Lunar distance method

A method of finding longitude at sea by measuring the Moon’s angular distance from a celestial body and comparing it with a predicted value for a reference meridian.

Lunar distance: The angular separation between the Moon and another celestial body as seen from Earth. This is the angle measured with a sextant before correction and comparison with tables.
Longitude problem: The historical challenge of determining longitude accurately, especially at sea. The lunar-distance method was one of the principal proposed solutions.
Celestial navigation: Navigation that determines position from observations of celestial bodies. Lunar distances formed one historical technique within this broader practice.
Nautical Almanac and Astronomical Ephemeris: A British annual publication of astronomical predictions, first issued for 1767. Its regular lunar-distance data made the method usable by navigators at sea.
Chronometer method: A longitude-finding method that compares local time with time kept at a known reference meridian. It provides longitude from a carried clock rather than lunar angular measurements.
Nautical almanac: An annual publication giving predicted positions of celestial bodies for navigation and astronomy. Its lunar-distance tables provide predicted angles tied to Greenwich time.
Johannes Werner: A German mathematician and astronomer who described lunar distances as a means of finding longitude in 1514. His proposal helped establish the method centuries before practical tables and instruments existed.
Nautical astronomy: The application of astronomical observations and calculations to navigation at sea. The method relies on nautical astronomical observations and predicted positions.
Board of Longitude: A British government body established in 1714 to reward solutions to the longitude problem. Its prizes and support shaped the competition between lunar distances and chronometers.
Latitude by noon altitude: A method of finding latitude from the Sun’s altitude at local apparent noon. Latitude could be found comparatively simply, while longitude required a time reference or lunar distances.

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