A laser pulse fired from Earth strikes a retroreflector array on the Moon and returns along the same path. The measured quantity is the round-trip travel time of that pulse.
Light travels at a known speed, so a travel time is a distance. Repeat that measurement over decades and the distance is seen to grow slowly.
The physical explanation is tidal: the Moon raises tides on Earth, Earth's rotation drags those bulges slightly ahead of the Moon, and the resulting gravitational tug transfers rotational energy outward — Earth's day lengthens, the Moon's orbit widens.
Lunar laser ranging against reflectors placed during crewed and robotic missions, repeated by multiple observatories over many years. Independent evidence for a longer-term trend comes from tidal rhythmites and coral growth bands recording shorter days in the geological past.
The Earth–Moon distance is increasing, and the rate is measured directly rather than inferred from theory.
The present rate cannot simply be extrapolated backwards or forwards. Tidal dissipation depends on the arrangement of oceans and continents, which changes over geological time, so long-range projections carry real uncertainty.
- NASA // 2020
Laser Beams Reflected Between Earth and Moon Boost Science ↗
- NASA Jet Propulsion Laboratory // 2019