Earth's Inner Core Gravitational Pull Affects Day Length

Variations in the length of Earth's day that occur over multidecadal timescales are attributed to the gravitational coupling between the planet's inner core and its solid mantle, according to research published online on September 23, 2026, in the journal Nature. This finding offers a new explanation for subtle yet significant shifts in Earth's rotation speed that have puzzled scientists for years.

The study, detailed in the publication Nature, posits that the immense gravitational forces exerted by the solid mantle on the inner core, and vice versa, create a complex interplay that influences the planet's rotational momentum. Earth's inner core is a solid sphere of iron and nickel, approximately 1,220 kilometers (760 miles) in radius, surrounded by a liquid outer core. The mantle, a thick layer of silicate rock, encases the outer core. The interaction between these distinct layers, particularly their relative movements and gravitational attractions, can subtly alter the distribution of mass within the Earth system, thereby affecting its rotation rate.

Scientists have long observed that Earth's rotation is not perfectly constant. While the primary drivers of day length changes are well-understood phenomena like the tidal pull of the Moon and Sun, atmospheric and oceanic currents, and the melting of glaciers, the multidecadal variations have been more challenging to explain. This new research suggests that the gravitational tug-of-war between the solid inner core and the overlying mantle is a key, previously underestimated, factor in these longer-term fluctuations. The precise mechanism involves how the inner core, which is thought to rotate slightly faster than the mantle, interacts gravitationally. As the inner core's rotation or position shifts relative to the mantle, its gravitational influence changes, leading to minute adjustments in the mantle's rotation and, consequently, the length of a day.

The implications of this discovery extend beyond understanding Earth's rotation. It highlights the dynamic and interconnected nature of Earth's internal structure and its influence on surface phenomena. By quantifying the gravitational coupling between the inner core and mantle, researchers can refine models of Earth's rotation and potentially gain deeper insights into the processes occurring deep within the planet. This research contributes to the broader scientific endeavor of comprehending planetary dynamics, which is crucial for fields ranging from geophysics to climate science and even astrobiology, as understanding planetary rotation is fundamental to habitability.