The Moon is gradually moving away from Earth at a rate of 3.83 centimetres each year. Scientists estimate the annual uncertainty in this figure at about 0.09 millimetres. Over the course of an individual's life, this slow drift translates to around three metres. While it may seem insignificant, the long-term effects on Earth could be substantial.
This change is not just theoretical. Over the last five decades, it has been confirmed repeatedly through the use of laser pulses directed at retroreflector arrays on the Moon's surface. These arrays were originally deployed by Apollo astronauts between 1969 and 1971. They consist of reflective prisms designed to bounce laser beams back to their source, allowing scientists to determine the distance to the Moon with high accuracy. The data collected over this time has shown the Moon’s steady retreat.
How the Distance Is Measured
Measuring the Moon’s distance with such precision relies on a network of observatories. Scientists in New Mexico, France, Italy, and Germany use these facilities to fire lasers at retroreflector arrays left on the Moon. The arrays, composed of corner-cube prisms, return the laser pulses along the same path they arrived. By timing how long the light takes to return, researchers can calculate the Earth-Moon distance.
The most advanced system for this kind of work is the Apache Point Observatory Lunar Laser-ranging Operation, or APOLLO. Located at a 3.5-metre telescope in southern New Mexico, APOLLO is capable of measuring the round-trip time of laser pulses to within a few picoseconds. This level of precision allows scientists to determine the distance to the Moon with accuracy down to about one millimetre.
This method has been used for more than thirty years, and it has consistently shown that the Moon is moving further away. The precision of the measurements has remained highly stable, confirming the reliability of the data. These results provide a clear picture of the Moon's long-term migration.
What's Pushing the Moon Away
The Moon's retreat is driven by tidal forces. Earth's gravity pulls on the Moon, but the Moon's gravity also affects Earth. This gravitational interaction causes Earth to bulge slightly, especially on the side facing the Moon. This tidal bulge extends into both the oceans and the solid crust, creating a subtle oval shape around the planet.
Because Earth rotates faster than the Moon orbits, these tidal bulges don’t line up directly with the Moon. They lag slightly behind, causing a gravitational tug on the Moon in the forward direction of its orbit. This small but constant force adds energy to the Moon’s orbit, pushing it farther away from Earth.
Meanwhile, Earth loses rotational energy as the Moon gains it. This loss causes Earth’s rotation to slow down, making each day a little longer. Over time, the effects will accumulate, reshaping the relationship between the two celestial bodies.
Longer Days on a Slower Earth
Currently, Earth's day is growing longer by approximately one to two milliseconds per century. However, this trend has been visible in Earth’s history. Studies of geological and fossil records suggest that about one billion years ago, a day on Earth lasted roughly 19 hours. At that time, the Moon was much closer, and the effects of tidal forces were different.
As the Moon continues to drift away, the strength of the tides will decrease, and the pace at which Earth’s rotation slows will change. The process will continue for as long as Earth rotates and the Moon remains in orbit. Eventually, the two bodies may reach a point where their rotations are perfectly synchronized, a scenario that could take billions of years to unfold.
Until then, the Moon’s slow retreat is a natural process that scientists are now able to study in great detail. The use of laser ranging has made it possible to track this movement with unprecedented accuracy, offering a clearer view of how the Earth-Moon system is evolving over time.
The Moon has been moving away for the past four and a half billion years and will likely continue to do so. As the tides weaken and the days lengthen, the dynamic between Earth and its only natural satellite will continue to shift. Scientists remain focused on understanding how this gradual change will shape the future of our planet.
The data collected so far, through efforts like the APOLLO project and the retroreflector arrays left by Apollo and Soviet missions, has given researchers a wealth of information. These observations not only confirm the Moon’s steady migration but also provide insights into the long-term evolution of the Earth-Moon system.
With continued monitoring, scientists hope to refine their models and better predict how this process will unfold in the distant future. For now, the Moon continues its slow journey away from Earth, inching toward a future where the relationship between the two bodies will be very different from what it is today.