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In celestial navigation, lunar distance, also called a lunar, is the angular distance between the Moon and another celestial body. The lunar distances method uses this angle and a nautical almanac to calculate Greenwich time if so desired, or by extension any other time. That calculated time can be used in solving a spherical triangle.
The lunar distance is on average approximately 385,000 km (239,000 mi), or 1.28 light-seconds; this is roughly 30 times Earth's diameter or 9.5 times Earth's circumference. Around 389 lunar distances make up an AU astronomical unit (roughly the distance from Earth to the Sun). Lunar distance is commonly used to express the distance to near ...
Lunar Laser Ranging Experiment from the Apollo 11 mission. Lunar Laser Ranging (LLR) is the practice of measuring the distance between the surfaces of the Earth and the Moon using laser ranging. The distance can be calculated from the round-trip time of laser light pulses travelling at the speed of light, which are reflected back to Earth by ...
The lunar distance method depends on the motion of the Moon relative to the "fixed" stars, which completes a 360° circuit in 27.3 days on average (a lunar month), giving an observed movement of just over 0.5°/hour.
An older but still useful and practical method of determining accurate time at sea before the advent of precise timekeeping and satellite-based time systems is called "lunar distances," or "lunars," which was used extensively for a short period and refined for daily use on board ships in the 18th century. Use declined through the middle of the ...
In the eighteenth and nineteenth centuries, navigational tables based on lunar theory, initially in the Nautical Almanac, were much used for the determination of longitude at sea by the method of lunar distances.
The Board of Longitude therefore decided that rewards should be given to Harrison (£10,000), Mayer (£3000, posthumously) and others involved in helping to develop the lunar-distance method. [ 16 ] [ 17 ] Harrison was told that a further reward of £10,000 would be forthcoming if he could demonstrate the replicability of his watch.
Before the 18th-century development of the marine chronometer by John Harrison and the lunar distance method, dead reckoning was the primary method of determining longitude available to mariners such as Christopher Columbus and John Cabot on their trans-Atlantic voyages.