Gravitational pull of the inner core drives the decades-long swings in the length of Earth's day
2 October 2026
A study in Nature, in the issue dated 1 October 2026, finds that multidecadal changes in the length of Earth's day are driven primarily by gravitational torque exerted by the planet's inner core. The length of day is not constant: fluctuations of several milliseconds occur on timescales of 10 to 70 years, and the authors analysed the record from 1964 to 2019. They report a broad oscillation of about 60 to 70 years in the rotation of the inner core. Because the inner core rotates slightly differently from the mantle, it becomes misaligned in longitude with mass anomalies in the lower mantle, and the resulting gravitational torque dominates the multidecadal fluctuation. Electromagnetic and topographic coupling at the core-mantle boundary act in the opposite direction, resisting the change rather than causing it. The finding assigns the main cause of these long-period changes to the deep interior of the planet rather than to the atmosphere or the oceans.
In plain words
LOD Length of day — The time Earth takes to complete one rotation, which varies slightly from day to day.
Inner core Earth's inner core — The solid innermost region of Earth, lying inside the liquid outer core.
Torque Torque — A turning force that changes the rotation of a body.
Core-mantle boundary Core-mantle boundary — The surface separating Earth's rocky mantle from its metallic core.