The Apollo 11, Apollo 14, and Apollo 15 missions left retro-reflectors on the Moon as part of a laser ranging experiment, the Lunar Laser Ranging Experiment.

Since then, the range from Earth to Moon has been measured at intervals, over a period of more than 30 years. Lunar ranging provides information that is potentially useful for a number of purposes, including providing tests of general relativity.

As of 2002 work is progressing on increasing the accuracy of the Earth-Moon measurements to near millimeter accuracy.

to be written

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