The ongoing Lunar Laser Ranging experiment or Apollo landing mirror measures the distance between surfaces of Earth and the Moon using laser ranging. Lasers at observatories on Earth are aimed at retroreflectors planted on the Moon during the Apollo program (11, 14, and 15), and the two Lunokhod missions. Laser light pulses are transmitted and reflected back to Earth, and the round-trip duration is measured. The lunar distance is calculated from this value.
The first successful tests were carried out in 1962 when a team from the Massachusetts Institute of Technology succeeded in observing laser pulses reflected from the Moon's surface using a laser with a millisecond pulse length. Similar measurements were obtained later the same year by a Soviet team at the Crimean Astrophysical Observatory using a Q-switched ruby laser. Greater accuracy was achieved following the installation of a retroreflector array on 21 July 1969, by the crew of Apollo 11, and two more retroreflector arrays left by the Apollo 14 and Apollo 15 missions have also contributed to the experiment. Successful lunar laser range measurements to the retroreflectors were first reported by the 3.1 m telescope at Lick Observatory, Air Force Cambridge Research Laboratories Lunar Ranging Observatory in Arizona, the Pic du Midi Observatory in France, the Tokyo Astronomical Observatory, and McDonald Observatory in Texas.
The uncrewed Soviet Lunokhod 1 and Lunokhod 2 rovers carried smaller arrays. Reflected signals were initially received from Lunokhod 1, but no return signals were detected after 1971 until a team from University of California rediscovered the array in April 2010 using images from NASA's Lunar Reconnaissance Orbiter. Lunokhod 2's array continues to return signals to Earth. The Lunokhod arrays suffer from decreased performance in direct sunlight—a factor considered in reflector placement during the Apollo missions.
The Apollo 15 array is three times the size of the arrays left by the two earlier Apollo missions. Its size made it the target of three-quarters of the sample measurements taken in the first 25 years of the experiment. Improvements in technology since then have resulted in greater use of the smaller arrays, by sites such as the Côte d'Azur Observatory in Grasse, France; and the Apache Point Observatory Lunar Laser-ranging Operation (APOLLO) at the Apache Point Observatory in New Mexico.
The distance to the Moon is calculated approximately using the equation: distance = (speed of light × duration of delay due to reflection) / 2
To compute the lunar distance precisely, many factors must be considered in addition to the round-trip time of about 2.5 seconds. These factors include the location of the Moon in the sky, the relative motion of Earth and the Moon, Earth's rotation, lunar libration, polar motion, weather, velocity of light in various parts of air, propagation delay through Earth's atmosphere, the location of the observing station and its motion due to crustal motion and tides, and relativistic effects. The distance continually changes for a number of reasons, but averages 385,000.6 km (239,228.3 mi) between the center of the Earth and the center of the Moon.
At the Moon's surface, the beam is about 6.5 kilometers (4.0 mi) wide[i] and scientists liken the task of aiming the beam to using a rifle to hit a moving dime 3 kilometers (1.9 mi) away. The reflected light is too weak to see with the human eye. Out of 1017 photons aimed at the reflector, only one is received back on Earth, even under good conditions. They can be identified as originating from the laser because the laser is highly monochromatic. This is one of the most precise distance measurements ever made, and is equivalent in accuracy to determining the distance between Los Angeles and New York to within 0.25 mm (0.01 in).
- The Moon is spiraling away from Earth at a rate of 3.8 cm/year. This rate has been described as anomalously high.
- The Moon probably has a liquid core of about 20% of the Moon's radius. The radius of the lunar core-mantle boundary is determined as 381±12 km.
- The universal force of gravity is very stable. The experiments have constrained the change in Newton's gravitational constant G to a factor of (2±7)×10−13 per year.
- The likelihood of any Nordtvedt effect (a differential acceleration of the Moon and Earth towards the Sun caused by their different degrees of compactness) has been ruled out to high precision, strongly supporting the validity of the strong equivalence principle.
- Einstein's theory of gravity (the general theory of relativity) predicts the Moon's orbit to within the accuracy of the laser ranging measurements.
- Gauge freedom plays a major role in a correct physical interpretation of the relativistic effects in the Earth-Moon system observed with LLR technique.
- The distance to the Moon can be measured with millimeter precision.
- The flattening of the lunar core-mantle boundary is determined as (2.2±0.6)×10−4.
- The free core nutation of the Moon is determined as 367±100 yr. 
Laser Ranging at Goddard Space Flight Center
- Apache Point Observatory Lunar Laser-ranging Operation
- Apollo Lunar Surface Experiments Package
- Tom Murphy (physicist) (principal investigator of Apollo's reflector experiment)
- Carroll Alley (previous principal investigator of Apollo's reflector experiment)
- Earth–Moon–Earth communication
- Lunar distance (astronomy)
- Lunokhod programme
- Satellite laser ranging
- Space geodesy
- Third-party evidence for Apollo Moon landings
- List of retroreflectors on the Moon
- During the round-trip time, an Earth observer will have moved by around 1 km (depending on their latitude). This has been presented, incorrectly, as a 'disproof' of the ranging experiment, the claim being that the beam to such a small reflector cannot hit such a moving target. However the size of the beam is far larger than any movement, especially for the returned beam.
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- "Lunar Retroreflectors" by Tom Murphy
- Station de Télémétrie Laser-Lune in Grasse, France
- Lunar Laser Ranging from International Laser Ranging Service
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