The three reflector arrays absorb a small sample of that light and reflect it back to earth in amounts gathered up and counted in photons per hour.
The Soviet Union also attached smaller reflectors on two rovers, one which has never been detected and another that is perched on Luna 21. Though small, it is regularly detected by laser ranging station on Earth. Together, the four sets of arrays on the Moon continue to provide enough science to inspire designs for future reflectors.
Using MacDonald LLR telescope, the distance to the Moon has been measured within three inches, enough to determine the Moon is presently pulling away from Earth at a rate of a few inches per century. At Apache Point, that horizon has been brought down to within one inch, "sort of," anyway, according to their website.
The three Apollo laser reflector arrays are the only remaining active experiments from the Apollo field expeditions to the lunar surface between 1969 and 1972. The same should also be credited to the Soviet unmanned sampling and survey program.
If the MacDonald 0.6 lunar laser ranging is soon ended, these experiments using the arrays left on the Moon will continue to be monitored.
Close is no longer good enough.
Finer monitoring of the distance between Earth and Moon took on greater significance after it was proposed the measurements, even over even such a relatively small distance, might constrain the range of possible answers to great cosmic questions.
Apache Point, upgraded by 2005, scientists have improved the accuracy (and photon count) of measures of the Earth-Moon distance with an ultimate goal being within centimenters, a threshold some believe will prove or disprove certain theories of the cosmos.
According to those at Apache Point, "Einstein's Equivalence Principle, upon which General Relativity rests, claims all forms of mass-energy experience the same acceleration in response to an external gravitational force." The inertial mass and gravitational mass are equal for all forms of mass and energy.
"This is very difficult to verify for gravitational energy itself," they write, "because laboratory masses have no appreciable gravitational binding energy." They need masses with gravity they can detect.
"One needs bodies as large as Earth to have any measurable self-energy content. Even then, the self-energy contribution to Earth's total mass-energy is less than one part-per-billion." The contribution of Earth to the inertia of the Sun plus Earth is hard to detect.
"If Earth's gravitational self-energy does not precisely obey this Equivalence Principle, the orbits of Earth and the Moon, around the sun, would be slightly displaced from one another, a modification of Kepler's Third Law) which would show up as a signal in our lunar range data."
"Various Cosmic String-theories, Quintessence, and other alternatives to General Relativity almost all predict a violation of the Equivalence Principle at some level. Recent hints there may be some new and mysterious modification to the laws of large-scale gravitational attraction, indicated by supernovae and the unequal distribution of the cosmic background microwave radiation, make it important to probe every available aspect of the basic nature of gravity."
"Lunar Laser Ranging also provides the best test of the stability of Issac Newton's gravitational constant, G, at this time limited to a variation of less than one part in 1012 each year."
"Relativistic geodetic precession is also best probed, currently by the more sensitive LLR installed in 2005, and verified at Apache Point t0 a 0.35% level of precision."
"The list goes on. Lunar Laser Ranging also provides the best test of the motional influence on gravitational attraction (called gravitomagnetism) to a 0.1% level of precision, and also sets the most stringent limits on deviations from the expected 1/r2 law of gravity."