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Earth’s day length changes because angular momentum and mass are constantly exchanged among the solid planet, atmosphere, oceans, ice, surface water and deep interior. The Moon’s tides produce the long-term average slowing, while winds, currents, tides, climate-driven mass movement and core–mantle interactions create shorter-term speedups and slowdowns that geodesists can measure in milliseconds.
Why a day is not exactly 24 hours
A civil day is defined as 86,400 SI seconds, but Earth does not rotate at a perfectly constant rate. The mean solar day is about 86,400.002 seconds on average, according to NASA Goddard. Individual rotation days vary around that average, sometimes becoming slightly shorter and sometimes slightly longer.
The reason is angular momentum. A rotating body tends to conserve its angular momentum, which depends on how much mass it has, how that mass is distributed relative to the spin axis, and how fast it rotates. If angular momentum moves from the solid Earth into the atmosphere or oceans, the ground rotates a little more slowly. If angular momentum moves back into the solid Earth, it rotates a little faster. Moving mass farther from the spin axis also increases the planet’s moment of inertia and slows the spin; moving mass inward has the opposite effect.
These changes are tiny compared with a 24-hour day, usually measured in milliseconds (thousandths of a second), but modern geodetic instruments detect them reliably.
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The main causes, compared
| Cause | Typical timescale | Possible direction | How it is established |
|---|---|---|---|
| Moon-driven tidal friction | Long-term, secular trend; periodic tidal variations also occur | Primarily lengthens the average day | Earth-orientation records, tidal theory and long-term geophysical models |
| Atmospheric winds and pressure | Daily, seasonal and generally shorter than two years | Can lengthen or shorten a day | Correlation with atmospheric angular-momentum measurements and geodetic observations |
| Ocean currents and tides | Periodic, seasonal and weather-related | Can lengthen or shorten a day | Ocean and tide models combined with Earth-orientation data |
| Ice, groundwater and other mass redistribution | Years to centuries | Recent polar-to-equatorial movement generally lengthens the day | Satellite gravity observations and mass-balance reconstructions |
| Liquid outer-core flow and mantle coupling | Multi-year to decadal; commonly five to 15 years | Can speed or slow the solid Earth | Earth-rotation records and indirect inference from magnetic-field and interior models |
Moon-driven tidal braking is the long-term baseline
How tidal friction removes spin
The Moon’s gravity raises tides in the oceans. Because Earth rotates faster than the Moon orbits, the tidal bulges are carried slightly ahead of the Earth–Moon line by friction. The resulting gravitational torque transfers angular momentum from Earth’s spin to the Moon’s orbit. Earth therefore loses rotational speed on the long-term average, while the Moon slowly moves outward.
The Sun also raises tides, and tides in the solid Earth contribute measurable periodic changes. The U.S. Naval Observatory identifies tidal friction as the chief cause of secular rotational-speed change. Its stated secular increase is about 0.0015–0.0020 seconds per day per century (1.5–2.0 milliseconds per day per century). A 2024 NASA Jet Propulsion Laboratory account gives an average lunar-tidal contribution of 2.4 milliseconds per century. These are source-specific estimates expressed for different summaries and should not be treated as identical measurements.
Why a long-term slowdown does not make every day longer
A secular trend is an average over a long interval. Superimposed on it are atmospheric, oceanic, tidal and interior fluctuations. A particular date can therefore have a shorter day even while the long-term average continues to lengthen. The long-term tidal trend and a short-term record are different claims about different timescales.
Atmosphere and oceans trade angular momentum with the solid Earth
Winds and pressure systems
Air moving east or west carries angular momentum. Strong westerly winds can take angular momentum from the solid Earth and make the ground rotate slightly more slowly; opposite circulation can return angular momentum and speed it up. Changes in atmospheric pressure also shift mass and alter the torque exchanged with the surface.
The U.S. Naval Observatory says variations with periods under two years are related largely to changes in the total angular momentum of the atmosphere. Seasonal wind and pressure patterns can change the length of a day by milliseconds over a year. The sign is not fixed: one season or weather pattern may lengthen the day, while another may shorten it.
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Ocean currents, tides and climate oscillations
Ocean currents move enormous amounts of water and exchange angular momentum with the seafloor through friction. Ocean tides add regular gravitational and frictional effects. Together with atmospheric circulation, these processes explain much of the predictable periodic structure in Earth-orientation records.
NASA reports that El Niño-related atmospheric variations can lengthen a day by as much as one millisecond. That figure describes the reported effect of this climate oscillation, not a permanent change to the length of every subsequent day.
Water, ice and climate-driven mass movement
Why moving water can slow rotation
Imagine a figure skater extending their arms: the mass moves farther from the rotation axis, the moment of inertia increases and the spin slows. Earth behaves similarly. When ice stored near the poles melts and its water spreads through the oceans toward lower latitudes, more mass is placed farther from the spin axis. The resulting increase in Earth’s oblateness tends to lengthen the day. Groundwater depletion can have a related effect when extracted water eventually reaches the oceans.
Observed attribution for 2000–2018
NASA’s 2024 summary describes a reconstruction that combined GRACE and GRACE-FO satellite gravity observations with earlier mass-balance work. For 2000–2018, the researchers estimated that ice and groundwater changes increased day length at 1.33 milliseconds per century. That rate was faster than their estimates for periods in the preceding century, which ranged from 0.3 to 1.0 milliseconds per century.
