The Moon is an astronomical partner, locked in a gravitational dance with Earth that has persisted for billions of years. This relationship is not static; our nearest celestial neighbor is slowly moving away from our planet. The Moon is currently receding from Earth at a rate of approximately 3.8 centimeters per year, a measurement confirmed through decades of precise observation. This phenomenon is a consequence of the interactions of gravity and motion between two orbiting bodies.
Measuring the Lunar Retreat
Scientists know the Moon is moving away thanks to the Lunar Laser Ranging Experiment (LLRE), a legacy of the Apollo missions. Astronauts from Apollo 11, 14, and 15 placed specialized mirror arrays, known as retroreflectors, on the lunar surface. These arrays are designed to reflect a laser beam fired from Earth directly back to its source.
Researchers fire high-powered laser pulses at these reflectors and measure the time it takes for the light to return, which is roughly 2.5 seconds. Since the speed of light is constant, this round-trip time allows the distance between Earth and the Moon to be calculated with millimeter-level precision. Repeated measurements over five decades have consistently demonstrated the Moon’s slow escape from Earth.
The Mechanism of Tidal Energy Transfer
The outward drift of the Moon is a direct result of the continuous gravitational interaction between it and the Earth’s oceans. The Moon’s gravity creates bulges in the Earth’s oceans, forming high tides on both the side facing the Moon and the side directly opposite. The Earth rotates on its axis much faster than the Moon completes an orbit, spinning beneath these bulges.
This faster rotation pulls the ocean bulges slightly ahead of the Moon’s direct line of sight. The gravitational attraction of the Moon attempts to pull these “leading bulges” back into alignment, which creates a torque, or twisting force, on the Earth. This mechanical resistance acts as friction, slowing the Earth’s rotation.
According to the principle of conservation of angular momentum, the total rotational and orbital energy of the Earth-Moon system must remain constant. As the Earth loses rotational angular momentum due to this tidal friction, that momentum is transferred to the Moon’s orbit. This input of energy accelerates the Moon, causing it to spiral outward into a higher, larger orbit. The Moon’s orbital speed decreases as it moves farther away, but its total orbital angular momentum increases, which is why it recedes.
The Resulting Lengthening of Earth’s Day
The transfer of angular momentum away from Earth manifests as a gradual slowdown of our planet’s rotational speed. The gravitational drag exerted by the Moon on the tidal bulges acts like a brake on the planet’s spin. This constant braking means the length of a day on Earth is slowly increasing over geological time.
Historical data compiled from ancient observations of solar and lunar eclipses, as well as geological records, confirms this long-term trend. On average, the Earth’s day lengthens by about 1.7 to 1.8 milliseconds every century. While this change is imperceptible over a human lifetime, it is significant across deep time.
Evidence from tidal sediments and ancient coral growth rings shows that in the past, Earth’s day was considerably shorter. For example, approximately 2.46 billion years ago, a full rotation of the Earth took only about 17 hours. This slow, consistent lengthening of the day is the price Earth pays for sending its moon into an ever-higher orbit.
Long-Term Consequences for the Earth-Moon System
The process of the Moon drifting away will continue for billions of years, fundamentally changing the dynamics of the Earth-Moon system. The transfer of angular momentum will eventually cause the Earth’s rotation to slow down until its day length matches the Moon’s orbital period. At this point, called tidal locking, Earth would show only one face to the Moon, just as the Moon currently shows only one face to Earth.
When this final state of equilibrium is reached, the Moon will cease to recede, and the lunar tides will become exceptionally weak. Scientists estimate this full tidal locking of the Earth might take as long as 50 billion years to complete.
Long before the Earth and Moon achieve complete tidal harmony, the life cycle of the Sun will intervene. In roughly five to seven billion years, the Sun will expand into a red giant star. This expansion is expected to engulf the inner planets, including Earth and the Moon, long before their gravitational dance reaches its natural conclusion.