Gravitational Forces Inside Earth Are Changing Our Days

Earth may look like a stable planet from the surface, but deep beneath our feet it is constantly moving. The planet's metallic core, rocky mantle and outer layers interact in ways that can influence something surprisingly familiar: the length of a day. New research published in Nature on September 23, 2026, provides evidence that gravitational forces acting deep inside Earth play a major role in multidecadal changes in the planet's rotation.

The discovery adds an important piece to the puzzle of Earth's rotation. Scientists have known for decades that a day is not exactly the same length every day. Changes can occur because of the atmosphere, oceans, tides, melting ice and processes inside Earth's core. The new study focuses on changes that occur over periods of roughly 10 to 70 years and last only a few milliseconds, but these tiny variations contain valuable information about Earth's deep interior.

According to research by Huifeng Zhang and Mathieu Dumberry, the gravitational interaction between Earth's solid inner core and the mantle can explain much of the observed multidecadal variation in the length of day. The researchers also found evidence that electromagnetic and topographic forces act against the gravitational torque, creating a complex geophysical tug-of-war.

Why Does the Length of a Day Change?

The familiar 24-hour day is based on Earth's rotation relative to the Sun, but Earth's rotation rate is not perfectly constant. The planet behaves more like a complicated system in which different components exchange angular momentum.

READ ALSO: AI Weather 2026–2027: Super El Niño Risks

When Earth's rotation becomes slightly faster, the length of the day decreases. When rotation slows, the day becomes slightly longer. These changes are extremely small. The new research examines variations measured in milliseconds rather than minutes or hours.

That may sound insignificant, but scientists can measure these changes with extremely precise astronomical and geophysical observations. Small changes in Earth's rotation provide a window into processes that cannot be observed directly hundreds or thousands of kilometres beneath the surface.

The latest study is particularly important because it addresses a question that has remained unresolved: how does Earth's core exchange angular momentum with the mantle?

Earth's Inner Core Is Part of the Story

Earth has several major layers. The crust forms the thin outer surface where humans live. Beneath it lies the mantle, a vast layer of hot rock that behaves over geological timescales like a very slow-moving material. Deeper still is the metallic core.

The outer core is liquid and consists largely of iron and other elements. Its movement helps generate Earth's magnetic field. At the centre is the solid inner core, another region dominated by iron and nickel.

The inner core is not simply locked permanently in place. It can rotate at a slightly different rate relative to the mantle and the rest of Earth. Scientists can investigate this motion indirectly using seismic waves generated by earthquakes.

The new research combines information about inner-core rotation with models of fluid motion in the outer core and changes in Earth's magnetic field. This approach allowed researchers to investigate how different forces could produce the observed variations in the length of Earth's day.

A Gravitational Tug-of-War Deep Beneath the Surface

Gravity is the key to the new explanation. Dense regions within Earth's inner core can interact gravitationally with dense structures near the base of the mantle. This creates a twisting force known as a gravitational torque.

A torque can change rotational motion. In simple terms, the gravitational interaction tends to pull certain structures toward preferred alignments. But Earth is not a static ball. The liquid outer core is continuously moving, and its flow can push the inner core away from that gravitational alignment.

This creates a geophysical tug-of-war.

The researchers found that their model matched observations particularly well when gravitational torque was treated as the dominant influence on multidecadal changes in Earth's rotation. At the same time, electromagnetic and topographic torques resist the gravitational effect.

The result is not a single force simply speeding up or slowing down Earth. Instead, several processes interact, with their relative strength changing over time.

What Is Electromagnetic Torque?

Earth's outer core is electrically conducting and constantly moving. These movements are closely connected to the planet's magnetic field. Because magnetic fields and electrically conducting materials interact, electromagnetic forces can transfer angular momentum between different parts of Earth's interior.

The Nature study suggests that electromagnetic torque works against the gravitational torque in the long-term rotational changes being investigated.

