How Earth's 1,400-Mile-Thick Outer Core Is Changing Day Length

Earth's rotation is not perfectly steady, and a study published this week in the journal Nature offers a new explanation for why: a gravitational tug-of-war between the planet's inner core and the surrounding mantle, according to reporting by WIRED.

The variations are tiny, often amounting to just a few milliseconds added to or subtracted from a day, WIRED reported. But geophysicists track them closely because they signal broader shifts in how fast the planet spins.

Why do Earth's days vary in length at all?

Scientists have already identified several drivers of short-term change, including atmospheric and ocean fluctuations, mass redistribution from major earthquakes, and water entering the oceans as continental ice melts, according to WIRED. Those factors explain sudden or recent shifts. But over timescales spanning several decades, part of the variation appears to originate far deeper — in the 1,400-mile-thick layer of molten metal known as the outer core, WIRED reported.

How does the inner core's gravity factor in?

The outer core is in constant motion, and as its flows change, they can alter the rotation speed of the rest of the planet. The problem, researchers have long noted, is that friction between the core and mantle is far too weak to transmit that influence upward, per WIRED's account of the study.

The new paper proposes a different mechanism: the inner core spins at a slightly different rate than the rest of Earth, which puts it out of alignment with mass irregularities in the mantle. Gravity then pulls the layers back toward alignment, and that pull nudges the rotation of Earth's surface, WIRED reported.

"The core 'wants to be aligned,'" Mathieu Dumberry, a geophysicist at the University of Alberta involved in the study, said in a press release, according to WIRED.

What data did the researchers use?

The team reconstructed how this gravitational interaction shaped Earth's spin between 1964 and 2019, drawing on seismic estimates of the inner core's rotation combined with models of liquid-metal flow in the outer core, WIRED reported. The resulting model closely reproduced both the timing and magnitude of gradual rotation changes actually observed over those 55 years.

Is gravity the only force at work?

No. WIRED noted the model did not account for every force competing inside the planet, and other unidentified factors are likely involved. Even so, the fit between the model and observed data was strong enough that researchers describe the gravitational push-and-pull as a major piece of the puzzle. As Dumberry put it, the competition is a tug-of-war, but "gravity still wins," per WIRED.

By the numbers

• 1,400 miles: thickness of Earth's liquid outer core
• 1964–2019: the 55-year period the study's model reconstructed

What to watch next

• Whether follow-up research identifies the additional, still-unknown forces the model left out
• Whether independent teams can replicate the gravitational-alignment mechanism using other seismic datasets
• How refined length-of-day models affect long-term timekeeping standards

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