Earth’s Internal Gravitational Tug-Of-War Is Changing The Length Of Our Days

A Gravitational Battle Deep Inside Earth Is Altering How Fast The Planet Spins

Your Day Is Not Always Exactly The Same Length — And Scientists Think They Know Why

A hidden gravitational struggle deep inside Earth is helping to change the length of our days, according to new research that explains mysterious variations in the planet's rotation over periods of decades.

The effect is extraordinarily small — typically only a few milliseconds — but scientists say interactions involving Earth's solid inner core, liquid outer core and rocky mantle can explain why the planet sometimes rotates slightly faster or slower.

The findings offer something far more valuable than a reason to check the clock.

They provide a new way to investigate the inaccessible interior of the planet.

Earth Does Not Rotate At One Perfectly Constant Speed

A day is treated as 24 hours for everyday life.

But Earth's real rotation is more complicated.

The planet's spin varies slightly as mass and momentum move through different parts of the Earth system.

Atmospheric winds, ocean currents, melting ice and interactions with the Moon can all affect rotation on different timescales.

Scientists have also known for years that changes occurring inside the planet contribute to longer-term variations.

Over decades, the difference can amount to several milliseconds in the measured length of a day.

That may sound insignificant.

But modern astronomical measurements are precise enough to detect it.

The Mystery Was Deep Inside Earth

Scientists suspected Earth's core was responsible for much of the multidecadal pattern.

The challenge was explaining exactly how.

Earth is structured in layers.

The rocky mantle lies beneath the crust.

Below that sits the liquid outer core, composed largely of iron and nickel.

At the centre is a solid inner core.

The liquid outer core flows and helps generate Earth's magnetic field.

The solid inner core can rotate at a slightly different rate from the rest of the planet.

That means these regions can exchange forces and angular momentum.

Three Forces Were Competing

Researchers Huifeng Zhang and Mathieu Dumberry of the University of Alberta developed a model examining three major forms of torque that could couple Earth's interior layers.

The first is mechanical.

Flowing material in the liquid outer core can interact with topographic irregularities where the core meets the mantle.

The second is electromagnetic.

Earth's magnetic field can interact with electrically conductive material and transfer rotational momentum.

The third is gravitational.

Dense regions inside the inner core and mantle can gravitationally attract one another.

The researchers found that the observed changes in Earth's day length were best reproduced when gravitational torque dominated — while electromagnetic and mechanical torques pushed in the opposite direction.

That creates the "tug-of-war".

Gravity Appears To Win

The gravitational interaction effectively encourages dense regions of the inner core to align with dense regions in the mantle.

But flows in the outer core can pull the inner core away from that alignment.

Gravity then acts to draw it back.

The competing forces influence how angular momentum moves between Earth's interior layers.

That changes the rotation of the solid Earth by a tiny amount.

Nature's accompanying summary of the research describes gravitational torque as driving multidecadal changes in day length while electromagnetic and topographic forces resist it.

Why Milliseconds Matter

No one is going to feel a day becoming a few milliseconds longer.

The importance is scientific.

Directly observing Earth's core is impossible.

The centre of the planet lies roughly 6,400 kilometres beneath the surface.

Researchers instead study it indirectly through seismic waves, the magnetic field and subtle changes in planetary rotation.

If a model of Earth's interior accurately reproduces changes measured at the surface, that provides evidence that the underlying physics is realistic.

In other words, variations in the length of a day become a probe of Earth's deep interior.

The Inner Core Has Its Own Strange Rotation

Previous seismic research has suggested that Earth's inner core does not rotate at exactly the same rate as the mantle and crust.

Nature reported that work analysing earthquake waves indicated the inner core rotated slightly faster than the rest of the planet for a period before beginning to move more slowly relative to the surface around 2010.

The new model incorporates that changing motion.

The researchers found that the same competition between forces that influences Earth's overall rotation can also help explain the behaviour of the inner core itself.

That links two previously difficult problems.

Earth Is A Dynamic Machine

The research is another reminder that the apparently solid ground beneath us is part of a constantly moving planetary system.

Rock circulates through the mantle over geological timescales.

Liquid metal flows through the outer core.

The inner core rotates.

Magnetic fields evolve.

Mass shifts across the planet.

All of those processes interact.

At the surface, their effects can be extraordinarily subtle.

Yet precise measurements of Earth's rotation allow scientists to detect traces of activity occurring thousands of kilometres below.

Does This Affect Clocks?

Not in any meaningful everyday sense.

The changes discussed in the study are measured in milliseconds over long periods.

They are scientifically important but imperceptible to humans.

Atomic timekeeping is enormously more stable than Earth's rotation, which is one reason modern time standards rely on atomic physics rather than treating the planet itself as a perfect clock.

Ironically, while scientists are learning that deep forces inside Earth can alter the planet's natural day, physicists are simultaneously building atomic clocks accurate enough to measure time with almost unimaginable precision.

A New Window Into The Planet

The most exciting part of the research is therefore not that days change length.

Scientists already knew they did.

The breakthrough is identifying a physical mechanism capable of explaining a significant part of the multidecadal pattern.

Gravity pulling between structures in the inner core and mantle appears to dominate.

Mechanical and electromagnetic forces resist it.

Together they create a vast, slow-motion contest inside the planet.

We cannot see that struggle directly.

But every few milliseconds added to or removed from Earth's rotation leave scientists another clue that it is happening.