Earth's day is commonly considered to last 24 hours, but the planet's rotation is not perfectly constant. Tiny changes deep inside Earth can make days slightly shorter or longer by a few milliseconds. A new study published in Nature offers a possible explanation for these variations, revealing how gravitational interactions between Earth's inner core and mantle may influence the planet's rotation.

Researchers at the University of Alberta investigated how changes in the inner core's rotation affect the rest of Earth. Scientists have long known that the liquid outer core can speed up or slow down over periods lasting decades, while the rocky mantle responds in the opposite direction. These exchanges occur because Earth's total angular momentum must remain constant.

The new research suggests that changes in the rotation of the solid inner core can generate a gravitational torque. Because the inner core is not perfectly spherical and the mantle contains uneven distributions of mass, their gravitational interactions can transfer rotational momentum between Earth's internal layers, slightly changing the mantle's speed.

Another force acts at the boundary between the liquid outer core and the mantle. Frictional effects and electromagnetic drag produce a resisting torque that counteracts the gravitational interaction. According to the researchers, shifts in the balance between these competing forces may explain the small variations in day length.

The findings also suggest that Earth's inner core may gradually deform under sustained forces over a timescale of approximately 10 years. This indicates that the planet's deepest interior may be more dynamic than previously assumed, despite its solid composition.

Although these changes are imperceptible in daily life, understanding them could help scientists improve models of Earth's interior and better explain how its core, mantle, and magnetic field interact.