The value is an attribution to specific mass changes over a defined interval, not a claim that every year adds exactly 1.33 milliseconds per century. Ice loss, groundwater movement, land uplift and other hydrological processes vary through time.
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A conditional future projection
The same NASA account reports that, if greenhouse-gas emissions continue to rise, climate-related lengthening could reach as much as 2.62 milliseconds per century. The projection is conditional on the emissions pathway and the researchers’ model; it is not an observed present-day rate or a guaranteed outcome. Under that scenario, the climate-related contribution would exceed the 2.4-millisecond-per-century average lunar contribution cited in the NASA account.
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Processes in Earth’s core create multi-year fluctuations
Core–mantle coupling
Earth’s liquid outer core contains electrically conducting iron. Flow in that layer can exchange angular momentum with the rocky mantle through electromagnetic and viscous coupling. If the core speeds up relative to the mantle, the mantle can lose a small amount of angular momentum; if the exchange reverses, the mantle can gain it.
The U.S. Naval Observatory associates decadal variations with characteristic periods of about five to 15 years with processes deep inside Earth. NASA explains that longer-period length-of-day changes can be linked to liquid outer-core flow interacting with the mantle. Scientists infer core behavior indirectly, using rotation records, magnetic-field observations and interior models; they do not observe the core’s flow directly.
Why no single interior process explains every record
NASA also lists inner-core dynamics among possible long-period contributors. Multiple mechanisms can overlap, and the short-term rotation record is not fully predictable. A measured change on one date should not be assigned to a single unconfirmed core process without a specific analysis that separates the competing effects.
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What the “shortest day” record actually means
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 does not mean Earth has entered a permanent speedup, and it is not the shortest day in the planet’s entire history. Geological evidence indicates that ancient Earth rotated much faster, producing substantially shorter days, although estimates from deep time are less precise. The 2022 value is best understood as one local minimum within a variable modern record.
NASA’s Space Geodesy Project characterizes the observation as follows: “While this is a record in the era of atomic clocks (since the 1960s), it is nothing out of the extraordinary.”
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How scientists measure changes in rotation
Very Long Baseline Interferometry
Earth orientation is measured against reference directions in space. The U.S. Naval Observatory describes very long baseline interferometry (VLBI), in which radio telescopes at widely separated locations observe the same distant quasar. Researchers correlate the arrival times of the radio signals. Because the quasar is effectively fixed for this purpose, tiny changes in the telescopes’ orientation reveal how Earth has rotated.
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This method contributes measurements of the angle through which Earth has turned, as well as polar motion and related orientation parameters. The International Earth Rotation and Reference Systems Service (IERS) coordinates observations and publishes Earth-orientation products. The U.S. Naval Observatory says its Earth-orientation values are recomputed at least daily.
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UT1, atomic time and leap seconds
UT1 is a time scale based on Earth’s actual rotation. Atomic clocks provide a much more uniform time scale, coordinated as UTC. Because the planet’s rotation speeds up and slows down, UT1 and UTC do not remain identical. Historically, leap seconds have been inserted when needed to keep their difference within the then-applicable limit; they are not added on a fixed annual schedule.
Daniel MacMillan of NASA Goddard Space Flight Center explained the purpose this way: “Earth’s rotation is gradually slowing down a bit, so leap seconds are a way to account for that.” The wording describes the long-term tendency, while the operational decision to insert a leap second depends on observed and predicted Earth orientation.
How to interpret a day-length number
- Check the reference: “Milliseconds shorter” normally means shorter than 86,400 SI seconds, not shorter than the mean solar day on every date.
- Check the interval: A value per century is a long-term rate, whereas a daily value describes a particular observation or short-period variation.
- Check the direction: Atmospheric and oceanic exchanges can either lengthen or shorten a day; polar mass moving toward lower latitudes generally lengthens it.
- Check the attribution: Satellite reconstructions and model correlations support estimates for particular causes, but no single mechanism explains every fluctuation.
- Check the uncertainty: Interior processes are inferred indirectly, and predictions become less certain as the timescale and number of interacting systems increase.
What changes in day length mean in practice
A millisecond-scale change is far too small for people to sense and does not alter ordinary clocks by itself. It matters to precision systems that combine astronomical and atomic time: satellite navigation, geodesy, radio astronomy, spacecraft tracking and global reference frames. Those fields need Earth-orientation parameters to relate an Earth-fixed coordinate system to a stable celestial reference.
For everyday timekeeping, the important distinction is between the uniform rhythm of atomic clocks and the irregular spin of the planet. Earth’s rotation is monitored continuously, and time standards can account for their small divergence without treating each short day as evidence that the long-term tidal trend has changed.
Bottom line: several clocks are interacting
The Moon’s tidal torque supplies the dominant long-term braking, but it is only one part of the observed record. Winds, pressure systems, ocean currents and tides shuffle angular momentum on daily and seasonal timescales. Melting ice and shifting groundwater redistribute mass and generally move it farther from the spin axis, while liquid-core flow exchanges angular momentum with the mantle over years and decades. Geodetic networks can measure the combined result, but separating each contribution—and predicting the exact length of a future day—remains an active problem in Earth science.
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