Earth’s day isn’t exactly 24 hours—scientists probe core’s hidden role

KUALA LUMPUR, 30 SEPTEMBER 2026 —

Scientists from the University of Alberta have proposed that subtle shifts in Earth’s solid inner core generate a gravitational torque that nudges the mantle, causing the planet’s day to lengthen or shorten by a few milliseconds, a study published in Nature on Sept. 23 said.

The research explains why the length of a day, long assumed to be a fixed 24 hours, varies over decades. Physicists have measured minute fluctuations in Earth’s rotation, noting that some days become slightly shorter while others lengthen by a few thousandths of a second.

The new model links these variations to a balance between two opposing forces: a gravitational torque produced by the non‑spherical inner core and a resisting core‑mantle boundary torque that includes friction and electromagnetic drag.

Huifeng Zhang, a physics PhD student, and Professor Mathieu Dumberry detailed how the inner core’s rotational speed changes can create a gravitational pull on uneven mass distributions in the mantle. When the inner core speeds up, the mantle slows, and vice‑versa, preserving the planet’s total angular momentum. The core‑mantle boundary torque counteracts this pull, limiting the magnitude of day‑length changes.

The researchers argue that the observed millisecond‑scale variations arise from subtle shifts in the balance between these two torques.

For roughly three decades, scientists have known that Earth’s liquid outer core does not rotate at a constant rate. Magnetic‑field observations have shown periods of gradual acceleration followed by deceleration over successive decades. The mantle, a roughly 3,000‑kilometre‑thick rocky layer that includes the crust, responds in the opposite direction, slowing when the core speeds up and speeding up when the core slows.

The new study adds that the solid inner core, despite its rigidity, deforms viscously on a timescale of about ten years, indicating a more dynamic deep interior than previously thought.

The findings provide a long‑sought explanation for the exchange of rotational momentum between core and mantle, a process that has puzzled geophysicists. By quantifying the gravitational torque and the opposing core‑mantle boundary torque, the study offers a mechanistic view of how Earth’s rotation can vary without violating conservation of angular momentum.

Understanding these internal dynamics has implications for precise time‑keeping and satellite navigation, sectors that rely on accurate Earth‑rotation models. For Malaysia and the broader region, where satellite‑based services underpin telecommunications, aviation and maritime operations, refined predictions of day‑length variations could enhance the reliability of positioning systems and timing infrastructure.

The authors suggest that continued monitoring of Earth’s magnetic field and rotational speed, combined with further modelling of inner‑core deformation, will improve forecasts of day‑length changes. Future research will aim to validate the proposed torque balance with observational data, potentially leading to more accurate Earth‑rotation models for scientific and practical applications.