A day is supposed to last 24 hours, but Earth does not keep time quite that neatly.
A new study found that forces deep inside the planet are subtly speeding up and slowing down Earth's rotation, changing the length of a day by milliseconds over the course of decades, Gizmodo reported.
Here's what to know
In a Nature paper published Wednesday, researchers argued that a long-running question about multidecadal changes in Earth's rotation may be explained by processes inside the planet.
The explanation involves three competing mechanisms — gravitational, electromagnetic, and topographic coupling — that help account for those changes.
According to Huifeng Zhang, a University of Alberta doctoral candidate and the study's first author, gravitational coupling between the inner core and the mantle appears to produce most of the shift, while the other two mechanisms push in the opposite direction, the outlet noted.
Zhang and co-author Mathieu Dumberry reached that conclusion by comparing statistical models with observational records spanning roughly six decades.
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The effect is tiny but detectable.
Observational data cited in the study shows core-driven processes changed day length by a few milliseconds between the early 1970s and 2021, per Gizmodo.
More background
Scientists already knew that Earth's rotation can vary and that the reason depends on the timescale being studied.
Over short periods, the atmosphere and oceans play a major role. Over millions of years, tidal interactions with the moon can drive long-term changes.
The middle-range shifts that unfold over several decades have been harder to explain.
For Zhang, that is one of the most compelling parts of the work.
"What I find particularly exciting is that these different pieces of information can come together to provide a more coherent picture of Earth's deep interior, a region that is extremely difficult to observe directly," she told Gizmodo.
A difference of a few milliseconds will not affect daily life, but it can give scientists insight into how the planet's inner layers interact.
What's being done?
Because each of the three proposed mechanisms depends on physical properties that are not fully understood, the new findings give researchers constraints to test as they build models of Earth's interior.
Building on earlier work from 1988 that linked angular momentum exchanges between Earth's core and mantle to changes in the planet's rotation, Zhang and Dumberry used the new study to narrow down which types of core-mantle interactions are most likely to matter over decades.
Zhang is also investigating whether the same deep-Earth processes could explain another long-standing mystery: a six-year oscillation in the length of the day.
"Even tiny changes in Earth's rotation can provide valuable information about processes occurring thousands of kilometers beneath our feet," Zhang told Gizmodo. "By combining these improved observations with measurements of Earth's rotation and inner core motion, we hope to develop a more detailed picture of how Earth's deep layers interact and how motions thousands of kilometers beneath the surface subtly change the length of our day."
Where can I learn more?
These stories examine other forces that can alter Earth's rotation, tilt, and day length. They show how melting ice, rising seas, and groundwater pumping are changing the planet's motion.
• In Antarctica, melting ice and shifting water are altering the dynamics of Earth's core.
• Climate-driven sea level rise is changing the length of our days by redistributing mass.
• Antarctica's ice sheets hold massive consequences for Earth's rotation as global sea levels climb.
• Heavy groundwater pumping has changed the Earth's tilt by shifting water around the planet.
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