Something More Than 3,000 Miles Down Keeps Adjusting the Length of Your Day

Today was not exactly 24 hours long. Neither was yesterday. For as long as instruments have been precise enough to check, Earth's spin has run a few milliseconds fast or slow, drifting one way for decades before reversing. The cause appears to be a gravitational argument more than 3,000 miles (about 5,000 kilometers) beneath the surface.

In a study published in Nature on September 23, Huifeng Zhang and Mathieu Dumberry of the University of Alberta traced the pattern to a tug between Earth's solid inner core and dense regions of the rocky mantle above it. The inner core isn't a perfect sphere. It's slightly elongated, and gravity holds that long axis pointed toward the mantle's heaviest patches, the way a compass needle settles toward north. Currents in the liquid outer core can nudge it out of line. Gravity drags it back, and that corrective pull is passed to the mantle. The planet's spin shifts, and the day changes length by a few milliseconds.

Two other forces push back. One is electromagnetic, generated where the outer core meets the mantle. The other is liquid metal pressing against bumps along that boundary. In the model, both resist the gravitational pull, and both lose. The researchers hadn't expected the forces to be working against each other at all.

Measuring these milliseconds is one of the few ways to learn anything about those depths. Zhang is now testing whether the same mechanism explains a separate, roughly six-year wobble in day length that remains unexplained.

The thing doing the pulling is a ball of solid iron roughly the size of Pluto. Nobody has ever seen it, and it is nudging your clock.

Read the full story at Wired, September 26, 2026

Hot Take: Sixty years of data is a single breath for a planet on a 60- to 70-year cycle. We have watched this oscillation roughly once, which should make everyone a little more careful with the word "drives."