  The use of the observed positions of celestial bodies to determine the
location of a navigator and to direct vessels, was an aspiration of ancient
seafarers. Various peoples, in the Mediterranean as much as in the Indian
Ocean, in China as much as in the Pacific, looked up to the sky as a guide for
ships, but there is no convincing evidence this was achieved until the Modern
Age. It was not before the Modern Age that two techniques were developed by the
Portuguese, the measurement of the altitude of the North Star and that of the
meridian altitude of the Sun, which, adapted and simplified for the use of
sailors, enabled them to achieve good accuracy at least as far as the
measurement of latitude was concerned. To this end, instruments that had
already been used by astronomers for several centuries were also adapted and
simplified: the quadrant, the astrolabe and, later, the cross-staff.
