Earth’s inner core may be quietly changing the length of our days

A day feels reliably fixed at 24 hours, but sufficiently precise clocks reveal that Earth does not rotate at exactly the same rate from decade to decade. The difference is tiny, usually measured in milliseconds, yet those fluctuations contain information about forces operating thousands of kilometers beneath our feet.

A new study suggests that one of the most important forces comes from Earth’s solid inner core. Researchers at the University of Alberta found that changes in its rotation can produce a gravitational torque on the mantle, subtly altering how quickly the rest of the planet turns.

The work, published in Nature, also indicates that competing forces at the boundary between the liquid outer core and mantle resist that gravitational pull. Together, the opposing torques reproduce broad changes in day length observed between 1964 and 2019.

The result provides a possible solution to a problem geophysicists have wrestled with for more than three decades: scientists knew angular momentum was moving between the core and mantle, but they could not explain precisely how.

Earth does not keep time like a perfect mechanical clock. Its rotation changes across timescales ranging from days to millions of years.

Atmospheric winds and ocean circulation create seasonal changes. Tidal friction caused largely by the Moon gradually lengthens the day over geological time, while movements of water and ice across Earth can also alter its rotation.

Another pattern occurs over decades. After researchers remove known atmospheric, oceanic and long-term effects, variations of several milliseconds remain on timescales of roughly 10 to 70 years.

Scientists have connected these fluctuations to movement inside Earth’s metallic core. Flows in the liquid outer core can gain or lose angular momentum, requiring the mantle to compensate because the angular momentum of the planet as a whole must be conserved.

If the core accelerates in one direction, the mantle can slow slightly. Because the crust rides on the mantle, that change alters the measured length of the day.

The missing piece has been the force that transfers that angular momentum.

Earth’s inner core is a solid sphere of iron-rich material surrounded by the much larger liquid outer core. It is not perfectly spherical, and it can rotate slightly differently from the mantle above it.

The mantle also contains unevenly distributed masses. Those density variations create a gravitational field that tends to keep the inner core’s slightly elongated shape aligned in a preferred direction.

Flows in the outer core can disturb that alignment by exerting electromagnetic forces on the inner core. As the inner core rotates slightly ahead of or behind its preferred position, gravity tries to pull it back.

That creates torque, a twisting force analogous to the force used to turn a wrench.

Huifeng Zhang and Mathieu Dumberry modeled how the reconstructed motion of the inner core would translate into gravitational torque on the mantle. Their calculations relied partly on seismic studies that tracked changes in inner-core rotation over recent decades.

The match with observed day-length changes was striking. A broad oscillation in inner-core rotation lasting about 60 to 70 years produced a gravitational torque with both the timing and magnitude needed to reproduce the multidecadal rotation record.

Gravity was not the only interaction the researchers considered.

Moving electrically conducting liquid iron produces Earth’s magnetic field. That field can interact with conductive material near the bottom of the mantle, creating electromagnetic coupling between the outer core and mantle.

Flowing liquid metal can also interact mechanically with variations in the shape of the core-mantle boundary, producing what researchers call topographic torque.

Neither mechanism reproduced the observed multidecadal variations well when acting alone. In the model, both tended to operate in the opposite phase from the torque required to explain the changing day length.

The researchers therefore interpret them primarily as resisting forces.

Their preferred picture resembles a planetary tug-of-war. Gravitational interaction involving the inner core drives the broad multidecadal changes, while coupling at the core-mantle boundary limits those changes.

The final movement depends on the relatively small difference between these large opposing torques.

The model also revealed something unexpected about the mechanical behavior of the solid inner core.

If the inner core behaved as a perfectly rigid object, the timing of the gravitational torque would not match observations. Its shape instead needs to relax gradually toward gravitational alignment with the mantle.

The researchers estimated a characteristic relaxation time ranging from roughly 2 to 31 years, depending on assumptions about the resisting torque. Their best-fitting models generally placed it around a decade.

That implies the inner core can deform viscously despite being solid. Over sufficiently long timescales, immense temperature and pressure can allow solid materials to flow slowly rather than behave like perfectly rigid objects.

The inferred viscosity is roughly 10^17 to 10^18 pascal-seconds if deformation occurs throughout the inner core. If movement is concentrated near its outermost portion, the effective viscosity could be considerably lower.

Those values broadly agree with recent laboratory experiments and theoretical calculations of iron alloys under extreme inner-core conditions.

No instrument can directly sample Earth's core. The center of the planet lies about 6,371 kilometers beneath the surface, leaving scientists dependent on indirect measurements such as seismic waves, magnetic-field changes and variations in planetary rotation.

That makes millisecond changes in day length surprisingly useful.

The new model places constraints not only on the inner core but also on the deepest mantle. Its results are consistent with large, chemically distinct structures near the core-mantle boundary and potentially with a thin, electrically conductive layer enriched in iron at the mantle's base.

Important uncertainties remain. Reconstructions of inner-core rotation and outer-core flow depend on incomplete seismic and magnetic records, and changing those models can shift some estimated parameters substantially.

The researchers also focused primarily on the broad 60-to-70-year signal. Faster variations lasting 10 to 30 years probably involve additional processes that existing models still do not fully reproduce.

Even so, the work offers a coherent explanation for a remarkable connection between Earth's deepest interior and an everyday measurement at its surface. A day may appear fixed, but hidden thousands of kilometers below us, the inner core, outer core and mantle are continually trading angular momentum and changing the clock by milliseconds.

These recent studies provide additional context on inner-core movement, deformation and the physical properties that govern Earth's deep interior.

Annual-scale variability in both the rotation rate and near surface of Earth’s inner core: Repeating earthquake signals reveal changes in both inner-core rotation and its outermost structure, providing evidence that the solid inner core can deform as well as move. (Nature Geoscience, 2025)

Depth-dependent anisotropy in the Earth’s inner core linked to chemical stratification: High-pressure experiments investigate how iron alloys deform inside the inner core and provide viscosity estimates relevant to its slow reshaping. (Nature Communications, 2025)

Inner core backtracking by seismic waveform change reversals: Seismic records show that the inner core moved faster than the mantle before reversing direction and retracing part of its previous path between 2008 and 2023. (Nature, 2024)

Tilted transverse isotropy in Earth’s inner core: Seismic tomography reveals complex crystal orientations inside the inner core, providing clues to its structure, deformation and long-term dynamics. (Nature Geoscience, 2024)

Multidecadal variation of the Earth’s inner-core rotation: This seismic study reconstructed an approximately seven-decade oscillation in differential inner-core rotation that closely tracks changes in Earth’s day length and magnetic field. (Nature Geoscience, 2023)

Research findings are available online in the journal Nature.

The original story "Earth’s inner core may be quietly changing the length of our days" is published in The Brighter Side of News.

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