Why Earth’s Days Keep Changing by a Few Milliseconds

Earth’s days are not always the same length. Scientists have recorded tiny fluctuations for decades—sometimes only a few milliseconds long. Although these changes are almost imperceptible, they reveal something important: Earth’s rotation is constantly speeding up and slowing down.

What causes Earth’s rotation to change?

Scientists have identified several complex causes. Shifts in the atmosphere and oceans can make Earth spin slightly faster or slower. Large earthquakes can redistribute some of the planet’s mass, while melting continental ice sends vast amounts of water into the oceans.

These processes explain some of the more abrupt, recent changes. But over timescales of decades, some fluctuations appear to originate much deeper inside Earth.

A hidden influence deep inside the planet

Beneath Earth’s rocky mantle lies the outer core, a roughly 1,400-mile-thick layer of super-hot liquid metal that is constantly in motion. Changes in these flows can slightly alter the rotational speed of the rest of the planet.

Exactly how these forces are transmitted from one region to another remains a mystery. Because friction between the core and mantle is too weak to explain everything, geologists have searched for other mechanisms operating inside Earth.

In new research published this week in Nature, researchers say they may have identified one of these hidden mechanisms. They propose that gravity from the inner core pushes Earth’s rotation in one direction, while interactions between the core and mantle counteract it. Over decades, this competition produces small accelerations and decelerations in the planet’s rotation.

Gravity and the moving inner core

The idea is based on the fact that Earth’s inner core does not rotate at exactly the same speed as the rest of the planet. Irregularities in the mantle’s mass can also cause the inner core to shift slightly.

Matthew Danbury, a geophysicist at the University of Alberta who worked on the study, said the planet’s nuclei “want to align,” according to a press release. In other words, gravity tries to pull Earth’s inner layers back into a more harmonious position. That tug affects the rotation of the planet’s surface, even if the effect is extremely small.

A model that matches decades of observations

The authors combined seismic estimates of the inner core’s rotation with models of liquid-metal flow in the outer core. They used the data to reconstruct how the interaction affected Earth’s rotation between 1964 and 2019.

Their gravity model closely reproduced both the timing and magnitude of the gradual changes observed over those decades.

Earth’s rotation is a long-running tug of war

Other unknown factors may still be involved, and the model did not include every competing force. Even so, the researchers say it produced convincing results and helps clarify the key processes behind day-to-day fluctuations in Earth’s rotation.

It may be a tug of war between forces inside Earth—but as Danbury put it, “gravity still wins.”

This story was originally published in the Spanish edition of WIRED and translated from Spanish.