Is the Moon’s slow retreat from Earth a cosmic time bomb in our planet’s relationship or an entry to the precision of modern science?
Fifty years ago, Apollo astronauts left more than footprints on the face of the moon. They placed retroreflectors strings of mirrors that bounce laser beams back to Earth with precision. This unassuming but enlightening test, lunar laser ranging, has since been a gold standard of planetary science. Shooting laser beams of incredibly high intensity at the retroreflectors and timing the time it takes for the light to return, scientists have determined the Moon is receding from Earth at a rate of about 3.8 centimeters per year, or about 1.5 inches the same rate human fingernails grow. The accuracy of current in such measurements is breathtaking: at observatories like New Mexico’s Apache Point, distance can be tracked to within a single millimeter, thanks to very advanced Nd:YAG lasers and photon counters that typically have 1-mm precision with a 7-picosecond round-trip travel-time error.
At the heart of this lunar drift is a gentle waltz of gravity. The Earth spins faster than the Moon revolves, and thus the Moon’s gravity drags Earth’s oceans into constantly slightly ahead bulges of the Moon. This is a displacement, which creates a torque, and angular momentum is transferred from Earth to the Moon. the Moon spirals outward, and Earth’s rotation gradually slows. This tidal friction lengthens the length of an Earth day by around one millisecond every century since the 17th century, as measured by observations of ancient geologic and biologic histories. Yet the tale is not one of unidirectional or uninterrupted change.
The Earth–Moon system has also gone through phases of uncommon stability and resonance. For example, during the period between two billion and 600 million years ago, the length of Earth’s day stalled at around 19.5 hours, according to cyclostratigraphic study of fossil sedimentary rocks. Then, a resonance between lunar-forced oceanic tides and solar-forced atmospheric tides created a “tidal impasse,” resisting the forces that otherwise would have continued to decelerate Earth’s rotation. Theoretical astrophysicist Norman Murray explained, “Sunlight also produces an atmospheric tide with the same type of bulges. The sun’s gravity pulls on these atmospheric bulges, producing a torque on the Earth. But instead of slowing Earth’s rotation like the Moon, it speeds it up.” But instead of making Earth rotate more slowly as the Moon does, it makes it rotate more quickly. This delicate balance, fine-tuned through atmospheric composition and temperature gradient, temporarily halted the Earth’s day from lengthening a finding corroborated by geological proxies and sophisticated atmospheric simulations of those used in climate modeling. Accuracy in lunar laser ranging not only elucidated the Earth–Moon orbital process but also became a vehicle for fundamental physics in general.
These were some of the most stringent tests of Einstein’s general relativity, confirming the Equivalence Principle to a part in a trillion per year. According to James Williams of the Jet Propulsion Laboratory at NASA, “LLR results are consistent with the expectations of Einstein’s general theory of relativity. It is remarkable that general relativity has survived a century of testing and that the Equivalence Principle is intact after four centuries of scrutiny.” The technology itself continues to be developed.
The Next Generation Lunar Retroreflector (NGLR), on future lunar missions, will yield results one order of magnitude better than the Apollo-era reflectors. NASA’s Marshall Space Flight Center’s NGLR payload manager Dennis Harris clarified, “NASA has more than half a century of experience with retroreflectors, but NGLR-1 promises to deliver findings an order of magnitude more accurate than Apollo-era reflectors.” The arrays are engineered to survive the harsh lunar environment and constrain thermal distortions, which will yield micron-level accuracy for measuring distance. Looking out into the far future, the fate of the Earth–Moon system is written in the language of tidal physics. Earth and Moon both will be tidally locked to one another in about 50 billion years if, that is, the Sun itself does not first evolve.
One side of Earth will forever look at the Moon, permanently fixed in the sky. But this boundary of space will likely be met by the Sun’s evolution into a red giant and engulfing both planets in a fire of ruin. The slowness of the Moon’s movement from Earth is not loss, but a testament written in light, stone, and time of the gentle forces that shape our world and its best friend. Ongoing advances in laser ranging and the resolute ingenuity of scientists ensure that each millimeter of lunar roaming brings fresh insight into the workings of the universe.
