Earth’s day wobbles by milliseconds and the inner core may be slowly bending

The length of Earth’s day drifts by a few milliseconds over decades, and a Nature paper from the University of Alberta argues that the culprit sits more than 5,000 kilometres down: a solid inner core that appears to deform slowly, on a timescale of about 10 years, and in doing so tugs gravitationally on the rocky mantle above it. Physics PhD student Huifeng Zhang and professor Mathieu Dumberry published the result on September 23.

The swings are tiny. Across the 1964 to 2019 record the fluctuations amount to roughly two to three milliseconds, far too small to notice on a clock but large enough to be measured against distant radio sources and to matter for the precise timekeeping that depends on knowing exactly how fast the planet turns.

Length of day as a few milliseconds of drift

Earth’s rotation is tracked by the International Earth Rotation and Reference Systems Service, which defines the excess revolution time as the length of day, or LOD. The U.S. Naval Observatory explains that the variations are measured in milliseconds, using very long baseline interferometry, in which several radio telescopes observe the same quasar at the same moment to fix the planet’s orientation.

Most short-term changes come from the atmosphere and oceans, whose angular momentum shifts with winds and the water cycle. Zhang and Dumberry removed atmospheric, oceanic and tidal contributions from the record covering 1964 to 2019, leaving the decade-scale swings that must come from deeper inside the planet. According to the paper in Nature, those multidecadal changes in length of day “are driven primarily by gravitational torque” from the inner core.

Gravitational torque between inner core and mantle

The mantle, the roughly 3,000-kilometre-thick rocky layer that includes the crust, sits on top of a liquid outer core, and inside that lies the solid inner core. Total angular momentum is conserved, so when one layer speeds up another must slow down. For about 30 years, scientists have followed changes in how the liquid outer core flows from observations of Earth’s magnetic field, and when that flow quickens the mantle compensates by slowing, which lengthens the day.

Zhang and Dumberry add a third player, and the University of Alberta release carried by ScienceDaily lays out how it works. The inner core is not a perfect sphere, and the mantle above it has uneven mass, with denser and lighter regions. When the inner core changes its rotation speed, its dense regions pull on dense regions of the mantle, a gravitational torque that changes how quickly the mantle spins. That pull is resisted by electromagnetic drag from the outer core’s magnetic field and by pressure pushing the molten outer core against bumps on the mantle boundary.

Dumberry, a geophysicist at the University of Alberta, described the earlier gap in a Nature news report: how the core influences rotational changes was “the part that’s unclear.” The new model weighs the three competing torques, and Zhang explained that the forces compete with one another rather than acting independently, so the observed millisecond changes reflect a shifting balance between gravitational pull and frictional resistance, not a single driver. Because the three torques oppose or reinforce each other, the authors constrained them together with a Bayesian inversion, a statistical method that samples many possible combinations and keeps the ones consistent with the data.

An inner core that deforms viscously over about a decade

For the gravitational link to work as the data require, the inner core cannot be a rigid ball. The study finds it “deforms viscously,” relaxing toward a rounder shape on a timescale the authors constrain to between 4 and 31 years, with a best-fit value of about 10 years. That implies an inner core viscosity between 10¹⁵ and 10¹⁸ pascal-seconds, a range that sets how fast a solid under enormous pressure can slowly flow.

The fit also supplies numbers for the torque, between 0.6 and 4.2 times 10¹⁹ newton-metres, and for the mantle’s uneven boundary, which would need relief of about 31 to 83 metres peak to peak. The inner core’s differential rotation across the multidecadal cycle comes out at about 2.35 degrees.

The work builds on seismic studies from 2023 that tracked the inner core’s rotation relative to the rest of the planet since the 1960s. Those studies suggested that the inner core rotated faster than the rest of the planet until about 2010 and then began to slow, which gives the new model a seismic history of the inner core to fit against the length-of-day record.

The claim rests on inference from three kinds of data: seismic histories of inner core rotation, core flow derived from the magnetic field, and length of day. The independent check will come from further seismic observations of the inner core, which can test whether it deforms on the timescale Zhang and Dumberry propose.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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