Earth’s Core: 3 Surprising Forces Shaping Our Day
A new Canadian study is shedding light on how Earth’s core can subtly influence the length of a day, revealing that the planet’s rotation is far more dynamic than it appears from the surface.
To people living on Earth, a day seems fixed at 24 hours. Clocks move from midnight to midnight, calendars follow predictable cycles, and the rotation of the planet appears almost perfectly constant. Yet scientists have long known that Earth’s rotation changes slightly over time.
The differences are extremely small. They are generally measured in milliseconds rather than minutes or hours. Still, those tiny variations provide researchers with valuable information about what is happening thousands of kilometers beneath the surface.
Researchers from the University of Alberta in Canada have now proposed a detailed explanation for multidecadal changes in the length of Earth’s day. Their study, published in Nature on September 23, 2026, identifies gravitational interactions involving the inner core and the mantle as an important part of the process.
The research helps explain a long-standing question in geophysics: How can movements deep inside Earth influence the rotation experienced at the surface?
The answer involves a complex exchange of angular momentum between Earth’s core and mantle.
Earth’s Core Is Not Simply Sitting Still
Earth is divided into several major layers. Beneath the crust lies the mantle, followed by the liquid outer core and the solid inner core at the center of the planet.
The inner core is composed primarily of iron and other dense materials. It is surrounded by the liquid outer core, whose movement plays an important role in generating Earth’s magnetic field.
For decades, scientists have observed that the rotation of Earth’s core does not always behave exactly like the rotation of the planet’s outer layers.
According to the University of Alberta, scientists have known for roughly 30 years that the rotation of Earth’s liquid core can speed up and slow down over periods lasting several decades. When the core’s rotational behavior changes, the mantle can respond in the opposite direction because the planet’s total angular momentum must be conserved.
That interaction creates a subtle but measurable effect on the length of a day.
The new research focuses particularly on the role of the solid inner core and its gravitational interaction with irregularities in the mantle.
How Earth’s Core Can Change the Length of a Day
The key concept behind the research is gravitational torque.
Torque is a twisting force. In this case, the researchers examined how the gravitational pull between Earth’s inner core and mass variations within the mantle can transfer angular momentum between different parts of the planet.
The inner core is not a perfectly uniform sphere. Similarly, Earth’s mantle contains variations in density and structure.
As these components interact, gravitational forces can influence how angular momentum is distributed inside the planet.
The result is a tiny change in the rotational speed of the mantle.
When the mantle rotates slightly faster, the length of the day becomes marginally shorter. When it rotates slightly more slowly, the day becomes slightly longer.
These changes occur on a scale of milliseconds.
The Nature study concludes that gravitational torque from the inner core can help explain multidecadal variations in Earth’s length of day, while other forms of coupling between the core and mantle can oppose the effect.
This is important because scientists have observed these fluctuations for decades but have not always been able to explain precisely how the angular momentum moves between Earth’s deep layers.
A Gravitational Tug-of-War Deep Inside Earth
The new research describes something resembling a gravitational tug-of-war.
On one side is the gravitational interaction between the inner core and the mantle. On the other are forces operating near the boundary separating Earth’s core from the mantle.
The core-mantle boundary is a critical region approximately 2,900 kilometers beneath Earth’s surface.
There, the liquid outer core meets the rocky mantle. The researchers considered forms of electromagnetic and mechanical coupling that can resist changes in rotation.
These opposing effects help prevent Earth’s rotational changes from becoming much larger.
According to the University of Alberta, the balance between gravitational torque and core-mantle boundary torque ultimately contributes to the small fluctuations observed in the length of a day.
This means Earth’s rotation is not controlled by one single mechanism.
Instead, several processes interact simultaneously.
Earth’s Core and the Millisecond Difference
The most important point for the public is that these changes are extraordinarily small.
A day does not suddenly become 23 hours or 25 hours because of activity inside Earth’s core.
The changes described by the research occur at the millisecond level over much longer periods.
A millisecond is one-thousandth of a second.
That means a person would never notice the effect while going about a normal day. There is no need to reset a watch or adjust a daily schedule because Earth’s core has changed its rotational behavior.
However, precision matters enormously to scientists.
Modern measurement systems can detect extremely small variations in Earth’s rotation. Researchers can then compare those variations with observations of the atmosphere, oceans, Earth’s magnetic field and processes occurring inside the planet.
Those measurements effectively allow scientists to use Earth’s rotation as another window into the otherwise inaccessible deep interior.
Why Scientists Study Earth’s Core Through Rotation
No spacecraft or drilling project has reached Earth’s core.
The deepest human-made boreholes extend only a tiny distance compared with the roughly 6,371-kilometer radius of Earth.
Scientists therefore rely heavily on indirect measurements to study the planet’s interior.
Seismic waves are one of the most important tools. Earthquakes generate waves that travel through the planet, and their behavior provides clues about the materials and structures they encounter.
Magnetic observations offer another source of information because Earth’s liquid outer core is closely associated with the planet’s magnetic field.
Rotational measurements add another piece to the puzzle.
