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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.
In contrast, the Lunar distance (LD or ), or Earth–Moon characteristic distance, is a unit of measure in astronomy. More technically, it is the semi-major axis of the geocentric lunar orbit . 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 .
At this location the selenographic colongitude at sunrise is defined as 0°. Thus, by the time of the Full Moon the colongitude increases to 90°; at Last Quarter it is 180°, and at the New Moon the colongitude reaches 270°. Note that the Moon is nearly invisible from the Earth at New Moon phase except during a solar eclipse.
Tidal rhythmites from 620 million years ago show that, over hundreds of millions of years, the Moon receded at an average rate of 22 mm (0.87 in) per year (2200 km or 0.56% or the Earth-moon distance per hundred million years) and the day lengthened at an average rate of 12 microseconds per year (or 20 minutes per hundred million years), both ...
The lunar distance is the angle between a suitable star and the Moon. The dotted lines show the distances between Aldebaran and the Moon, 5 hours apart. Moon not to scale. "Lunars" or lunar distances were an early proposal for the calculation of longitude, having been first made practical by Regiomontanus in his 1474 Ephemerides Astronomicae.
A diagram of a typical nautical sextant, a tool used in celestial navigation to measure the angle between two objects viewed by means of its optical sight. Celestial navigation, also known as astronavigation, is the practice of position fixing using stars and other celestial bodies that enables a navigator to accurately determine their actual current physical position in space or on the ...
Although the Moon's Hill sphere extends to a radius of 60,000 km (37,000 mi), [6] the gravity of Earth intervenes enough to make lunar orbits unstable at a distance of 690 km (430 mi). [7] The Lagrange points of the Earth-Moon system can provide stable orbits in the lunar vicinity, such as halo orbits and distant retrograde orbits.
On that trip, it is said that every man of the crew of 12, including the ship's cook, became competent to take and calculate lunar observations and to plot the correct position of the ship. The New Practical Navigator was published in 1799, followed by a second edition in 1800.