Lunar laser ranging

Lunar laser ranging measures the Earth–Moon distance by timing laser pulses returned from retroreflectors placed on the Moon. Repeated measurements test gravitational theories and reveal changes in the Earth–Moon system.

Lunar retroreflector: An optical device that returns incoming light toward its source, largely independent of the light’s angle of arrival. Apollo and Lunokhod missions left these arrays on the Moon to send laser pulses back toward Earth.
Equivalence principle: The principle that inertial and gravitational mass are equivalent, so bodies fall alike in a gravitational field. Lunar ranging tests whether Earth and Moon fall toward the Sun at the same rate.
Apollo 11: The 1969 NASA mission that first landed astronauts on the Moon. Its astronauts deployed the first lunar laser-ranging retroreflector array.
Lunar orbit: The path the Moon follows around Earth, shaped by Earth’s gravity and perturbations from other bodies. Ranging records the orbit’s changing size, shape, and orientation.
Time-of-flight measurement: A measurement of distance or motion inferred from the time a signal takes to travel to a target and return. The round-trip travel time of each laser pulse provides the range to a lunar reflector.
Strong equivalence principle: The claim that gravitational experiments and the motion of self-gravitating bodies obey the same laws as nongravitational experiments. The Earth–Moon orbit can reveal whether the gravitational binding energy of each body affects its fall toward the Sun.
Apollo 14: The 1971 NASA mission that landed astronauts near Fra Mauro on the Moon. Its crew installed a second Apollo laser-ranging array.
Tidal acceleration: A change in orbital motion caused by tidal interactions that transfer angular momentum between bodies. Earth’s tides transfer angular momentum to the Moon and alter its orbital motion.
Corner-cube retroreflector: An optical element made of three mutually perpendicular reflecting surfaces that sends light back toward its source. The lunar arrays use corner cubes to return light despite changes in reflector orientation.
Earth–Moon distance: The separation between the centers of Earth and the Moon, which varies over the lunar orbit. Ranging provides repeated, high-precision measurements of this changing separation.

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