Earth’s inner core is changing the length of your day by milliseconds

A day on Earth is not a fixed 86,400 seconds. Over spans of decades it stretches and shrinks by a few thousandths of a second, and a new study traces most of that drift to a gravitational tug-of-war more than 5,000 kilometres beneath our feet.

Geophysicists Huifeng Zhang and Mathieu Dumberry of the University of Alberta report in Nature that Earth’s solid inner core, a ball of iron that is slightly out of round, pulls on dense regions of the mantle as its orientation shifts. That pull speeds up or slows down the rocky mantle and the crust we live on, and with them the length of the day. Two other forces at the boundary between the core and the mantle push back.

The result answers a question geophysicists have circled for decades. They knew the core was trading spin with the mantle on these timescales, but not which physical link carried the exchange. It also brings a surprise about the planet’s centre: the inner core appears to flow, very slowly, instead of sitting there as a rigid lump.

Why a swinging iron ball can move the clock

Earth’s total spin, its angular momentum, stays almost constant. When the liquid outer core speeds up, the mantle has to slow down to compensate, and the reverse. Those tiny swings in the mantle’s rotation are what make the length of the day wobble over multi-decade cycles.

The open question was the coupling, and there were three candidates. Electromagnetic coupling works through the magnetic field and depends on how well the lowest layer of the mantle conducts electricity. Topographic coupling comes from pressure pushing against bumps on the core-mantle boundary. Gravitational coupling comes from mass: the inner core is not a perfect sphere, and the mantle holds uneven lumps of dense rock.

When flows in the liquid outer core nudge the inner core so that it turns slightly relative to the mantle, its bulges fall out of line with those dense regions. Gravity pulls them back toward alignment, and that restoring pull, a gravitational torque, hands spin to the mantle. According to the study, the inner core swings back and forth by about 2.35 degrees relative to the mantle, on a cycle of 60 to 70 years.

“The gravitational torque acts in the direction required to explain the observed changes in Earth rotation, whereas the electromagnetic and topographic torques generally oppose it,” the two researchers wrote in a summary of the work for Springer Nature’s research community.

How they did it

No instrument can reach the core, so the team combined three indirect records. The first is the length of the day itself, tracked with atomic clocks and astronomical observations. The second is a set of models of how the liquid outer core flows, reconstructed from satellite measurements of Earth’s magnetic field. The third is seismic evidence on the structure of the deep Earth.

They fed these into a statistical technique called Markov chain Monte Carlo, which tries out a vast number of possible values for the unknown quantities and keeps the combinations that fit the observations. The unknowns included the strength of each of the three couplings. The model reproduced about six decades of day-length data best when gravitational torque did most of the work and the other two acted as brakes.

Testing the three mechanisms together matters because they operate at the same time and partly cancel each other. A model that looks at one coupling alone can credit it with an effect that, in the real Earth, is being offset by another.

“These different pieces of information can come together to provide a more coherent picture of Earth’s deep interior, a region that is extremely difficult to observe directly,” Zhang said.

A few milliseconds, measured against what

The effect is tiny next to the day itself. A few milliseconds out of 86,400 seconds is a change of a few parts in 100 million, far shorter than the blink of an eye. Nobody feels it, but atomic clocks and the systems that track Earth’s orientation for satellite navigation do.

Other forces change the day too. Winds and ocean currents shift it over days and seasons, while the Moon’s tides slowly brake Earth’s spin over millions of years. The decade-scale swings sat in between, with the core long suspected as the source and the mechanism unresolved.

The study also puts a number on how soft the centre of the planet is. The best-fitting models imply that the inner core relaxes its shape over roughly a decade, within a possible range of 2 to 31 years. Under crushing pressure it is still solid iron, yet over years it gives way like extremely stiff putty. The researchers report that this viscosity is consistent with recent laboratory experiments on iron alloys.

The strength of the gravitational pull says something about the mantle as well. It supports the view that the two continent-sized piles of rock beneath Africa and the Pacific, where seismic waves travel unusually slowly, are hot and also chemically denser than the rock around them.

What it doesn’t settle

The reconstruction depends on records that are incomplete. Seismic coverage of the deep Earth is patchy, and core-flow models derived from magnetic data only see the largest structures in the outer core. The inferred shape of the inner core and the density of the mantle piles come out of the fit rather than from direct measurement, and the 2-to-31-year range for the inner core’s softness is wide.

The model also leaves part of the signal unexplained. Faster variations on timescales of 10 to 30 years likely involve additional processes, and a separate six-year oscillation in the length of the day remains an open problem that Zhang is now investigating.

Then there is the inner core’s rotation itself, one of the most argued-over topics in seismology. Studies by a Peking University team in 2023 and a University of Southern California team in 2024 concluded that the inner core’s motion relative to the mantle stalled and began to reverse around 2009 to 2010. The new model, with a change of direction around 2010, fits that picture, but other seismologists have argued that changes at the inner core’s surface, not its rotation, explain the same seismic signals. The authors also list as unresolved whether future shifts in the inner core’s rotation can be predicted at all.

Common questions about Earth’s day length

Why is a day not exactly 24 hours long?

The 24-hour day is a convention, and Earth’s actual rotation varies. Winds and ocean currents change it over days and seasons, the Moon’s tides lengthen it over millions of years, and exchanges of spin between the core and the mantle change it by a few milliseconds over decades.

Is Earth’s inner core reversing its spin?

No. Seismic studies suggest the inner core turns slightly faster or slower than the mantle, so relative to the surface it drifts one way and then back. The new study models this as a swing of about 2.35 degrees over a cycle of 60 to 70 years, with a change of direction around 2010.

Will the inner core make days noticeably longer or shorter?

No. The changes add up to a few milliseconds over decades, far below anything a person can notice. They matter for precise timekeeping, satellite navigation and geophysics, where Earth’s rotation is tracked to fractions of a millisecond.

How do scientists know what happens inside Earth’s core?

No probe can reach it. Researchers combine seismic waves from earthquakes that cross the core, satellite measurements of the magnetic field generated in the liquid outer core, and precise records of Earth’s rotation. This study used all three.

Nature published the study on 23 September 2026. Between the early 1970s and 2021, the team found, core processes changed the length of the day by a few milliseconds. The next test is the six-year cycle: a separate group reported in a June 2026 preprint, not yet peer-reviewed, a matching six-year signal in satellite gravity data that it links to the same coupling between the inner core and the mantle. If both records show the same swing, the pull of a metal ball at the centre of the planet will be readable from orbit as well as from the clock.

Reference: Zhang and Dumberry, “Gravitational torque drives multidecadal variations in length of day,” Nature, 2026. DOI: 10.1038/s41586-026-10999-2

Tags: earth science, Earth's rotation, geophysics, inner core, length of day, Mathieu Dumberry