The Moon is slowly moving away from Earth, a phenomenon that might seem counterintuitive for a gravitationally bound system. This gradual recession is an established scientific fact, continuing a process underway for billions of years.
Though the effect is imperceptible in a human lifetime, the Moon’s distance from Earth increases by approximately 3.8 centimeters each year. This yearly drift is caused by a constant transfer of energy within the Earth-Moon system, explained by the laws of physics.
Confirming Lunar Recession
Scientists rely on the Lunar Laser Ranging Experiment (LLRE) to precisely measure the increasing distance between our two celestial bodies. This technique involves powerful telescopes on Earth firing laser pulses toward the Moon. These pulses are reflected back by special instruments left on the lunar surface. The time it takes for the light to return allows researchers to calculate the distance with millimeter accuracy.
The reflective targets used for the LLRE are retroreflector arrays, placed on the Moon by the Apollo 11, 14, and 15 missions, and the Soviet Lunokhod 1 and 2 rovers. Repeated measurements over decades have confirmed the current rate of recession at about 3.8 centimeters annually.
The Mechanism of Tidal Braking
The physical mechanism driving the Moon away is known as tidal braking, which relies on the gravitational interaction between the Earth and Moon. The Moon’s gravity pulls on the Earth’s oceans, creating a bulge of water on the side facing the Moon and another bulge on the opposite side. If the Earth did not rotate, these tidal bulges would align perfectly with the Moon.
However, the Earth rotates much faster than the Moon orbits, spinning beneath these bulges and dragging them slightly ahead of the direct Earth-Moon line. This misalignment is the origin of the recession. The mass in the bulge that is now ahead of the Moon exerts a forward gravitational tug, constantly pulling the Moon in its direction of orbit. This gravitational tug acts like a tiny, continuous rocket burn for the Moon.
When an orbiting body receives forward acceleration, it moves into a higher orbit, causing it to move slower and farther away from Earth. The kinetic energy required for this boost is supplied by the Earth’s rotational energy through a transfer of angular momentum. This constant exchange means the Earth is losing rotational energy, which is converted into the Moon’s orbital energy.
The Effect on Earth’s Rotation
The transfer of angular momentum from the Earth’s rotation to the Moon’s orbit has a reciprocal consequence for our planet. As the Earth loses rotational energy, its spin gradually slows down, increasing the length of a day. This deceleration is minimal on a human timescale, lengthening the day by approximately 2.3 milliseconds per century. Over geological time, this effect is profound, meaning the Earth’s day was significantly shorter millions of years ago.
As the Moon moves farther away, the tidal forces it exerts on Earth weaken because gravitational force decreases with distance. This weakening will eventually cause the rate of lunar recession to slow down. Tidal braking will continue until tidal locking is reached, where the Earth’s rotation period and the Moon’s orbital period are synchronized. At that point, the recession would stop, though this final state is estimated to be billions of years in the future.