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If you look up at the night sky, the Moon seems like a permanent, unchanging fixture. But our nearest neighbour is actually spiralling away from us. This subtle drift has profound implications for the future of our planet, the length of our days, and our understanding of orbital mechanics.
How do we know the Moon is moving away?
The proof comes from a project that started during the Apollo program. On 21 July 1969, during the first manned lunar mission, Apollo 11, astronauts placed the first retroreflector array on the lunar surface. For decades, observatories on Earth have been firing laser beams at these panels to measure the distance between the two bodies. (If you have ever calculated the Number of photons emitted per second by a standard laser, you can imagine how intense these pulses must be to make a round trip across space and still be detected by instruments on Earth.)
By timing exactly how long the light takes to return, scientists can determine the Moon's distance down to the millimetre. A landmark study of this Lunar Laser Ranging data confirmed that the Moon is drifting away at a rate of 3.8 centimetres per year (Dickey et al., 1994). The same laser ranging experiments have turned the Earth-Moon system into a laboratory for a broad range of investigations. They have led to a three-orders-of-magnitude improvement in the accuracy of the lunar ephemeris and allowed researchers to test the principle of equivalence for massive bodies with unprecedented accuracy (Dickey et al., 1994). The analysis has also provided measurements of the Earth's precession and lunar rotational dissipation.
Why is the Moon moving away from the Earth?
The explanation lies in the oceans. The gravitational pull of the Moon draws Earth's water towards it, creating a tidal bulge. If the Earth did not spin, this bulge would sit directly beneath the Moon.
However, the Earth rotates on its axis daily, which is much faster than the 27.32 days the Moon takes to complete its sidereal orbit (NASA). As the Earth spins, it drags the tidal bulge slightly ahead of the Moon's position. This "leading" bulge of water exerts its own gravitational pull on the Moon, effectively tugging it forward in its orbit.
This forward tug transfers energy and angular momentum from the Earth to the Moon. In orbital mechanics, when an object gains energy, it moves into a higher, wider orbit. Visualising how tidal bulges lead the Moon can be tricky; you can model this transfer of angular momentum in the Fermi Simulation Lab.
The Moon's orbit is also highly complex, which makes tracking its exact distance a continuous mathematical challenge. According to NASA, the Moon's distance is not fixed but ranges from a mean perigee (closest point) of 363,396 km to a mean apogee (farthest point) of 405,504 km. Over a 5,000-year period, these extremes can stretch even further, reaching a minimum distance of 356,355 km and a maximum of 406,725 km (NASA). Interestingly, the Sun's gravitational pull on the Moon is actually more than two times larger than the Earth's pull on the Moon (NASA).
Does this affect life on Earth?
Because the Earth is transferring its rotational energy to the Moon, our planet's spin is slowing down. This tidal friction acts like a brake, gradually lengthening an Earth day.
The interplay of gravity in the Earth-Moon system also affects how we measure time on a lunar scale. While the sidereal month is 27.32 days, the time from one new moon to the next, known as the synodic month, is longer at 29.53 days (NASA). The Moon's orbit is tilted at a mean inclination of 5.1 degrees, and its nodes slowly regress over a cycle of 18.6 years (NASA). These orbital cycles are what make eclipses possible.
If you are studying Kepler's second law from angular momentum or want to understand the deeper mathematics of gravity, Fermi's AI tutor can build a plan for your study session and teach you the concepts step by step.
