The Moon is moving away from Earth by about 3.8 centimeters a year.

The Moon's average orbital distance is increasing by about 3.8 centimeters per year under present conditions.

Tides couple Earth's rotation to the lunar orbit, transferring angular momentum and gradually slowing Earth's spin.

Laser measurements reveal this tiny long term trend within much larger ordinary variations in the Moon's distance.

The Moon does not maintain one fixed distance from Earth. Its elliptical orbit brings it closer and carries it farther away each month. Superimposed on those repeating changes is a much smaller trend: the orbit is gradually expanding. The often quoted 3.8 centimeters per year refers to this average recession, not a steady outward motion that could be read from any two nightly observations.

The underlying mechanism is tidal interaction. The Moon's gravity deforms Earth, and Earth rotates more quickly than the Moon completes an orbit. Because the tidal response is not perfectly aligned with the line connecting the two bodies, gravitational forces exchange angular momentum. Earth loses some rotational angular momentum while the lunar orbit gains it. The Moon moves into a larger orbit, and Earth's rotation gradually slows.

This exchange does not create energy from nothing. Tidal friction also dissipates energy as heat. Angular momentum provides the essential link between the slowing spin and the expanding orbit, while energy accounting includes both orbital changes and dissipation. The oceans play a major role in the tidal response, although the solid Earth also deforms. Coastlines, ocean depths and the geometry of ocean basins affect the details.

To measure the distance, observatories send laser pulses toward retroreflectors on the lunar surface. Apollo astronauts and Soviet robotic missions placed reflectors that return some incoming light toward its source. By timing a pulse's round trip and using the speed of light, scientists determine a distance. Repeated observations over decades allow researchers to separate the long term recession from regular orbital motions and other effects.

Precision ranging is not simply a matter of pointing a laser and dividing one travel time by two. The observatory moves as Earth rotates, the Moon moves during the measurement, and the atmosphere affects the light. Researchers model these effects and combine many observations. The method supports studies of lunar motion and tests of gravity as well as the familiar measurement of recession.

The present rate should not be multiplied backward across the Moon's entire history. Continents have moved, ocean basins have changed and tidal dissipation has varied. Those changes alter the rate at which angular momentum is transferred. A modern measurement describes the current system very well without implying that the same annual increase applied billions of years ago. Understanding earlier conditions requires geological evidence and dynamical models.

The Moon’s present recession rate describes a long-term trend superimposed on a changing orbit. It does not mean the distance increases smoothly by the same tiny amount every day. Ordinary orbital variation is much larger than the annual trend, which is why precise observations and a physical model are needed to separate the effects.

Not necessarily. Its elliptical orbit produces much larger recurring changes in distance. The outward drift emerges as a long term trend after those ordinary orbital variations are taken into account.

No. Retroreflectors send some incoming laser light back toward the direction it came from. The observatory supplies the pulse and records the faint returning signal to measure its travel time.

No. That would assume an unchanging tidal system. Past ocean configurations and other conditions affected tidal dissipation, so reconstructing lunar history requires more than extending today's rate backward.