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Random freezes, missing sound and display glitches usually trace back to one bad driver. Find and replace yours safely.Free scan · under a minuteEarth’s rotation changes because angular momentum and mass are continually exchanged among the solid planet, atmosphere, oceans, ice, surface water and deep interior. The Moon’s tides produce a persistent long-term braking effect, while winds, ocean currents, moving ice and groundwater, and flows in the core make individual days slightly longer or shorter.
These changes are tiny—usually measured in milliseconds—but modern geodesy can detect them. A short day in one year does not overturn the much slower long-term trend, and a climate-related estimate is not the same thing as a direct observation of every day’s rotation.
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Why is Earth’s day not always exactly 24 hours?
“Twenty-four hours” is a convenient civil-time definition, not a guarantee that every rotation takes exactly the same interval. NASA describes the mean solar day as approximately 86,400.002 SI seconds. Individual solar days fluctuate around that average because the planet is not a rigid, isolated sphere: fluids move, ice and water shift location, and the core and mantle exchange angular momentum.
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Clear out junk files and repair common Windows errorsFree Scan →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →The governing idea is conservation of angular momentum. In simplified form, L = I × ω, where L is angular momentum, I is the mass distribution’s moment of inertia and ω is rotation rate. If angular momentum is transferred from the solid Earth to another reservoir, the solid planet spins more slowly. If the solid Earth receives angular momentum, it spins faster. Moving mass farther from the rotation axis increases I and tends to slow the spin; moving mass inward tends to speed it up.
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Because these exchanges happen simultaneously, no single mechanism determines every day. A day can be shortened by one process and lengthened by another, with the measured result being their combined effect.
The causes and their timescales
| Cause | How it changes rotation | Typical timescale | Possible direction | How it is supported |
|---|---|---|---|---|
| Lunar and solar tides | Tidal friction transfers angular momentum from Earth’s spin to ocean and solid-Earth tides. | Periodic cycles and a long-term secular trend | Long-term slowing; periodic departures in either direction | Geodetic records and tidal theory |
| Atmosphere and oceans | Winds, pressure systems and currents exchange angular momentum with the solid Earth. | Daily, seasonal and sub-two-year variations | Shortening or lengthening | Observed Earth-orientation changes correlated with fluid angular momentum |
| Ice, groundwater and surface water | Redistributed mass changes the planet’s moment of inertia. | Years to centuries, depending on the process | Often lengthening when polar ice moves toward lower latitudes | Satellite mass measurements and reconstruction models |
| Core–mantle coupling | Liquid outer-core flow exchanges angular momentum with the mantle. | Multi-year to decadal, commonly five to fifteen years | Either direction over a cycle | Earth-orientation data and indirect magnetic-field evidence |
| Inner-core and other interior processes | Changes in the deep interior can affect rotation over long periods. | Long periods, not reliably assigned to a single day | Not fixed | Geophysical inference rather than direct observation |
1. Tidal friction: the long-term brake
The Moon’s gravity raises tides in the oceans. Because Earth rotates faster than the Moon orbits, tidal bulges are displaced from the exact Earth–Moon line. Friction in moving seawater and in the solid Earth dissipates energy and transfers angular momentum from Earth’s spin to the Moon’s orbit. The result is a gradual average increase in day length.
The U.S. Naval Observatory identifies tidal friction as the chief cause of the secular change in rotational speed and gives an estimated day-length increase of about 0.0015–0.0020 seconds per century. NASA’s 2024 climate-rotation summary states an average lunar-tidal contribution of 2.4 milliseconds per century. These are source-specific estimates using different descriptions of the long-term contribution; they should not be treated as identical measurements or as the change on any particular date.
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The Sun also raises tides, and solid-Earth tides create measurable periodic variations. Tidal braking therefore supplies the background trend, while shorter oscillations are superimposed on it. A temporarily shorter day is compatible with a long-term average that is still increasing.
2. Atmosphere and oceans: weather can change a day by milliseconds
Air and seawater are moving masses. A strong jet stream, a seasonal pressure pattern or a changing ocean current carries angular momentum. Through friction and other interactions, the solid Earth gains or loses an equal amount, so its rotation adjusts.
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Seasonal circulation
The U.S. Naval Observatory reports that variations with periods shorter than two years are related largely to changes in the atmosphere’s total angular momentum. NASA explains that seasonal and day-to-day weather variations can alter day length by a few milliseconds over the course of a year. The sign is not fixed: winds and pressure arrangements can either accelerate or retard the solid Earth.
El Niño and other climate patterns
Large-scale ocean–atmosphere events reorganize winds and pressure over broad regions. NASA reports that El Niño-related atmospheric variations can lengthen a day by as much as one millisecond. That figure describes an effect associated with the event, not a permanent annual increase.
The “ice-skater” analogy helps with the direction. A skater who extends their arms increases the distance of mass from the spin axis and turns more slowly; pulling the arms inward speeds the spin. Earth’s atmosphere and ocean are more complicated than a skater, but the same angular-momentum accounting applies.
3. Ice, groundwater and climate-driven mass redistribution
When glaciers and ice sheets lose mass, and when groundwater is pumped and eventually reaches the ocean, water is redistributed across the planet. Moving mass from high latitudes toward the equatorial oceans places more mass farther from the rotation axis. Earth becomes slightly more oblate and its rotation slows, lengthening the day.