This is significant because Earth's magnetic field is not only important for protecting the planet from charged particles from the Sun. Its variations also provide scientists with information about movements deep inside Earth.

By combining magnetic observations with seismic measurements, researchers can reconstruct aspects of processes occurring in regions that cannot be reached by conventional drilling or direct observation.

What Does the Inner Core Have to Do With Earth's Day?

The inner core contains a huge amount of mass. Even though the rotational changes involved in this study are tiny, the enormous scale of Earth's interior means that small exchanges of angular momentum can influence the rotation of the entire planet.

Researchers used a history of inner-core rotation derived from seismic observations dating back to the 1960s. Earlier research found that the inner core appeared to rotate slightly faster relative to the rest of Earth for part of that period before its relative motion changed around 2010.

The new model uses those observations to examine how the core's changing motion could be connected with changes in the length of Earth's day.

This is an example of how seemingly unrelated measurements can be combined. Earthquakes reveal information about the inner core, magnetic-field observations reveal information about the outer core, and astronomical measurements reveal changes in Earth's rotation.

Why Scientists Care About Milliseconds

For everyday life, a change of a few milliseconds is impossible to notice. Human sleep, work and daily schedules are not affected by these tiny variations.

For geophysics, however, milliseconds matter.

Precise measurements of Earth's rotation help scientists understand the distribution and movement of mass throughout the planet. They can also improve models of Earth's core, mantle and magnetic field.

Earth rotation measurements have been used for decades to study the planet's orientation in space. NASA has also explained how variations in Earth's rotation are important for precise timekeeping and astronomical measurements.

The broader lesson is that Earth's rotation acts as a kind of global sensor. When the rotation changes slightly, scientists can investigate what caused the change and learn more about processes occurring throughout the planet.

This Is Not About 25-Hour Days Arriving Soon

Headlines about Earth's changing rotation sometimes create the impression that humans will soon experience noticeably longer or shorter days. The new research does not support such an interpretation.

The changes examined in the Nature study are measured in milliseconds and occur over decades. They are not changes that would turn today's 24-hour day into a 25-hour day in the near future.

Earth's rotation is influenced by many different processes operating on different timescales. Lunar tides, atmospheric circulation, ocean movements, changes in ice distribution and processes in the core can all contribute to variations in rotation.

The gravitational interaction described by the new study is therefore one component of a much larger system.

The Moon Is Still Important

The discovery of gravitational coupling inside Earth does not mean that the Moon has stopped influencing Earth's rotation. Lunar tides remain a major long-term factor in the gradual slowing of Earth's rotation.

As the Moon raises tides in Earth's oceans, friction associated with tidal motion transfers rotational energy and angular momentum. Over very long geological periods, this contributes to longer days.

The new research addresses a different phenomenon: relatively short-term, multidecadal fluctuations in the length of day.

This distinction is important. The Moon's tidal influence operates over very long periods, while the newly investigated core-mantle interaction helps explain variations occurring over decades.

Climate Change Can Also Affect Earth's Rotation

Earth's rotation can also be influenced by processes at the surface. Scientists have found that melting polar ice redistributes mass toward lower latitudes. Because the distribution of mass affects rotational dynamics, large-scale changes in Earth's ice can influence the length of the day.

Research reported by Nature has shown that climate-driven ice loss can slow Earth's rotation and influence the timing of adjustments to global timekeeping.

This demonstrates how interconnected Earth's systems are. The atmosphere, oceans, cryosphere, mantle and core can all contribute to changes in planetary rotation, although they operate on different timescales and with different magnitudes.

How Scientists Measure Earth's Rotation

Measuring Earth's rotation with sufficient precision is a major scientific achievement. Astronomers can compare Earth's orientation with extremely distant astronomical objects. These observations allow researchers to determine tiny changes in the planet's rotation rate and orientation.

Modern geodesy combines several techniques, including very long baseline interferometry, satellite observations, seismic measurements and observations of Earth's magnetic field.

The result is an increasingly detailed picture of how Earth behaves as a dynamic planet rather than a perfectly rigid sphere.

