Brief
Study links Earth's inner core torque to millisecond day length changes
Physicists propose that gravitational torque from the solid inner core, combined with core‑mantle boundary forces, explains tiny variations in Earth's rotation.
By Felo News Desk · Published
Physicists from the University of Alberta have presented a mechanism that may explain why Earth's day length varies by a few milliseconds, according to a study published in Nature on Sept. 23.
The research, led by PhD student Huifeng Zhang and professor Mathieu Dumberry, suggests that small changes in the rotational speed of Earth's solid inner core generate a gravitational torque. Because the inner core is not perfectly spherical, its motion interacts with uneven mass distributions in the mantle, slightly altering the mantle's rotation and thus the length of the day.
The study also notes that core‑mantle boundary torque—friction and electromagnetic drag at the core‑mantle interface—opposes this gravitational pull, limiting the magnitude of day‑length changes.
These findings build on decades of observations that the liquid outer core and mantle exchange angular momentum, with the core speeding up while the mantle slows, and vice versa, to conserve the planet's total angular momentum.
Key facts
- Earth's day length can vary by a few milliseconds due to internal dynamics. (sciencedaily.com)
- A new Nature study proposes gravitational torque from the inner core as a cause. (sciencedaily.com)
- Core‑mantle boundary torque provides opposing friction and electromagnetic drag. (sciencedaily.com)
- The study was authored by Huifeng Zhang and Mathieu Dumberry of the University of Alberta. (sciencedaily.com)
- The paper was published on Sept. 23, 2026. (sciencedaily.com)
Sources
- [1] sciencedaily.com — originally reported as “Earth’s day isn’t exactly 24 hours. The reason may lie deep inside the planet”