NASA’s Jet Propulsion Laboratory used GRACE and GRACE-FO satellite observations together with earlier mass-balance work to reconstruct this contribution. For 2000–2018, the estimated increase in day length attributable to ice and groundwater changes was 1.33 milliseconds per century. Comparable estimates for periods earlier in the twentieth century ranged from 0.3 to 1.0 milliseconds per century, so the 2000–2018 estimate was higher than those earlier intervals.
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NASA’s 2024 account also reports a conditional projection: if greenhouse-gas emissions continue to rise, climate-related lengthening could reach as much as 2.62 milliseconds per century. This is a model-based future scenario, not an observed present-day rate or a guaranteed outcome. It would exceed the 2.4-millisecond-per-century average lunar contribution quoted in the same NASA account.
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Surendra Adhikari, NASA Jet Propulsion Laboratory geophysicist and co-author
Mass redistribution is only one component of the measured rotation record. Atmospheric angular momentum, ocean circulation, tides and the interior can offset or reinforce it at a given time.
4. The core and mantle: hidden sources of multi-year variation
Earth’s outer core is a layer of liquid iron. Its flow interacts electromagnetically and mechanically with the overlying mantle, allowing angular momentum to pass between the fluid core and the solid surface. When the mantle loses angular momentum to the core, the observed rotation slows; when it receives angular momentum, it speeds up.
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The U.S. Naval Observatory associates decadal changes with characteristic periods of roughly five to fifteen years with processes deep inside Earth. NASA’s Jet Propulsion Laboratory explains that longer-period length-of-day changes can be linked to liquid-iron flow in the outer core interacting with the mantle. Scientists infer that flow indirectly, including from changes in Earth’s magnetic field; they do not watch the core’s motion directly.
NASA also lists inner-core dynamics among possible long-period contributors. Because several mechanisms overlap, assigning one interior process to an individual daily record would be unwarranted. Rotation forecasts remain difficult precisely because the relevant exchanges are only partly understood and are not all predictable in advance.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What was the shortest day on record?
NASA’s Space Geodesy Project reports that June 29, 2022, was 1.59 milliseconds shorter than a standard 86,400-SI-second day. The page describes it as the shortest day in the modern atomic-clock era, which begins in the 1960s.
That record has two important qualifications. First, it is a comparison with a uniform SI-second standard, not evidence that Earth has stopped experiencing long-term tidal braking. Second, it is not the shortest day in all of geological history: Earth rotated faster in the distant past, and estimates for ancient rotation are less precise.
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“While this is a record in the era of atomic clocks (since the 1960s), it is nothing out of the extraordinary.”
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NASA Space Geodesy Project
How scientists measure changes in Earth’s rotation
Very Long Baseline Interferometry
Earth orientation is measured against objects whose directions are effectively fixed on human timescales. In Very Long Baseline Interferometry (VLBI), radio telescopes at different locations observe the same distant quasar. Scientists correlate the arrival times of the signals; the pattern reveals how the observatories—and therefore Earth—are oriented and rotating.
Earth-orientation products
The International Earth Rotation and Reference Systems Service (IERS) coordinates observations and publishes rotation-angle, polar-motion and related products. The U.S. Naval Observatory says its Earth-orientation values are recomputed at least daily. These observations resolve changes far smaller than one second and provide the record against which geophysical explanations are tested.
UT1, atomic time and leap seconds
UT1 is a time scale based on Earth’s actual rotation. Atomic clocks provide a highly uniform reference, represented for civil use by Coordinated Universal Time (UTC). Because Earth’s spin is irregular, UT1 and UTC do not remain perfectly synchronized. Leap seconds have historically been inserted when needed to keep their difference within the applicable limit; they are not added on a fixed annual schedule.
Daniel MacMillan of NASA’s Goddard Space Flight Center summarized the purpose this way: “Earth’s rotation is gradually slowing down a bit, so leap seconds are a way to account for that.” The operational need for such adjustments reflects the mismatch between variable astronomical rotation and uniform atomic seconds.
How to interpret a reported day-length change
- Check the reference. A statement may compare a date with 86,400 SI seconds, with a long-term average, or with a modeled baseline. Those comparisons answer different questions.
- Check the timescale. A daily or seasonal fluctuation does not describe the secular trend produced mainly by tides.
- Check the attribution. A measured change can be correlated with atmospheric angular momentum, while a climate estimate may come from satellite mass observations and reconstruction models.
- Check the direction. Winds, currents and interior coupling can either shorten or lengthen a day; polar ice loss generally acts in the lengthening direction through mass redistribution.
- Keep the scale in perspective. Milliseconds are negligible for ordinary daily activities but significant for satellite navigation, astronomy, geodesy and global timekeeping.
Can climate change affect the length of a day?
Yes. Ice loss and groundwater redistribution alter the moment of inertia, while warming-driven changes in atmosphere and ocean circulation alter angular-momentum exchange. The NASA estimates for recent ice and groundwater movement and for a higher-emissions scenario quantify one part of that effect. They do not replace the Moon’s tidal contribution or account for every short-term fluctuation.
The most accurate overall picture is therefore a superposition: lunar tides set a long-term slowing trend; atmosphere and oceans create weather and seasonal variability; surface-water and ice movement can add a persistent lengthening signal; and core–mantle processes contribute multi-year and decadal departures. Earth’s rotation is changing all the time, but the size and cause of a change depend on the interval being measured.
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