What the New Research Reveals About Earth's Deep Interior

One of the most interesting aspects of the new study is that it does more than explain changes in day length. The researchers say their model provides clues about the physical properties of Earth's deepest regions.

The results are consistent with a picture of the lowermost mantle containing dense thermochemical structures and a relatively low-viscosity layer beneath it. The research also suggests that the inner core may deform over relatively short timescales compared with geological processes.

These conclusions remain dependent on the models and measurements used. The researchers note that uncertainties in reconstructed inner-core rotation and outer-core flow affect estimates of the torque parameters.

That uncertainty is normal in deep-Earth science. Scientists cannot directly observe the core, so they must combine indirect evidence and test whether models reproduce measurements collected independently.

A New View of Earth's Hidden Engine

The latest findings reinforce the idea that Earth's deep interior is an active mechanical system. The inner core, liquid outer core and rocky mantle are not isolated layers. They interact through gravity, electromagnetism and mechanical forces.

These interactions can influence the rotation of the entire planet.

For the average person, a millisecond variation in the length of a day will never be noticeable. But for scientists, it is a valuable signal. By studying these tiny changes, researchers can investigate the behaviour of material thousands of kilometres below Earth's surface.

The study also demonstrates why long-term scientific measurements are so important. Observations collected over many decades can reveal patterns that would be invisible in a short experiment.

What Happens Next?

Future research will likely focus on improving models of the inner core, outer-core flow and the boundary between the core and mantle. Better seismic observations and increasingly precise measurements of Earth's magnetic field could help scientists determine how these forces vary over time.

Researchers may also be able to refine estimates of the physical properties of the deepest mantle and the inner core.

The bigger scientific goal is not simply to predict whether a particular day will be a few milliseconds longer. It is to understand the physical engine operating inside Earth.

Every earthquake, magnetic-field measurement and astronomical observation can provide another piece of the puzzle.

Key Facts About Earth's Changing Day Length

• Earth's rotation is not perfectly constant. The length of a day changes slightly over different timescales.
• The new Nature study was published on September 23, 2026.
• Gravitational torque between the inner core and mantle is identified as the dominant mechanism in the studied multidecadal variations.
• Electromagnetic and topographic torques act in opposition to the gravitational effect.
• The studied variations are generally on the scale of milliseconds over decades, not hours.
• Seismic observations and Earth's magnetic field are important tools for investigating the planet's deep interior.
• The Moon continues to influence Earth's rotation through long-term tidal effects.
• Atmospheric, oceanic and climate-related processes can also affect Earth's rotation.

Conclusion

Earth's day may appear fixed at 24 hours, but the planet is constantly adjusting its rotation. New research provides evidence that a gravitational tug-of-war deep inside Earth helps explain some of the mysterious changes that occur over decades.

The interaction between the solid inner core, liquid outer core and rocky mantle creates a complex system in which gravity, electromagnetic forces and physical structures compete to influence Earth's rotation. The resulting changes are tiny, but they offer scientists an extraordinary way to study the inaccessible interior of our planet.

The most important message is not that our days are suddenly becoming much longer. Instead, the research shows how much information can be hidden inside a difference of only a few milliseconds. Earth's rotation is a sensitive record of activity taking place from the atmosphere to the deepest parts of the planet.

As scientists continue combining seismic data, magnetic observations and high-precision measurements of Earth's rotation, the hidden dynamics of our planet's core may become clearer. The length of a day is therefore more than a measure of time — it is also a scientific clue to what is happening thousands of kilometres beneath Earth's surface.

Sources and Related Articles

Primary research: Zhang, H. & Dumberry, M., Gravitational torque drives multidecadal variations in length of day, Nature, September 23, 2026.

Nature: Gravitational tug-of-war inside Earth is changing the length of our days

Nature – Earth and planetary science research

NASA – Earth Science and Planetary Research

International Earth Rotation and Reference Systems Service

USGS – Earth Science Research