The Moon is losing roughly 6–11 centimeters of radius per million years—about 0.1 micrometers (0.0001 millimeters) annually if the long-term average has stayed constant. That is far too slow to notice year to year, but over geological time it compresses the crust, reactivates faults and can produce damaging moonquakes.
How fast is the Moon shrinking?
The best current figure is a geological estimate, not a direct annual measurement. Researchers infer the rate from contractional landforms, the distances that crustal blocks have moved and the ages assigned to those features. The estimate describes loss of 6–11 cm of radius per million years. Dividing that range by one million years gives approximately 0.06–0.11 micrometers per year, commonly rounded to 0.1 micrometers.
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| Quantity | Value | How it was established | Important qualification |
|---|---|---|---|
| Long-term radius decrease | 6–11 cm per million years | Inferred from mapped contractional faults and their estimated ages | Assumes the average rate remained constant over the interval represented by the landforms |
| Cumulative diameter decrease | About 50 m (150 ft) | NASA’s public synthesis of contraction over the last several hundred million years | This is a diameter total over a different, broad time interval; it should not be compared directly with the radius-rate estimate without matching units and dates |
Those figures describe the same broad process at different scales. The cumulative total is large enough to reshape the surface, while the present-day change is imperceptible without geological records.
What makes the Moon contract?
Cooling inside the Moon
The Moon formed nearly 4.5 billion years ago and is still losing internal heat. As its interior cools, it occupies slightly less volume. The rigid outer crust cannot contract smoothly, so it is compressed and broken into blocks.
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Thrust faults squeeze the crust
On a shrinking world, one crustal block can be pushed up and over another along a thrust fault. The resulting cliff-like scarps and wrinkle ridges are the surface expression of shortening. Some of these faults are geologically young, showing that contraction continued long after the Moon’s formation.
Earth adds tidal stress
Earth’s gravity constantly flexes the Moon. Tidal forces do not provide the main source of contraction, but they add stress to faults and can help determine when an already loaded fault slips. The changing Earth–Moon distance during each orbit matters: tidal stress is greatest near lunar apogee in the Apollo-era analysis.
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What evidence shows that contraction is recent?
Lunar Reconnaissance Orbiter mapping
NASA’s Lunar Reconnaissance Orbiter and its LROC camera have mapped thousands of young thrust-fault scarps. A NASA survey published in 2019 examined more than 12,000 images and identified young wrinkle ridges in Mare Frigoris. Some features formed within the last billion years, and others may be no older than about 40 million years. Their sharp, relatively fresh appearance and cross-cutting relationships indicate that lunar tectonics did not end in the distant past.
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Four Apollo seismometers operated from 1969 through 1977 and recorded 28 shallow moonquakes. Reanalysis placed eight of those 28 events within 30 kilometers of mapped faults. Six of those eight occurred near lunar apogee, when tidal stress is strongest. In 10,000 simulations, the chance of obtaining that pattern by coincidence was less than 4 percent. The result links at least some seismic activity to known contractional structures rather than treating every quake as unrelated background noise.
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Is the Moon still getting smaller today?
Probably, but the exact present rate is not directly measured. No modern instrument has provided a year-by-year radius change precise enough to replace the geological inference. Instead, young scarps, recent-looking ridges and fault-associated seismicity indicate that the interior is still cooling and that some faults remain capable of movement. The 6–11 cm-per-million-year range should therefore be read as a long-term average, not a promise that every million-year interval—or every part of the Moon—contracts at the same speed.
Can a shrinking Moon cause moonquakes?
Yes. Compression from cooling can load a thrust fault until it slips, releasing a shallow moonquake. NASA describes shallow events reaching approximately magnitude 5 to 5.5; some lasted more than 10 minutes. Because lunar seismic waves can persist and travel through dry, fractured rock differently from earthquakes on Earth, a moderate event can still create substantial shaking near a fault.
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The Apollo record provides the strongest statistical connection between shallow quakes, mapped scarps and tidal timing. Thomas Watters of the Smithsonian National Air and Space Museum said the analysis offers “the first evidence that these faults are still active and likely producing moonquakes today as the Moon continues to gradually cool and shrink.” Nathan Williams of NASA’s Jet Propulsion Laboratory summarized the implication more broadly: “The Moon is still quaking and shaking from its own internal processes.”
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Why the south pole matters for future missions
Contractional faults are not evenly distributed hazards, and the south polar region contains steep slopes as well as locations of interest for potential water ice. NASA’s south-pole modeling found that shallow quakes could produce strong shaking tens of kilometers from a scarp. Slopes already close to failure could then shed rocks or trigger landslides.
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Tom Watters of the Smithsonian Institution said, “Our modeling suggests that shallow moonquakes capable of producing strong ground shaking in the south polar region are possible from slip events on existing faults or the formation of new thrust faults.” For landing-site and outpost design, that means resource maps are not enough. Planners also need fault maps, seismic scenarios, slope-stability assessments and safe distances from scarps.
Quick Recap
What the estimate does—and does not—tell us
- It establishes a very slow but geologically meaningful contraction driven primarily by interior cooling.
- It is consistent with a roughly 50-meter reduction in diameter over several hundred million years, as described by NASA.
- It does not mean the Moon visibly shrinks from one year to the next.
- It does not provide a single guaranteed rate for every region or future interval.
- It does show that lunar faults can remain active and that seismic hazards must be considered in future south-polar exploration.
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