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Lunar Laser Ranging experiment

Measuring the distance between the Earth and the Moon with laser light / From Wikipedia, the free encyclopedia

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Lunar Laser Ranging (LLR) is the practice of measuring the distance between the surfaces of the Earth and the Moon using laser ranging. The distance can be calculated from the round-trip time of laser light pulses travelling at the speed of light, which are reflected back to Earth by the Moon's surface or by one of five retroreflectors installed on the Moon during the Apollo program (11, 14, and 15) and Lunokhod 1 and 2 missions.[1]

Lunar Laser Ranging Experiment from the Apollo 11 mission

Although it is possible to reflect light or radio waves directly from the Moon's surface (a process known as EME), a much more precise range measurement can be made using retroreflectors, since because of their small size, the temporal spread in the reflected signal is much smaller.

A review of Lunar Laser Ranging is available.[2]

Laser ranging measurements can also be made with retroreflectors installed on Moon-orbiting satellites such as the LRO.[3][4]