Astronomical navigation consisted in calculating the ship’s position by observing heavenly bodies. The most important instruments were the quadrant and the astrolabe. Such instruments made it relatively easy to determine latitude by observing the Sun, the Moon, and the planets.
It was distinctly more difficult to measure longitude, which depended on time-keeping. Mechanical clocks and other time-keeping devices such as water clocks and sandglasses were extremely imprecise. Ideally, they should have been able to keep the time of the port of departure, which would have been compared with the time determined on board the ship. The time difference multiplied by the length of the degree of longitude at a given latitude would then yield the distance traveled.
As one could not rely on the precision of mechanical clocks, the most accurate method was that of lunar distances. Because of the speed at which the Moon moves across the night sky, its revolution around the Earth can serve as an efficient indicator of time. Navigators could predict the Moon’s position relative to the stars and planets in a given location whose geographic coordinates were known. Thus, the Moon’s distance from the reference stars, measured on board the ship, provided the longitude of the observation location.