page_title: Lunar distance (astronomy)
hierarchical_section_title: Lunar distance (astronomy)
caption_attribution_description: English: Lunar perigee and apogee apparent size comparison (from April and October 2007, when events occurred near full-phases) thumb|FullSky Observatory software logo External links Inconstant Moon: The Moon at Perigee and Apogee Apogee Moon, Perigee Moon NASA APOD October 25, 2007 (Feb/Sept 2006 photographic comparison)
context_page_description: Lunar distance, also called Earth–Moon distance, Earth–Moon characteristic distance, or distance to the Moon, is a unit of measure in astronomy. It is the average distance from the center of Earth to the center of the Moon. More technically, it is the mean semi-major axis of the geocentric lunar orbit. It may also refer to the time-averaged distance between the centers of the Earth and the Moon, or less commonly, the instantaneous Earth–Moon distance. The lunar distance is approximately 400000 km, or a quarter of a million miles.
  The mean semi-major axis has a value of 384,402 km. The time-averaged distance between Earth and Moon centers is 385,000.6 km. The actual distance varies over the course of the orbit of the Moon, from 356,500 km at the perigee to 406,700 km at apogee, resulting in a differential range of 50,200 km.
  Lunar distance is commonly used to express the distance to near-Earth object encounters.
context_section_description: Lunar distance (LD or ), also called Earth–Moon distance, Earth–Moon characteristic distance, or distance to the Moon, is a unit of measure in astronomy. It is the average distance from the center of Earth to the center of the Moon. More technically, it is the mean semi-major axis of the geocentric lunar orbit. It may also refer to the time-averaged distance between the centers of the Earth and the Moon, or less commonly, the instantaneous Earth–Moon distance. The lunar distance is approximately 400000 km, or a quarter of a million miles.
  The mean semi-major axis has a value of 384,402 km (238,856 mi). The time-averaged distance between Earth and Moon centers is 385,000.6 km (239,228.3 mi). The actual distance varies over the course of the orbit of the Moon, from 356,500 km (221,500 mi) at the perigee to 406,700 km (252,700 mi) at apogee, resulting in a differential range of 50,200 km (31,200 mi).
  Lunar distance is commonly used to express the distance to near-Earth object encounters. Lunar distance is also an important astronomical datum; the precision of this measurement to a few parts in a trillion has implications for testing gravitational theories such as general relativity, and for refining other astronomical values such as Earth mass, Earth radius, and Earth's rotation. The measurement is also useful in characterizing the lunar radius, the mass of the Sun and the distance to the Sun.
  Millimeter-precision measurements of the lunar distance are made by measuring the time taken for light to travel between LIDAR stations on the Earth and retroreflectors placed on the Moon. The Moon is spiraling away from the Earth at an average rate of 3.8 cm (1.5 in) per year, as detected by the Lunar Laser Ranging Experiment.
caption: A lunar distance, 384402 km, 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 closest and farthest from Earth.
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