The ship’s position is the geographic position of the ship at a given moment and is measured in latitude and longitude coordinates. The ship’s position is thus the intersection between a meridian and a parallel. Calculating the ship’s position on the open sea was often a hard problem to solve because of the difficulty of determining the exact longitude. By measuring latitude, mariners could be practically sure that they were at a given parallel, but they could not be certain of their distance from the point of departure and the point of arrival.
They could, however, combine the measurements of the altitude of the heavenly bodies whose zenithal projection was known at the time of measurement. In this way, mariners could obtain a fairly accurate estimate of the ship’s position, but this depended to a significant extent on the precision of the instruments and on the measurer’s skill.