Lunar distance

Lunar distance
Lunar distance
	A lunar distance, 384,399 km (238,854 mi), is the Moon's average distance to Earth. The actual distance varies over the course of its orbit. The image compares the Moon's apparent size when it is nearest and farthest from Earth.
General information
Unit system	astronomy
Unit of	distance
Symbol	LD,
Conversions
1 LD in ...	... is equal to ...
SI base unit	3.84399×108 m
Metric system	384399 km
English units	238854 mi
Astronomical unit	0.002569 au
Lightsecond	1.28 ls

The instantaneous Earth–Moon distance, or distance to the Moon, is the distance from the center of Earth to the center of the Moon. Lunar distance (LD or ${\textstyle \Delta _{\oplus L}}$ ), 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 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-Earth object encounters.^[1] Lunar semi-major axis is an important astronomical datum; the few millimeter precision of the range measurements determines semi-major axis to a few decimeters; it has implications for testing gravitational theories such as general relativity,^[2] and for refining other astronomical values, such as the mass,^[3] radius,^[4] and rotation of Earth.^[5] The measurement is also useful in characterizing the lunar radius, as well as the mass of and distance to the Sun.

Millimeter-precision measurements of the lunar distance are made by measuring the time taken for laser beam light to travel between stations on Earth and retroreflectors placed on the Moon. The Moon is spiraling away from Earth at an average rate of 3.8 cm (1.5 in) per year, as detected by the Lunar Laser Ranging experiment.^[6]^[7]^[8]

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