Lunar distance

The instantaneous Earth–Moon distance, or distance to the Moon, is the distance from the center of Earth to the center of the Moon. In contrast, the Lunar distance (LD or Δ ⊕ L {\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 average lunar distance is approximately 385,000 km (239,000 mi), or 1.3 light-seconds. It is roughly 30 times Earth's diameter; to cover the same distance on earth would take a non-stop airplane flight more than two weeks. Around 389 lunar distances make up an astronomical unit (roughly the distance from Earth to the Sun). Lunar distance is commonly used to express the distance to near-Earth object encounters. Lunar semi-major axis is an important astronomical datum. It has implications for testing gravitational theories such as general relativity and for refining other astronomical values, such as the mass, radius, and rotation of Earth. The measurement is also useful in measuring the lunar radius, as well as the distance to the Sun. Millimeter-precision measurements of the lunar distance are made by measuring the time taken for laser light to travel between stations on Earth and retroreflectors placed on the Moon. The precision of the range measurements determines the semi-major axis to a few decimeters. 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.