Lunar distance (navigation)

In celestial navigation, lunar distance is the angle between the Moon and another celestial body. The lunar distances method uses this angle, also called a lunar, and a nautical almanac to calculate Greenwich time. By comparing that calculated time to the measured local time, the navigator can determine longitude. The method was published in 1763 and used until about 1850 when it was superseded by the marine chronometer. It saw limited usage all the way up to the beginning of the 20th century on smaller vessels that could not afford a chronometer or had to rely on this technique for correction of the chronometer. A similar method uses the positions of the Galilean moons of Jupiter.

In celestial navigation, knowledge of the time at Greenwich and the measured positions of one or more celestial objects allows the navigator to calculate latitude and longitude. Reliable marine chronometers were unavailable until the late 18th century and not affordable until the 19th century. After the method was first published in 1763 by British astronomer royal Nevil Maskelyne, for about a hundred years (until about 1850) mariners lacking a chronometer used the method of lunar distances to determine Greenwich time as a key step in determining longitude. Conversely, a mariner with a chronometer could check its accuracy using a lunar determination of Greenwich time. The method saw usage all the way up to the beginning of the 20th century on smaller vessels that could not afford a chronometer or had to rely on the this technique for correction of the chronometer.

...
Wikipedia