, orientation methods relied on a measurement of distances traveled. This was done by empirically estimating the ship's mean velocity. These basic techniques were improved, beginning in the thirteenth century, with the introduction of the magnetic compass and later with the preparation of coastline charts known as portolans. Navigation systems remained largely unchanged until the fifteenth-sixteenth centuries, when nautical mapmaking received a powerful stimulus—largely thanks the Portuguese. Reliable tables of solar declination were produced, and the nautical astrolabe was developed.

Shortly after the mid-sixteenth century there appeared the log, an instrument that provided a more accurate measurement of the ship's speed. Traditional methods, which allowed seafarers to find their way with relative ease in the confined space of the Mediterranean, showed their limits when ocean navigation came into its own. One particularly acute handicap was the lack of a reliable method for determining longitude at sea. After the failure of proposals to solve the problem by astronomical methods, in which even Galileo was involved, the solution was found in the second half of the eighteenth century by an ingenious English craftsman, John Harrison. He developed an extremely accurate chronograph that made it possible to compare local time with the time of the port of departure, whose longitude was known.