Earth’s core is quietly changing the length of your day, study finds

A day lasts 24 hours, right? Almost. Earth’s spin is not perfectly steady, and a new study helps explain why.

Scientists have long known that the length of a day drifts by a few milliseconds over decades. Geophysicists track this by measuring Earth’s position against distant objects in space. The changes are far too small for you to notice, but they are real, and researchers have suspected for years that the cause lies deep underground, in the way Earth’s metal core and its rocky mantle interact. The puzzle was which force was doing the work.

What the study found

In a paper published in Nature on 23 September, Huifeng Zhang and Mathieu Dumberry of the University of Alberta in Canada argue that gravity is the main driver. Dense regions of the solid inner core and dense regions of the mantle pull on each other, and that pull, known as a torque, shifts spin back and forth between the core and the mantle.

It is not a one-way push. Electromagnetic forces, and so-called topographic forces where the core meets the mantle, pull in the opposite direction. Nature’s news team describes it as a geophysical tug-of-war.

To reach this conclusion, the researchers combined seismic observations of how the inner core rotates, including earlier work using earthquake waves, with estimates of how fluid moves in the core, worked out from Earth’s magnetic field.

Explain it like I’m in SS2

Picture a spinning wheel with a heavy, slightly lopsided ball rolling around inside it. As the ball shifts, it tugs on the wheel, speeding it up or slowing it down a touch. Earth’s inner core plays the ball; the mantle and crust, where we live, play the wheel. Other forces act like friction, pushing back.

Why care? Precise timekeeping, from satellites to the clocks that keep networks in sync, has to allow for Earth’s uneven spin. The findings also give scientists new clues about the hidden structure deep inside our planet, a place no one can visit.

As Dumberry put it, scientists already knew the core was responsible for these decade-long changes. The open question was simple: “But how does it do it?”