28 SEP 2026 — Earth's day is not exactly 24 hours. Over decades it drifts by a few milliseconds, longer for a while and then shorter. A study published in Nature on 23 September says the main cause is gravity pulling the planet's inner core back into line with the rock around it.
The finding also offers a way to test ideas about the deep interior.
A clock that wanders
Astronomers measure Earth's rotation against distant cosmic objects. Against that fixed background, the length of a day varies on several timescales. The decadal variations are only a few milliseconds, Nature's news report says, but they have long been linked to the core.
Mathieu Dumberry, a geophysicist at the University of Alberta, told Nature that the core is known to be responsible for these decadal changes. "But how does it do it?" he said. "This is the part that's unclear."
The core is a Mars-sized ball of mostly iron. Its inner part is solid and can turn independently inside a molten outer core, which is itself wrapped in the rocky mantle.
Three candidate forces
Three candidate forces could let the core speed up or slow down the mantle, and with it the planet's surface. Molten iron flowing past bumps on the core's boundary could push on the mantle, like a river on rocks. The magnetic field made by that flow could tug on iron-rich parts of the mantle. Or gravity could pull dense regions of the inner core towards dense regions of the mantle.
Dumberry and his colleague Huifeng Zhang built a model that tested combinations of these forces against the day-length record. Their paper is titled "Gravitational torque drives multidecadal variations in length of day".
Gravity wins, but not alone
The model matched the day-length record best when gravitational pull was dominant, with the magnetic and flow forces pushing the other way. "Before we obtained the result, we didn't know they are competing with each other," Zhang told Nature.
A key input was earlier seismic work, published in 2023, that used earthquake waves to track the inner core's spin since the 1960s. It found the inner core turned slightly faster than the rest of the planet until around 2010, and then more slowly.
A core that wants to line up
In the model, the inner core tends to sit aligned with the mantle, because its dense patches are drawn to the mantle's dense patches. The generally westward flow of the molten outer core drags it out of line, and gravity slowly pulls it back.
Each swing moves angular momentum between the core and the mantle. Because the planet's total spin has to stay the same, a change in the core's rotation shows up as a small change in how fast the surface turns, and so in the length of the day.
In an accompanying Nature commentary, the result is framed as gravitational forces dragging some decades out and making others pass more quickly.
What it means and what it does not
The changes are far too small to notice. They matter to people who keep time and track satellites, where milliseconds add up, and to geophysicists, for whom the rotation record is one of the few measurements that reaches the deep interior.
The result is a model fit, not a direct observation. It depends on the seismic record of the inner core's spin, which is itself debated, and on assumptions about how dense material is arranged deep in the mantle. If those assumptions hold, day-length measurements become a check on them.
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