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Lunar Laser Ranging ExperimentThe ongoing Lunar Laser Ranging Experiment was first made possible by a lunar laser ranging retroreflector array planted on the Moon on July 21, 1969, by the crew of the Apollo 11. Since then, the distance between the Earth and the Moon has been measured repeatedly over a period of more than 35 years. Three more retroreflector arrays, left by the Apollo 14 and Apollo 15 missions and by the unmanned Soviet Lunokhod 2 rover, have contributed to the experiment. 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 McDonald Observatory and the OCA Laser-Lune telemetry station affiliated with the Cte d'Azur Observatory. Lunokhod 1 also left an array, but it was poorly positioned and has not been of use to the experiment. Lunar ranging also provides data useful for other experiments, including tests of general relativity designed by physicists such as Carrol Alley. As of 2002 work is progressing on increasing the accuracy of the Earth-Moon measurements to near millimeter accuracy. See also External links and sources
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