Lunar laser ranging

A laser pulse is transmitted from an Earth-based observatory toward a reflector array on the lunar surface. Corner-cube retroreflectors return part of the light toward its source, and instruments record the round-trip travel time. Since the pulse travels at the speed of light, this timing gives the range to the target after accounting for the observing system and the motion of Earth and Moon. The measurement is challenging: the lunar distance is large, the returned signal is weak, and the result depends on careful calibration and modelling.

Apollo astronauts deployed reflector arrays during Apollo 11, Apollo 14 and Apollo 15; Soviet Lunokhod rovers also carried reflectors to the Moon. The first successful lunar laser ranging observations date to 1969. Over time, improvements in lasers, detectors, timing and analysis have increased measurement precision from the decimetre scale in early work to millimetre-scale results at specialized modern facilities. Measurements have been collected by observatories in several countries, creating a long record rather than a single experiment.

Repeated ranging measurements constrain the Moon’s orbit, orientation and physical response, as well as parameters describing Earth’s rotation and the Earth–Moon system. Analyses of the long record show that the Moon is receding from Earth at about 3.8 centimetres per year on average, largely because tidal interactions transfer angular momentum within the system. The data also support tests of general relativity and the equivalence principle. These are model-based scientific analyses: the reflector provides a precise ranging target, while the conclusions depend on fitting observations with dynamical models.

A reported ranging precision is not identical to the accuracy of every derived orbital parameter. Researchers must account for reflector orientation, lunar rotation and tides, station locations, atmospheric and instrumental effects, and calibration. Observations are also distributed unevenly over lunar phases and time. Long time spans and independent observatories help separate small physical effects from measurement systematics. The resulting record is valuable precisely because it can be reanalysed as measurement techniques and lunar models improve.