Our Moon is steadily moving farther away from Earth. This subtle but continuous retreat occurs at an approximate rate of 3.8 centimeters (about 1.5 inches) each year. While this might sound significant, it is a very slow process unfolding over vast geological timescales, making its effects imperceptible in human lifetimes.
The Role of Tidal Forces
The primary driver behind the Moon’s recession is the gravitational interaction between Earth and the Moon, known as tidal forces. The Moon’s gravity exerts a differential pull across Earth. This means the side of Earth facing the Moon experiences a stronger gravitational attraction than the side farthest away, creating bulges of water and solid rock on both the near and far sides of Earth.
Earth rotates much faster than the Moon orbits it. As Earth spins, it carries these tidal bulges slightly ahead of the Moon’s direct line of gravitational pull. This creates a gravitational tug-of-war, where the Moon’s gravity attempts to pull the bulges back into alignment, but the bulges are constantly being pulled forward by Earth’s rotation.
This continuous gravitational tug from Earth’s leading tidal bulge acts like a slight forward acceleration on the Moon. This constant push increases the Moon’s orbital energy, causing it to speed up and gradually spiral outwards into a higher orbit. The interaction therefore transfers energy from Earth’s rotation to the Moon’s orbital motion.
The Principle of Angular Momentum
The Moon’s outward spiral is linked to the principle of conservation of angular momentum within the Earth-Moon system. Angular momentum is an object’s tendency to continue rotating or revolving. In a closed system like Earth and its Moon, the total angular momentum must remain constant.
As the Moon’s gravity pulls on Earth’s tidal bulges, it creates a drag on Earth’s rotation. This drag causes Earth’s spin to gradually slow down, leading to a slight increase in the length of our days. This transfer of rotational angular momentum from Earth to the Moon’s orbit is a continuous process.
To compensate for Earth’s slowing rotation, the Moon gains angular momentum, moving it into a higher, larger orbit. Objects in higher orbits move more slowly, so the Moon’s orbital speed around Earth decreases as it moves farther away. This exchange ensures the total angular momentum of the Earth-Moon system remains balanced.
Measuring the Moon’s Retreat
Scientists have precisely measured the Moon’s recession using Lunar Laser Ranging (LLR). This method involves firing powerful laser beams from observatories on Earth towards retroreflectors placed on the Moon’s surface. These mirrors were left by Apollo missions and Soviet Lunokhod rovers.
The laser light travels to the Moon, bounces off these retroreflectors, and returns to Earth. By measuring the time it takes for the light to complete this round trip, scientists can calculate the distance to the Moon with millimeter precision. Repeated measurements over decades have confirmed the Moon’s steady retreat.
While LLR provides the most accurate current data, geological evidence also supports the Moon’s historical recession. Analysis of ancient tidal rhythmites, which are layers in sedimentary rock formed by tidal cycles, indicates that days were shorter and the Moon was closer in Earth’s distant past. This independent evidence corroborates the findings from modern laser ranging.
Long-Term Effects on Earth
The Moon’s gradual retreat will have several long-term consequences for Earth, though these changes will unfold over billions of years. As the Moon moves farther away, its gravitational influence on Earth’s rotation will continue to diminish. This will lead to Earth’s rotation slowing even further, resulting in longer days.
For instance, projections suggest that in approximately 200 million years, a single day on Earth could last as long as 25 hours. Eventually, the Earth and Moon are expected to become tidally locked to each other. This means Earth’s rotation period would match the Moon’s orbital period, causing one side of Earth to permanently face the Moon, similar to how the Moon always shows the same face to Earth now. These changes are far in the future, extending well beyond the timescale of human existence or even the lifespan of our Sun.