The Moon is slowly drifting away from Earth—about 3.8 cm per year—because tidal forces transfer a tiny bit of Earth’s spin into the Moon’s orbit. We measure this with lasers, watch it play out as tides, and even see its fingerprints in eclipses, ancient rocks, and the length of our day. Here’s the full story—plus 30 quick facts to make you a Moon expert.
How We Know the Moon Is Moving
Scientists fire laser pulses at retroreflector mirrors left on the Moon by Apollo astronauts and Soviet Lunokhod rovers. By timing the 2.5–2.7 second round trip of light, we track the Earth–Moon distance down to millimeters. That distance isn’t fixed: the Moon’s slightly elliptical orbit means it swings from perigee (closer) to apogee (farther) each month, centered around an average of ~385,000 km. When a full Moon lands near perigee, it looks bigger and brighter—hello, supermoon.
Why the Moon Is Drifting Away
It’s all about tides. The Moon pulls harder on Earth’s near side than its far side, stretching the oceans into two bulges. Because Earth rotates, these bulges sit a little ahead of the Moon’s position. Their gravity tugs the Moon forward, adding energy to its orbit and nudging it outward. The tradeoff: Earth’s rotation slows slightly, making days longer by a few milliseconds per century. Over geologic time, that adds up.
Should We Worry?
Nope. We’ll still have tides, eclipses, and 24-hour days for many millions of years. The Moon likely formed ~4.5 billion years ago after a Mars-sized impactor hit early Earth, and it started out much closer—looming large in our ancient sky. In the unimaginably distant future—tens of billions of years—Earth’s spin may slow enough that the Earth–Moon system changes character, but that’s far beyond any practical concern.
30 Fast (but Mighty) Facts
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Recession rate (~3.8 cm/yr): Tidal friction steals a sliver of Earth’s spin and hands it to the Moon’s orbit.
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Laser round trip (~2.5–2.7 s): Measure the light’s out-and-back time; get the distance.
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Retroreflectors: Corner-cube mirrors on the Moon bounce light straight back for ultra-precise ranging.
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Perigee vs. apogee: Closest and farthest points differ by ~42,000 km thanks to orbital ellipticity.
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Supermoon vs. micromoon: Full Moon near perigee looks larger/brighter; near apogee looks smaller/dimmer.
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Tidal locking: The Moon spins once per orbit (~27.3 days), so we see nearly the same face.
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Synodic month (29.53 days): Full-to-full is longer than one orbit because Earth–Moon also orbit the Sun.
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Longer days (ms/century): Tidal friction lengthens Earth’s day very slowly.
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Shorter ancient days (~18 h): Geologic “tidal rhythmites” show Earth used to spin faster.
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Sun’s tides (30–45%): Weaker than the Moon’s but still shapes spring and neap tides.
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Solid-Earth tides: The crust itself rises/falls by up to ~30 cm under lunar/solar gravity.
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Barycenter inside Earth: Earth and Moon orbit a shared center of mass beneath Earth’s surface.
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Eccentricity (~0.055): Slightly oval orbit drives monthly size/brightness changes.
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Apsidal precession (~8.85 yr): The orbital ellipse slowly rotates, shifting when perigee happens.
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Inclination (~5.1°): The tilt explains why we don’t get eclipses every month.
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Eclipse seasons (~173 days): Windows twice a year when alignments (and eclipses) are possible.
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Saros cycle (~18 yr 11 d): Predictable families of eclipses repeat with this rhythm.
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Changing basins, changing friction: Plate tectonics reshapes oceans, tweaking tidal energy loss over time.
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Tidal rhythmites: Layered sediments record ancient daily/monthly cycles—nature’s timecards.
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Libration (~59% view): Small wobbles let us peek beyond a strict 50% of the lunar surface.
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Total vs. annular eclipses: Near perigee, the Moon can fully cover the Sun; near apogee, a ring remains.
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End of total eclipses (~600 Myr): As the Moon recedes, it will look too small to fully cover the Sun.
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Anomalistic month (27.55 d): Perigee-to-perigee is longer than the sidereal month due to precession.
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Angular momentum transfer: Bulges tug the Moon forward (higher orbit) and slow Earth’s spin.
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Relativity tests: Millimeter-level ranging checks Einstein’s predictions and bounds any drift in G.
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Near vs. far side: The far side’s thicker crust hints at early tidal heating and uneven cooling.
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Past tidal heating: When closer, the Moon flexed and warmed more—shaping its interior history.
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Coasts & planning: Long-term tidal range shifts matter for habitats, sediments, and infrastructure.
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Mutual tidal locking (far future): One Earth hemisphere could one day always face the Moon.
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Giant-impact origin: Lunar rocks share isotopic “fingerprints” with Earth’s mantle—strong evidence.
FAQs
Why don’t we see eclipses every month?
Because the Moon’s orbit is tilted ~5.1°. Most months it passes above or below the Sun from our viewpoint.
What makes a supermoon “super”?
A full Moon near perigee looks bigger and brighter simply because it’s closer.
Is the Moon’s drift constant?
Not exactly. Ocean shapes and seafloor roughness change over geologic time, so the recession rate has varied.
Will total solar eclipses really end?
In roughly ~600 million years, yes—once the Moon appears too small to cover the Sun completely.
Key Takeaways
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The Moon is receding at ~3.8 cm/year due to tidal friction.
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We measure the effect with laser ranging to retroreflectors on the Moon.
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Earth’s day lengthens slowly; tides, eclipses, and ocean dynamics all carry the Moon’s signature.
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Big-picture changes unfold over millions to billions of years—amazing to study, nothing to fear.
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