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Earth’s rotation varies, so the length of a day is not exactly 24 hours every time. The solid inner core, about 3,000 miles beneath the surface, may contribute to some of those changes as it moves relative to the mantle. But it is one influence among several—not a proven single cause of day-length variation.
Is Earth’s day exactly 24 hours?
No. Twenty-four hours is the familiar civil-day convention; the planet’s actual rotation varies slightly. Scientists track those differences as changes in length of day, measured in milliseconds. A change of a few milliseconds is tiny in everyday life, but measurable with precision observations of Earth’s rotation.
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Day length can change when mass moves around or within the planet. Those movements alter how angular momentum is distributed, affecting the rate at which Earth turns. The causes operate on different timescales, from seasonal changes to variations spanning decades or longer.
What is the object 3,000 miles beneath the surface?
It is Earth’s solid inner core, enclosed by a liquid outer core. Researchers cannot observe it directly: they infer its properties and motion from seismic waves generated by earthquakes as those waves pass through Earth.
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Because the inner core is surrounded by liquid, it can move at a different rate from the mantle—the broad rocky layer above the outer core. Claims that the inner core is “slowing down” describe this relative motion, not a slowdown or reversal of Earth as a whole.
Is the inner core really slowing down or backtracking?
That is one interpretation of changing seismic records, not a claim that the whole planet has reversed its spin. A 2024 paper in Nature interpreted reversals in seismic waveforms as evidence that the inner core had begun backtracking relative to the mantle after its relative rotation slowed.
Other observations add complexity. A 2023 Nature Geoscience study reported an approximately seven-decade oscillation in the inner core’s differential rotation, with timing that coincided with changes in length of day and Earth’s magnetic field. Coincidence supports a possible relationship, but does not by itself establish that inner-core motion caused those changes.
A 2025 Nature Geoscience study examined 121 pairs of repeating earthquakes recorded at two arrays in northern North America from 1991 to 2023. It reported annual-scale variability and evidence of possible changes near the inner core’s surface. The findings suggest that changes in seismic signals may reflect near-surface structural change as well as rotation; a simple model of the inner core as a perfectly rigid spinning ball may not explain everything.
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How much can a day change, and what else changes it?
The scale depends on the mechanism and timescale. The figures below describe distinct effects; they should not be added together as though they were measurements of one event.
| Influence | Timescale and reported change | What the figure means |
|---|---|---|
| Lunar tidal friction | Average increase of 2.4 milliseconds per century | NASA’s 2024 summary gives this as the long-term average effect of friction from lunar tides. |
| Ice and groundwater movement | 1.33 milliseconds per century over 2000–2018 | NASA’s 2024 estimate attributes this rate to the redistribution of ice and groundwater; it is not an estimate of inner-core effects. |
| Climate-related change under high emissions | Up to 2.62 milliseconds per century | NASA’s 2024 figure is a conditional high-emissions scenario, not an unconditional forecast. |
| Atmosphere and ocean exchanges | About 1 millisecond over a year | NASA’s Jet Propulsion Laboratory described this scale of seasonal day-length variation in 2010. |
| Longer-period variations involving core dynamics | A 65–80-year mode; about 4 milliseconds at the beginning of the 20th century | NASA’s Jet Propulsion Laboratory reported this longer-timescale mode in 2010 and attributed fluctuations beyond atmosphere and ocean effects to flow in the liquid outer core. |
| A major earthquake | 2.68 microseconds | NIST reported NASA’s estimate that the 2004 earthquake shortened the day by this amount—a much smaller scale than the millisecond changes above. |
These comparisons also show why no single headline figure explains every change in Earth’s rotation. Lunar tides gradually lengthen the day; the atmosphere, oceans, moving surface water and ice, and dynamics inside the core contribute variation on other timescales. Directly measuring the rotation is different from inferring what is happening inside the core from seismic records.
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What does the inner core’s motion mean for day length?
Seismic studies support changing motion between the inner core and mantle, and some studies identify timing relationships between inner-core variation and length-of-day changes. That makes the core a plausible contributor to changes in Earth’s rotation, but the evidence does not show that the inner core is the sole cause or establish a simple one-to-one effect on the day.
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1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errorsFor a broader comparison of measured rotation changes, NASA’s Jet Propulsion Laboratory discussed seasonal and longer-period variations in its 2010 overview, while NASA’s 2024 summary focused on climate-related effects. NIST’s 2025 explainer describes how the liquid outer core can affect day length. Those surface, fluid, tidal, and deep-interior influences together make Earth’s day a subtly changing quantity rather than a perfectly fixed 24-hour interval.
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