Earth’s deep interior subtly changes the length of each day, offering new clues for future geothermal energy
A day is not exactly 24 hours. Earth’s rotation drifts slightly — speeding up, slowing down — by a few milliseconds over decades. The difference is invisible to daily life, but precision physicists track it closely, watching the planet’s spin wobble in ways that have never been fully explained.
Now a new study suggests the answer may lie thousands of kilometers beneath our feet — in a place so remote and extreme that scientists are only beginning to understand what moves there, and why it might be quietly nudging time itself.
A day that keeps shifting
Earth’s rotation has never been perfectly constant. Days can run a few milliseconds shorter or longer depending on subtle processes happening far below the surface. That sounds trivial, but milliseconds accumulate — and over decades, these tiny variations add up to shifts that matter for satellite navigation, for fundamental physics, for how we keep time at all.
Scientists have tracked this phenomenon for roughly 30 years using precision instruments sensitive enough to detect the planet’s faintest wobbles.
Scientists have tracked this phenomenon for roughly 30 years using precision instruments sensitive enough to detect the planet’s faintest wobbles. They knew the variations were real. What they couldn’t fully explain was why — what mechanism was quietly stealing or returning fractions of a second from each rotation.
One piece of the puzzle was already in place. Earth’s liquid outer core, which surrounds the solid inner core, was known to influence rotation through the planet’s magnetic field. Changes in that field revealed the liquid core doesn’t spin at a fixed rate — it speeds up and slows down over multi-decade cycles. That, however, only explained part of the story.
How the core and mantle swap rotational energy
The liquid outer core and the mantle don’t operate independently. They’re locked in a slow, continuous exchange of rotational energy — a kind of planetary give-and-take governed by physics.
When the liquid outer core speeds up, the mantle — the roughly 3,000-kilometer-thick rocky layer beneath the crust — slows down slightly in response. The reverse is equally true: when the core slows, the mantle compensates by speeding up. This happens because Earth’s total angular momentum must remain constant. The planet can’t simply gain or lose spin energy; it redistributes it internally.
Even very small changes in the mantle’s rotation rate are enough to shift day length by a few milliseconds. Identifying the precise mechanism transferring that momentum between layers has always been the hard part.
A gravitational tug from the planet’s innermost core
A study published September 23, 2026, in the journal Nature offers a compelling answer. University of Alberta physics PhD student Huifeng Zhang and professor Mathieu Dumberry propose that Earth’s solid inner core — the dense sphere at the planet’s very center — is the key driver.
The inner core isn’t a perfect sphere. Because of that irregularity, changes in its rotation rate produce what researchers call a “gravitational torque.” This torque interacts with uneven mass distributions in the mantle, subtly altering how fast the mantle spins — and, by extension, how long each day lasts. The inner core, in other words, is quietly tugging on the layers above it — and we feel the effect in every single rotation.
Competing forces keep the drift in check
The gravitational torque doesn’t act alone. A second force operates at the boundary between the outer core and the mantle: the core-mantle boundary torque, which generates friction and electromagnetic drag, pushing back against the gravitational pull.
The millisecond-scale variations we observe in day length arise from the shifting balance between these two opposing forces. Neither dominates permanently — they oscillate over decades, which is why the changes stay small and never spiral out of control. Understanding this balance isn’t just an academic exercise. It’s essential for building accurate models of how Earth’s rotation will behave long-term, models that underpin GPS systems, astronomical timekeeping, and our broader understanding of planetary dynamics.
A surprisingly dynamic interior
Perhaps the most striking implication of the study is what it suggests about the inner core itself. Zhang and Dumberry’s findings indicate that the inner core appears to “deform viscously” on a timescale of roughly 10 years — meaning Earth’s deepest solid layer may be far more fluid in its behavior than its composition would suggest.
This challenges long-held assumptions about a rigid, static inner core. A more dynamic interior could reshape how scientists model Earth’s magnetic field, which originates in the outer core, and how they think about the planet’s long-term evolution.
It also raises a quieter, more unsettling question: if the inner core is subtly shifting and deforming on decade-long timescales, what other slow, invisible processes might be unfolding at those extreme depths — processes we haven’t yet found the tools to detect?
All the information is available here: Huifeng Zhang, Mathieu Dumberry. Gravitational torque drives multidecadal variations in length of day. Nature, 2026; DOI: 10.1038/s41586-026-10999-2
Daniel García is an Editor-in-Chief with strong expertise in structural work and engineering principles. He combines this technical foundation with deep knowledge of energy, spatial design, and emerging technologies, bringing a forward-thinking and analytical approach to editorial leadership.
