These differences are too small for people to notice, but physicists can detect them. Over time, subtle changes deep inside Earth can make some days slightly shorter or longer.

How Earth’s core and mantle affect the length of a day

For about 30 years, scientists have known that Earth’s liquid outer core does not always rotate at the same speed. Changes in the planet’s magnetic field can cause the liquid core to gradually accelerate over several decades before slowing down over the following decades.

At the same time, the mantle—the roughly 3,000-kilometre-thick layer of rock that contains Earth’s crust—responds in the opposite direction. When the core speeds up, the mantle slows slightly. When the core slows down, the mantle speeds up.

This exchange occurs because Earth’s total angular momentum must remain constant. Even tiny changes in the mantle’s rotation are enough to alter the length of a day by a few milliseconds.

A hidden gravitational force inside Earth

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

In a new study published on September 23 in Nature, University of Alberta physics doctoral student Huifeng Zhang and professor Matthew Danbury propose a possible explanation.

Their research suggests that small changes in the rotational speed of Earth’s inner core—the solid region at the planet’s centre—can create what scientists call a gravitational torque.

Because the inner core is not perfectly spherical, changes in its motion can interact gravitationally with the uneven distribution of mass in the mantle. This interaction can slightly change the mantle’s rotation rate, leading to small variations in the length of a day.

Competing forces help regulate Earth’s rotation

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

Known as core–mantle boundary torque, it includes friction and electromagnetic resistance that oppose the effects of gravity. This resistance helps limit how much the length of Earth’s days can vary.

According to the researchers, the small changes in day length that scientists observe result from subtle shifts in the balance between gravitational torque and core–mantle boundary torque.

Earth’s inner core may be more dynamic than expected

The discovery could help explain the long-standing mystery of why Earth’s day length changes. It also offers new insight into the processes taking place deep inside the planet.

The researchers said the inner core appears to be viscously deforming over a timescale of about 10 years. This suggests that Earth’s deepest interior can change shape and respond to forces more dynamically than its solid composition might indicate.