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.

For about a hundred years (from about 1767 until about 1850) mariners lacking a chronometer used the method of lunar distances to determine Greenwich time, an important step in finding longitude.