Earth's Core May Change Day Length
Tuesday, October 6, 2026217 words3 minutes1961 reads
A day is commonly set at 24 hours. Yet Earth's rotation is never perfectly constant. Scientists can detect millisecond changes. Causes differ by time scale. This Nature study covers broad, multidecadal changes.
The authors, Huifeng Zhang and Mathieu Dumberry of the University of Alberta, used reconstructions of inner-core rotation and flows in the liquid core. Their analysis covers 1964 to 2019. It supports an explanation in which the solid inner core exerts a gravitational torque on the mantle. The inner core is slightly uneven rather than perfectly spherical, and it can become misaligned with uneven mass patterns in the mantle. That misalignment creates a small turning force.
Angular momentum is exchanged between the core and mantle. A tiny change in the mantle's rotation can change day length. The researchers found that gravitational torque may be the leading cause of the multidecadal pattern, while electromagnetic and topographic torque at the core-mantle boundary mainly resist it. Their result depends on the accuracy of the inner-core and core-flow models. It is not a final answer to every rotation change.
The model also suggests that the inner core may deform viscously over only a few years. This solid region can slowly change shape under force. The work gives scientists another way to study Earth's deepest interior, which cannot be observed directly.
