A day on Earth does not always last exactly 24 hours. Over several decades, the planet repeatedly speeds up and slows down, quietly changing the length of a day by several milliseconds.
Scientists have long known that these shifts involve exchanges of angular momentum between Earth’s mantle and its liquid outer core. What remained uncertain was how two regions separated by a boundary roughly 1,800 miles below the surface might exert enough force on one another to produce the measured changes.
A new study published in Nature points to an unexpected source: Earth’s solid inner core. Researchers found that its shifting rotation generates a gravitational force against density irregularities in the mantle, producing a tug powerful enough to alter the planet’s rotation over a roughly 70-year cycle.
“The relative ease with which the gravitational torque can explain the multidecadal [length-of-day changes] suggests that it is the main driver of these changes,” researchers write. “The electromagnetic and topographic torques may play a role, but their opposite phase suggests that they act primarily to resist, not drive, the [length-of-day changes].”
More than a solution to a decades-old geophysical problem, the results effectively turn the changing length of a day into a probe of places humans can never directly reach. The researchers’ model delivers clues about the shape and softness of the inner core, a potentially iron-rich layer at the bottom of the mantle and continent-sized structures sitting above the core.
Earth’s rotation changes for many reasons. Atmospheric winds and ocean circulation produce seasonal variations, while friction from the Moon’s gravitational pull gradually lengthens days across millions of years.
The new research focused on a different signal: fluctuations of several milliseconds occurring over periods of roughly 10 to 70 years.
Previous studies showed that flows inside the liquid outer core could account for the angular momentum involved. When the core speeds up, the mantle slows down to compensate, and vice versa. Because Earth’s crust rides on the mantle, that exchange changes the time required for the surface to complete one rotation.
The missing piece was the mechanism transferring that momentum.
Viscous friction at the core-mantle boundary is too weak. Other proposed explanations involved electromagnetic forces generated where the electrically conducting outer core meets the mantle or pressure from core flows pushing against topographic irregularities along the boundary.
However, physics PhD candidate Huifeng Zhang and Dr. Mathieu Dumberry of the University of Alberta found that those forces do not fit well with the timing of the observed multidecadal changes. Their calculations instead indicate that electromagnetic and topographic forces primarily resist the change.
The driving force appears to originate deeper.
Despite its name, Earth’s inner core is not a perfect sphere. Its boundary contains subtle irregularities, giving it a slightly elongated orientation. Density changes in the mantle likewise create an uneven gravitational field.
Under normal conditions, gravity tends to keep the inner core’s long axis aligned with those mantle structures. But flows in the surrounding liquid metal can rotate the inner core slightly east or west relative to the mantle. That misalignment produces gravitational torque, the rotational equivalent of a pulling force, as gravity tries to restore alignment.
To test the idea, researchers combined seismic reconstructions of inner-core rotation with models of outer-core flows derived from changes in Earth’s magnetic field. They compared the resulting torque with corrected length-of-day records from 1964 through 2019.
The inner core appears to oscillate relative to the mantle by about 2.35 degrees, with its rotation changing direction around 2010. Once the researchers allowed its uneven boundary to deform, the ensuing gravitational torque reproduced both the timing and extent of the observed multidecadal variations.
The reconstruction suggests the core-flow pattern switched between two broad states centered around 1967 and 2002. In each state, flows closer to the planet’s axis moved opposite those farther away, alternately displacing the inner core and reversing the gravitational pull on the mantle.
Whether this represents a persistent 70-year cycle remains unknown. Magnetic records from before the mid-20th century are not detailed enough to show conclusively whether the pattern previously repeated.
The model nevertheless exposes physical details that cannot be measured directly.
For the torque to match observations, the inner core cannot behave as a completely rigid object. Its irregular surface must gradually deform toward gravitational equilibrium over approximately two to 31 years, with the most likely estimates falling near eight to 10 years. That points to a relatively low-viscosity inner core whose outermost portion may be considerably softer than previously assumed.
The torque strength also constrains structures at the base of the mantle. The calculations imply that broad variations in the core-mantle boundary’s gravitational topography may rise and fall by only about 102 to 272 feet.
Those values support models in which the enormous low-velocity regions beneath Africa and the Pacific, often described as continent-sized “blobs,” contain chemically distinct material that is hotter but intrinsically denser than its surroundings. The opposing effects could make the structures nearly neutrally buoyant.
If electromagnetic drag provides much of the resisting force, the results also support a globally distributed, electrically conductive layer of iron-enriched material about 1.2 miles thick at the mantle’s base.
The researchers caution that their conclusions depend on reconstructions of inner-core motion and liquid-core flows that contain significant uncertainties. Different flow models shifted some of their torque estimates by as much as 30 percent.
Their analysis also explains the broad multidecadal signal better than shorter fluctuations lasting 10 to 30 years, which may involve core-mantle forces that present models do not yet capture.
Still, the findings show how changes measured in thousandths of a second at Earth’s surface can reveal the behavior of material thousands of miles below it.
“Better explaining the nature of the core–mantle torque driving the [length-of-day changes],” the researchers write, “contributes to sharpening our understanding of the structures, material properties and dynamics in the deep interior of Earth.”
Study: Zhang, H., and Dumberry, M. “Gravitational Torque Drives Multidecadal Variations in Length of Day.” Nature (2026).
Tim McMillan is a retired law enforcement executive, investigative reporter and co-founder of The Debrief. His writing typically focuses on defense, national security, the Intelligence Community and topics related to psychology. You can follow Tim on Twitter: @LtTimMcMillan. Tim can be reached by email: tim@thedebrief.org or through encrypted email: LtTimMcMillan@protonmail.com