Gravity Between Earth's Inner Core and Mantle May Explain Day-Length Variations

Scientists have long recorded millisecond fluctuations in the duration of Earth’s days, attributing most changes to atmospheric, oceanic and seismic events. A recent paper in Nature proposes that a previously hidden driver lies deep within the planet: the gravitational pull of the solid inner core on the surrounding mantle. The study, led by geophysicist Mathieu Dumberry, shows that the inner core rotates at a slightly different rate, becoming misaligned with mantle mass anomalies and generating a restoring gravitational torque.

Using seismic data spanning 1964-2019 and models of liquid-metal flow in the outer core, the authors recreated the timing and magnitude of observed day-length variations. While other unknown factors likely also play a role, this mechanism offers a major piece of the puzzle explaining why Earth’s rotation is not perfectly constant.