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Earth’s physical day is not exactly 24 hours every time: the planet’s rotation rate varies, changing the measured length of a day by milliseconds. A 2026 Nature study proposes that changing motion of the inner core helps explain one broad, multidecadal part of that variation—not every change in day length.
What does “24 hours” mean?
Twenty-four hours is the familiar civil-time reference for a day. Earth’s rotation, however, is not perfectly steady, so the measured length of a day (LOD) can be a little shorter or longer than that reference. The differences discussed in the 2026 study are on the scale of several milliseconds, not changes a person would notice in ordinary life.
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Day length also varies over different timescales. Daily changes are associated with tides; seasonal and annual changes are mainly linked to atmospheric winds; and lunar tidal friction contributes to the long-term increase in day length. The inner-core explanation addresses a distinct, slower signal.
What did the 2026 inner-core study find?
Huifeng Zhang and Mathieu Dumberry’s Nature paper examines decadal and multidecadal LOD variations from 1964 to 2019. It reports fluctuations of several milliseconds over timescales of 10 to 70 years. The authors propose that changing inner-core motion can help account for the broad multidecadal component of those observations.
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In their reconstruction, the inner core’s differential rotation follows a broad oscillation lasting about 60 to 70 years. As the inner core turns relative to the mantle, topography at the inner-core boundary shifts alignment with mass anomalies in the mantle. The authors model the resulting gravitational torque as pulling on the mantle and changing its rotation rate. That change in mantle rotation can lengthen or shorten the day by a few milliseconds over time, as the University of Alberta’s summary of the study explains.
How the proposed torque works
- Relative motion changes alignment. The model tracks inner-core rotation relative to the mantle and the changing alignment of inner-core boundary topography with mantle mass anomalies.
- Gravity transfers torque. The changing alignment produces a gravitational torque on the mantle, altering its rotation rate.
- Other core-boundary torques resist the pull. Electromagnetic or topographic torque at the core–mantle boundary can oppose the gravitational torque, so the net effect depends on the balance between them.
The model uses an inner-core differential-rotation amplitude of about 2.35° relative to the mantle. This is a reconstructed model value, not a direct observation that the entire inner core moves as a rigid sphere through that angle.
How strong is the evidence—and what remains uncertain?
The proposed mechanism is an interpretation built from seismic histories of inner-core rotation and core-flow models derived from geomagnetic secular variation. The torque itself is not directly observed. Zhang and Dumberry note that their torque histories are only as accurate as the underlying inner-core rotation and core-flow models; changing the core-flow model can change the best-fit torque parameters.
The authors’ reconstruction places a change in the direction of inner-core rotation near 2010. They also report inferred model parameters rather than direct measurements: in the electromagnetic-coupling case, the best-fit inner-core-boundary viscous relaxation time is approximately 10.2 years, while the combined best-fit range is 2 to 31 years.
Limited temporal resolution means the analysis supports broad multidecadal changes, not a complete account of every decadal or shorter fluctuation. Nor is a repeating cycle established. 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.”
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Does the inner core explain every change in day length?
No. The study offers a model-supported explanation for a particular multidecadal signal in observations from 1964 to 2019. It does not replace other known contributors to rotation changes: tides are associated with daily variations, atmospheric winds mainly drive seasonal and annual changes, and lunar tidal friction contributes to the long-term trend. The timescale matters when interpreting any claim about why a day is shorter or longer.
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Sources
- Huifeng Zhang and Mathieu Dumberry, “Gravitational torque drives multidecadal variations in length of day,” Nature, published 23 September 2026.
- Geoff McMaster, University of Alberta, “Earth’s deep secret: why your day isn’t quite 24 hours,” published 23 September 2026.
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