When Earth’s rotation changes, researchers can investigate whether those changes correspond with movements or exchanges of angular momentum in the core, mantle, oceans or atmosphere.
The new Canadian research is significant in this context because it provides a physical mechanism linking deep-Earth processes with measurable changes at the surface.
Earth’s Core Is Part of a Much Larger System
It would be misleading to suggest that Earth’s core is solely responsible for every variation in the planet’s rotation.
Earth is a complex rotating system.
The atmosphere, oceans, ice sheets, earthquakes and other processes can also influence how mass and angular momentum are distributed around the planet.
The National Institute of Standards and Technology, or NIST, notes that the unpredictable rotation of Earth’s liquid outer core affects the length of the day. If the core rotates faster, the solid portions of Earth can compensate by rotating more slowly, and vice versa.
The Moon also has a major long-term influence through tidal interactions.
That makes Earth’s rotation the result of several overlapping effects operating across different time scales.
The Canadian study is therefore not suggesting that one newly discovered process completely determines the length of every day. Instead, it helps clarify how deep-Earth interactions can contribute to longer-term variations.
What the Study Reveals About the Inner Core
The research also offers information about the physical behavior of Earth’s inner core itself.
According to the University of Alberta, the study indicates that the inner core can undergo viscous deformation on a time scale of about a decade.
That finding is important because scientists have traditionally treated the inner core as a largely rigid body when constructing models of Earth’s interior.
The emerging picture is more complicated.
The inner core remains solid, but its physical behavior over long periods can be more dynamic than a simple rigid sphere.
Understanding those properties could help researchers improve models of Earth’s internal structure and explain how different layers exchange angular momentum.
It could also contribute to broader research into the planet’s magnetic field and the processes that have shaped Earth’s deep interior over geological time.
Why the Length of a Day Matters to Science
For everyday life, a change of a few milliseconds is essentially irrelevant.
For precision science, however, it is valuable information.
Scientists use highly accurate measurements of Earth’s rotation for astronomy, satellite navigation, geodesy and timekeeping.
Even tiny changes have to be understood when measurements are made at extremely high precision.
The behavior of Earth’s rotation also provides clues about processes that cannot be observed directly.
In this sense, the length of the day becomes a scientific signal.
Rather than simply asking how long it takes Earth to rotate once, researchers can examine why that rotation changes and what those changes reveal about the planet’s interior.
The new study strengthens the connection between observations made from the surface and physical processes occurring deep inside Earth.
Could Earth’s Core Change Our Clocks?
For most people, the answer is no.
The changes identified by the research are far too small to produce an obvious difference in everyday timekeeping.
Earth’s rotation is constantly experiencing tiny variations, and scientists have sophisticated systems for monitoring them.
The important issue is not whether people will suddenly experience a different length of day. Instead, researchers want to understand the underlying mechanisms well enough to model Earth’s rotation accurately.
That is particularly important for scientific time standards and highly precise measurements.
Previous research has already shown that changes in Earth’s inner core can influence rotational behavior. A 2024 study, for example, provided evidence that the inner core had slowed relative to Earth’s surface beginning around 2008.
A separate 2025 study also reported evidence that the near-surface region of the inner core can undergo structural changes.
Together with the new Canadian research, these findings point toward an increasingly detailed picture of Earth’s deep interior.
A New View of a Hidden Part of Our Planet
Perhaps the most intriguing aspect of the study is what it says about how much remains unknown beneath our feet.
Earth’s core is inaccessible to direct observation. Yet researchers can detect its influence through seismic waves, magnetic measurements and changes in planetary rotation.
The latest study adds another layer to that understanding.
By connecting gravitational torque, core-mantle coupling and changes in the length of the day, researchers have developed a model that helps explain a phenomenon scientists have been investigating for decades.
The findings do not mean Earth’s rotation is suddenly becoming unstable. Nor do they suggest that human schedules or clocks are about to change.
Instead, they reveal something much more subtle — that the planet behaves as an interconnected system in which processes occurring deep below the surface can leave measurable fingerprints on the world above.
The new research therefore turns a tiny variation in the length of a day into a powerful scientific clue.
As scientists continue monitoring Earth’s rotation and studying seismic and magnetic data, future observations could help determine how these deep-Earth cycles evolve and how different forces interact over decades.
For now, the central lesson is straightforward: Earth’s core is dynamic, and even changes occurring thousands of kilometers below the surface can influence the planet we experience every day.
Key Takeaway
The new Canadian research does not suggest that Earth’s day is suddenly becoming dramatically longer or shorter. Instead, it shows that Earth’s core can participate in subtle, multidecadal changes in planetary rotation.
The study identifies gravitational torque between the inner core and mantle as an important mechanism, while electromagnetic and other core-mantle interactions can counteract the effect. The resulting changes in the length of a day are measured in milliseconds, making them invisible in everyday life but highly valuable to scientists studying Earth’s interior.
The research was led by University of Alberta scientists Huifeng Zhang and Mathieu Dumberry and published in Nature on September 23, 2026.
