The specific number, on the current best interpretation of five decades of lunar laser ranging data, is 3.83 centimetres per year, with a formal uncertainty of approximately 0.09 millimetres. It has been directly measured, thousands of times over, from multiple observatories on three continents, by firing laser pulses at retroreflector arrays that Apollo astronauts left on the lunar surface between 1969 and 1971. The result is one of the most precisely known physical quantities in all of contemporary planetary science.
The rate is small on any human timescale. It is roughly the rate at which human fingernails grow. Over a human lifetime, it accumulates to approximately three metres. Over the span of recorded human civilisation, roughly five thousand years, it accumulates to approximately 190 metres. On these timescales the Moon is, for all practical purposes, sitting where it has always sat.
On geological timescales the picture is substantially different, and the specific consequences for Earth over the next several hundred million years are worth setting out plainly.
The measurement
The Apollo 11 astronauts Neil Armstrong and Buzz Aldrin deployed the first laser retroreflector array on the lunar surface in July 1969, at the Sea of Tranquillity. The array contained one hundred quartz-glass corner-cube prisms, small pyramidal reflectors with the specific optical property that any incident beam of light is returned along its exact incoming direction. The Apollo 14 and Apollo 15 missions deployed additional arrays in 1971. The Apollo 15 array, the largest, contains three hundred corner cubes and remains the workhorse of modern ranging campaigns. Two Soviet lunar rovers, Lunokhod 1 and Lunokhod 2, carried French-built retroreflector panels of their own.
According to a summary of the ongoing programme published by NASA’s Jet Propulsion Laboratory, four observatories currently perform routine lunar laser ranging: one in New Mexico, one in France, one in Italy, and one in Germany. The most precise of these is the Apache Point Observatory Lunar Laser-ranging Operation, or APOLLO, at the 3.5-metre telescope in southern New Mexico. According to the APOLLO project’s own documentation at the University of California San Diego, the ranging precision is now approximately one millimetre in Earth-Moon distance, based on timing the round-trip travel of a laser pulse to a precision of a few picoseconds.
Every year that measurement returns a slightly larger number. The Moon is receding. The rate is 3.83 centimetres annually. The value has been robust to sub-millimetre precision for approximately three decades.
To understand what’s really going on, we came across a short video that breaks it down.
Click here to watch the video.
The mechanism
The physical process behind the recession is tidal friction, and it operates through the specific combination of Earth’s rotation, the Moon’s gravity, and the geometry of Earth’s oceans and crust.
The Moon’s gravity is stronger on the side of Earth facing it than on the far side, because gravity falls off with distance. The differential pull stretches Earth into a subtle oval, with tidal bulges rising on both the near and far sides. On Earth’s oceans, those bulges are what produce the tides. In Earth’s rocky interior, the same process produces smaller but measurable body tides in the solid crust.
Earth rotates faster than the Moon orbits. One Earth rotation takes 24 hours. One lunar orbit takes 27.3 days. As Earth spins on its axis, it drags the tidal bulges slightly ahead of the line between Earth’s centre and the Moon. The near-side bulge does not sit directly beneath the Moon. It sits slightly ahead of the Moon in the direction Earth is rotating.
That off-axis bulge exerts a small gravitational tug on the Moon in the forward direction of its orbit. Adding energy to an orbit widens it. So the Moon climbs, imperceptibly, into a higher orbit each year. By the conservation of angular momentum, the energy the Moon gains has to come from somewhere, and it comes from Earth’s rotational spin. Earth slows down. The day lengthens. The Moon recedes.
A 2021 peer-reviewed analysis by Andre Maeder of Geneva Observatory and Vesselin Gueorguiev of the Institute for Advanced Physical Studies, published in Astrophysics and Space Science, examined the relationship between the observed lunar recession and the observed rate of Earth’s rotational slowing across multiple independent data sources. The paper’s specific finding is that the recession rate has not been constant over Earth’s history. If the current 3.83 centimetres per year is projected naively backwards, the Moon would have been in contact with Earth approximately 1.5 billion years ago. Given that the Moon formed approximately 4.5 billion years ago in the giant impact that produced it, the naive projection is clearly wrong. The recession rate depends on the specific configuration of Earth’s ocean basins, because ocean basins resonate at particular tidal frequencies and either amplify or damp the tidal friction. The current configuration, with a narrow Atlantic and a wide Pacific, produces unusually strong tidal friction. For most of Earth’s history, the recession rate was probably substantially slower than today.
The lengthening of the day
The first observable consequence of the lunar recession, on the current best measurements, is that Earth’s rotational day is getting longer at a rate of between one and two milliseconds per century. The specific value depends on which data source is used. Ancient solar and lunar eclipse records dating from 720 BC to 1600 AD support a rate closer to 1.78 milliseconds per century. Lunar occultations of stars, timed from 1680 to the present, support a rate closer to 1.09 milliseconds per century. The difference between the two rates is real, and probably reflects long-term oscillations in the coupling between Earth’s core and its mantle, which alter the rate at which angular momentum is transferred to the Moon.
Coral fossils and layered sedimentary deposits called tidal rhythmites preserve the historical record of Earth’s rotation directly. On the accumulated evidence from those records, Earth’s day was approximately 19 hours long around one billion years ago, when the Moon was closer and the tidal friction was probably weaker but the geometry was different. The day has been getting longer ever since, and it will continue to lengthen for as long as the Moon continues to recede.
The end of total solar eclipses
The second observable consequence has a specific deadline. Total solar eclipses on Earth are only possible because the Moon happens to appear almost exactly the same angular size in Earth’s sky as the Sun does. The Sun’s diameter is approximately four hundred times larger than the Moon’s, but the Sun is also approximately four hundred times further away. The two figures cancel almost perfectly, producing the specific geometrical coincidence that allows the Moon to occasionally cover the Sun’s disc entirely.
That coincidence is not a permanent feature of the solar system. It is a specific product of the Moon’s current orbital distance. As the Moon recedes, its angular size shrinks. On the current best calculations, the last total solar eclipse observable from Earth’s surface will occur approximately 600 million years from now. After that, the Moon will appear too small in Earth’s sky to fully cover the Sun’s disc, and total solar eclipses will become physically impossible.
Annular eclipses, in which the Moon covers only the central portion of the Sun’s disc and leaves a bright ring visible around the edge, will continue to occur for far longer.
The tidal weakening
The third consequence is that the tidal amplitude produced by the Moon on Earth’s oceans will decline over deep time. Spring tides and neap tides will continue to exist, but the range between them will shrink as the Moon’s gravitational grip on Earth weakens with distance. The specific rhythms that structure coastal ecosystems, marine reproduction, and fisheries will slowly attenuate. On the accumulated evidence from marine geology, this process has been running throughout Earth’s history and is expected to continue for as long as the Moon continues to recede.
What the case shows
The lunar recession is not a hypothesis about the deep future. It is a directly measured phenomenon, precisely quantified over five decades, and it is the specific result of a well-understood physical mechanism operating on a two-body system that has been in place since the formation of the Moon. The consequences over the next several hundred million years are calculable, and on the strongest current reading of the evidence, they include a modest lengthening of Earth’s day, the end of total solar eclipses on Earth, and a slow weakening of the ocean tides.
The Moon is going away.
It has been going away for four and a half billion years.
And it will continue to go away for as long as Earth continues to rotate.