A day on Earth is not always exactly 24 hours, and new research points to a hidden cause thousands of miles beneath our feet. A study published Sept. 23 in the journal Nature concludes that a gravitational tug between Earth's solid inner core and its rocky mantle is the main driver of small changes in day length that unfold over several decades.

The changes are tiny, a few milliseconds, but scientists can measure them precisely. Geophysicists have known for about 30 years that the core and mantle trade rotational momentum, yet the mechanism behind that exchange has been uncertain.

The finding matters beyond curiosity. Precise knowledge of Earth's rotation underpins global timekeeping, satellite navigation and astronomy, and the study also offers a new look at how the planet's deepest layers behave.

A Spinning Planet That Keeps Imperfect Time

Earth's liquid outer core does not rotate at a constant rate. Changes in the magnetic field show that it speeds up over a few decades, then slows down over a few more, according to the University of Alberta's account of the research, where the work was done.

The mantle, the roughly 3,000-kilometer-thick (1,860-mile) rocky shell that includes the crust, responds in the opposite direction. When the core speeds up, the mantle slows slightly and the day grows a little longer. When the core slows, the mantle speeds up and the day gets slightly shorter. This balancing act happens because the planet's total angular momentum must stay constant.

Physics PhD student Huifeng Zhang and professor Mathieu Dumberry set out to identify which force actually transfers that momentum. Zhang told Interesting Engineering the goal was to investigate "the relative contributions of different core-mantle coupling mechanisms." Dumberry said scientists already knew the core was responsible for the decadal changes, but the mechanism was not settled. "This is the part that's unclear," he said.

The Inner Core's Gravitational Pull

The answer, the researchers report, lies in the inner core's shape. The solid inner core is not perfectly round. As its rotation changes, its uneven mass shifts relative to dense regions in the mantle above it. Gravity pulls the two back toward alignment, and that restoring pull, called a gravitational torque, speeds up or slows down the mantle.

A second effect works against it. At the boundary between the core and mantle, friction and electromagnetic drag create what scientists call core-mantle boundary torque, which resists the gravitational tug and limits how much the day can change. The small variations observed in day length come from subtle shifts in the balance between the two.

"Before we obtained the result, we didn't know they are competing with each other," Zhang said, as reported in Nature's news coverage of the study.

The study also suggests the inner core can slowly change shape. Zhang and Dumberry report that it appears to "deform viscously" on a time scale of about 10 years, unexpected behavior for a solid ball of iron.

Strong Evidence With Clear Limits

The research is peer-reviewed, but it is a modeling study. The team tested combinations of the competing forces against records of Earth's rotation covering roughly six decades, and the model matched the observed changes most closely when the gravitational torque had the strongest influence.

That is strong evidence, not direct observation. No instrument can watch the inner core move. Its motion is inferred from seismic waves that travel through it, and those inferences carry their own uncertainties and remain debated among seismologists.

The study also addresses changes over several decades. Shorter-term variations are influenced mainly by the atmosphere and oceans, and Zhang is examining whether similar deep-Earth mechanisms could explain a separate six-year cycle in day length.

Over much longer periods, a different process is at work. Tidal friction from the Moon has gradually lengthened Earth's day over millions of years, a slow trend distinct from the decades-long variations this study examines. The new findings do not change that long-term picture.

Nothing in the findings suggests any risk to people. The changes are a few thousandths of a second spread over decades, far too small to notice in daily life.

Milliseconds Matter to Clocks and Satellites

Even milliseconds add up for precise technology. Atomic clocks keep time far more steadily than the planet spins, so since 1972 timekeepers have periodically inserted leap seconds into Coordinated Universal Time to keep clocks close to Earth's actual rotation. In 2022, the world's metrology body voted to stop adding leap seconds by or before 2035, partly because Earth's rotation is hard to predict.

Better models of the deep forces that shift day length could eventually improve long-range forecasts of Earth's rotation, though the study does not claim to have produced such forecasts. Zhang has said the next step is to combine improved observations of the inner core with continued measurements of Earth's rotation, so the gravitational explanation can be tested against new data rather than only the historical record.

For now, the practical takeaway is simple. A clock that reads 24 hours is tracking atomic time, while the planet keeps slightly imperfect time of its own, shaped in part by a slow gravitational exchange deep underground.

What Readers Want to Know

Is a day on Earth exactly 24 hours?

Not quite. The length of a day varies by tiny amounts, measured in milliseconds, because Earth's rotation is not perfectly constant.

What did the new study find?

Researchers at the University of Alberta concluded that a gravitational tug between the solid inner core and dense regions of the mantle is the main driver of day-length changes over several decades.

How large are the changes?

The changes amount to a few milliseconds and play out over decades, far too small to notice in daily life.

Does this pose any danger?

No. The variations are extremely small and have no effect on people's safety or daily routines.

Why do scientists care about milliseconds?

Precise knowledge of Earth's rotation matters for global timekeeping, satellite navigation and astronomy, which is why leap seconds have been added to clocks since 1972.

Is the explanation proven?

It is a peer-reviewed modeling study that fits about six decades of rotation records well, but the inner core's motion is inferred indirectly, so further data will test the result.