The difference is tiny, measured in milliseconds, but physicists can detect it. Over time, some days become slightly shorter or longer depending on subtle changes deep inside the planet.

Earth's Core and Mantle Trade Momentum

For roughly three decades, scientists have known that Earth's liquid core does not always rotate at the same rate. Changes in the planet's magnetic field have revealed that the liquid core can gradually speed up over several decades, then slow down during the following decades.

At the same time, the mantle, the roughly 3,000 kilometer thick rocky layer that includes the crust, responds in the opposite direction. When the core speeds up, the mantle slows slightly. When the core slows, the mantle speeds up.

This exchange happens because Earth's total angular momentum must remain constant.

Even very small changes in the mantle's rotation are enough to alter the length of a day by a few milliseconds.

A Hidden Gravitational Tug Inside Earth

Scientists have long struggled to explain exactly how the core and mantle exchange this rotational momentum.

In a new study published Sept. 23 in Nature, University of Alberta physics PhD student Huifeng Zhang and professor Mathieu Dumberry present a possible answer.

Their work shows that small changes in the rotational speed of Earth's inner core, the solid region at the planet's center, can create what scientists call a "gravitational torque." Because the inner core is not perfectly spherical, changes in its motion can gravitationally interact with uneven distributions of mass in the mantle.

That interaction can slightly alter how quickly the mantle rotates, producing small changes in the length of a day.

Competing Forces Control Earth's Rotation

Another force also acts at the boundary between the core and mantle.

Known as core mantle boundary torque, it produces friction and electromagnetic drag that oppose the gravitational pull. This resistance helps limit how much Earth's day length can change.

According to the researchers, the small observed variations in the length of a day arise from subtle shifts in the balance between gravitational torque and core mantle boundary torque.

Earth's Inner Core May Be More Dynamic Than Expected

Beyond helping explain a long standing mystery about Earth's changing day length, the findings also offer new insight into what is happening deep inside the planet.

Zhang and Dumberry say the inner core appears to "deform viscously" on a time scale of about 10 years, suggesting that Earth's deepest interior may change shape and respond to forces far more dynamically than its solid composition might imply.