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Earth’s day is conventionally 24 hours, but the planet does not rotate at a perfectly constant rate. A 2026 Nature study proposes that changes in the inner core’s rotation may help explain one broad pattern: variations of several milliseconds in day length over decades. The mechanism is a model-supported explanation for that multidecadal signal, not a complete explanation for every change in Earth’s rotation.
What it means to say a day is not exactly 24 hours
Twenty-four hours is the familiar civil-time reference. In physical measurements, length of day (LOD) describes how long Earth’s rotation takes relative to a reference 24-hour rotation rate. Because the rotation rate varies, measured LOD can be a little longer or shorter than that reference.
The variations discussed in the 2026 study are measured in milliseconds. They are geophysical changes, not perceptible changes to an ordinary day for people going about their lives.
What the 2026 study found
Huifeng Zhang and Mathieu Dumberry’s Nature paper, published on 23 September 2026, examines decadal and multidecadal LOD variations from 1964 to 2019. It describes fluctuations of several milliseconds over timescales of 10 to 70 years and proposes that changing inner-core rotation can help account for the multidecadal component.
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In the authors’ reconstruction, a broad inner-core rotation oscillation of about 60–70 years changes the gravitational torque acting on the mantle. Their modeled inner-core rotation changes direction near 2010. The study uses seismic histories of inner-core rotation and core-flow models derived from geomagnetic secular variation; it does not directly observe the entire inner core moving as a rigid sphere or directly measure the torque.
How inner-core motion could affect the length of a day
Changing alignment can create a gravitational pull
Earth’s solid inner core is surrounded by the liquid outer core, with the mantle above it. The inner-core boundary is not perfectly smooth, and the mantle contains uneven distributions of mass. As the inner core rotates differently from the mantle, the alignment between inner-core boundary topography and mantle mass anomalies changes. The study proposes that this changing alignment produces a gravitational torque on the mantle.
The mantle’s rotation can respond
A torque can transfer angular momentum between Earth’s interior components. If the torque changes the mantle’s rotation rate, it changes the measured length of day: a slightly faster mantle rotation corresponds to a slightly shorter day, while a slightly slower rotation corresponds to a slightly longer one. The University of Alberta’s 23 September 2026 summary describes the effect as shortening or increasing a day by a few milliseconds over time.
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Other core-boundary torques can oppose the effect
The Nature authors’ model also includes torques at the core–mantle boundary, including electromagnetic or topographic coupling, that resist the gravitational pull. Their inferred inner-core-boundary viscous relaxation time is about 10.2 years in the electromagnetic-coupling case; across the combined best-fit models, the reported range is 2–31 years. These are inferred model parameters, not direct measurements of a physical relaxation time.
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Day-length changes have more than one cause
| Timescale | Influence described in the Nature paper | How it relates to this study |
|---|---|---|
| Daily | Tidal variations | Part of the broader variation in Earth’s rotation; not the multidecadal signal the paper focuses on. |
| Seasonal and annual | Mainly atmospheric winds | Another source of LOD variation, distinct from the proposed inner-core explanation. |
| Decadal and multidecadal | Core–mantle angular-momentum exchange; the paper proposes changing inner-core alignment and gravitational torque as an explanation for the studied broad pattern. | The paper analyzes observations from 1964–2019 and reports fluctuations of several milliseconds over timescales of 10–70 years. |
| Long term | Lunar tidal friction | A long-term influence on Earth’s rotation, separate from the study’s proposed explanation for multidecadal changes. |
What remains uncertain
The paper’s torque histories depend on its reconstructions of inner-core rotation and core flow. The authors note that changing the core-flow model can alter the best-fit torque parameters. Their limited temporal resolution also means the analysis addresses broad multidecadal changes, not every decadal or shorter fluctuation.
The roughly 70-year pattern should not be treated as a confirmed repeating cycle. Zhang and Dumberry write: “Whether this flow structure is periodic, and repeats over time, or whether it only reflects the dynamics over the past seven decades, is unknown.” The study supports a possible explanation for the pattern observed in the period it analyzed; it does not establish that the same pattern will recur.
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What to take away
- Twenty-four hours is the everyday reference, while measured physical day length varies as Earth’s rotation rate changes.
- A 2026 Nature study proposes that changing inner-core alignment and gravitational torque may help explain multidecadal LOD variations of several milliseconds.
- The mechanism is inferred from seismic and geomagnetic-derived models, and it does not account for all daily, seasonal, annual, or long-term changes in Earth’s rotation.
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