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Earth’s day is close to 24 hours, but its actual length varies by milliseconds. The solid inner core—about 3,000 miles beneath the surface—may contribute to some of those changes as its motion shifts relative to the mantle. It is not the only influence: the Moon, atmosphere, oceans, and redistribution of water and ice also affect Earth’s rotation.
Is Earth’s day exactly 24 hours?
No. Twenty-four hours is the rounded length used for the civil day; Earth’s physical rotation varies slightly. Scientists describe those differences as changes in the length of day, or LOD, and measure them in milliseconds. A millisecond is one-thousandth of a second, so the changes are tiny in daily life but measurable with precise observations.
There is no single mechanism behind every fluctuation. Earth’s rotation responds to exchanges of angular momentum within the planet and between the solid Earth, its fluids, and the Moon. The effects occur on different timescales, from seasonal shifts to changes accumulated over centuries.
Can Earth’s core change the length of a day?
Yes. Motion in Earth’s core can affect the planet’s rotation, and changes in rotation show up as changes in day length. The inner core sits inside the liquid outer core, deep beneath the mantle. It cannot be observed directly; researchers infer its behavior by studying how seismic waves from earthquakes travel through and around it.
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Seismic studies have found changes in the inner core’s motion relative to the mantle and have identified timing relationships between core behavior and length-of-day variation. Those findings support a possible contribution, not proof that the inner core alone causes day-length changes. NASA and the National Institute of Standards and Technology describe other important drivers, including lunar tides, atmosphere and ocean circulation, and shifts in surface water and ice.
Is the inner core really slowing down?
“Slowing down” needs a reference point. The 2024 study Inner core backtracking by seismic waveform change reversals interpreted changes in seismic waveforms as evidence that the inner core slowed its rotation relative to the mantle and then began backtracking relative to it. That does not mean the whole Earth reversed its spin. The planet continues rotating; the claim concerns the inner core’s motion compared with the surrounding mantle.
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What the seismic evidence shows
A 2023 Nature Geoscience paper reported an approximately seven-decade oscillation in the inner core’s differential rotation. It also noted that the timing coincided with changes in length of day and Earth’s magnetic field. Coincidence in timing is an observed association, not by itself proof that the core caused those changes.
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Because the inner core is inferred from seismic records rather than directly watched, its changing motion and shape remain interpretations of those observations. The findings are being refined as researchers analyze additional evidence.
How much can a day change?
Effects range from microseconds for some individual events to milliseconds across seasonal or longer trends. The figures below describe different mechanisms and timescales; they are not interchangeable measurements or amounts that can simply be added together.
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| Influence or observation | Reported day-length effect | Timescale and qualification |
|---|---|---|
| Lunar tidal friction | Average increase of 2.4 milliseconds per century | Long-term average reported by NASA’s Jet Propulsion Laboratory in 2024. |
| Ice and groundwater movement | Increase of 1.33 milliseconds per century | NASA’s 2024 estimate for the 2000–2018 period; this figure is attributed to surface mass redistribution, not inner-core motion. |
| Climate-related changes under high emissions | Up to 2.62 milliseconds per century | NASA’s 2024 conditional high-emissions scenario, not an unconditional forecast. |
| Seasonal atmosphere and ocean exchanges | About 1 millisecond over a year | NASA Jet Propulsion Laboratory’s 2010 description of seasonal variation. |
| Longer-period variation associated with outer-core flow | About 4 milliseconds | NASA Jet Propulsion Laboratory reported a 65–80-year mode that changed day length by about this amount at the beginning of the twentieth century; its account attributes longer fluctuations beyond atmosphere and ocean effects to liquid outer-core flow. |
| 2004 earthquake | Shortened the day by 2.68 microseconds | Estimate for that earthquake, reported by NIST in 2025. A microsecond is one-thousandth of a millisecond. |
The comparisons show why “the day is changing” can describe very different things: a seasonal swing, a long-term rate, or a tiny shift associated with one event. The inner-core studies add evidence about deep-Earth dynamics, but they do not assign the core a single universally applicable millisecond value.
What does “the core changes the day” mean in practice?
Earth’s length of day is measured from the planet’s rotation, while inner-core motion is inferred from seismic waves. Researchers compare patterns in those records to understand whether internal dynamics may be linked. The 2023 paper’s roughly seven-decade oscillation and its timing relationship with length-of-day changes are one piece of evidence; other processes, such as lunar tidal friction and movement of water or air, operate alongside it.
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For scale, NASA’s 2024 account of climate-related rotation changes quoted JPL geophysicist Surendra Adhikari: “The common thread between the two papers is that climate-related changes on Earth’s surface, whether human-caused or not, are strong drivers of the changes we’re seeing in the planet’s rotation.” That finding concerns surface changes, not the inner core, and underscores why day-length variation should not be assigned to one cause.
How scientists study a part of Earth 3,000 miles down
Earth’s inner core is solid and lies within the liquid outer core. Scientists study it indirectly through seismic waves generated by earthquakes. When similar earthquakes recur, researchers can compare waveforms recorded at the same seismic arrays; subtle changes can reveal that the waves interacted differently with deep structures.
Keith Koper, a University of Utah geophysicist, described detecting “very subtle differences” in seismic waves that sample shallow parts of the inner core. Such measurements do not amount to a direct view of the core. They provide evidence that researchers interpret alongside other records, including observations of Earth’s rotation and magnetic field.
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