Earth's day length changes by milliseconds because the planet's rotation is influenced not only by surface events but also by deep interactions between the inner core, outer core and lowermost mantle. Zhang and Dumberry combined seismic estimates and core-flow models to reconstruct how angular-momentum exchange altered day length from 1964 to 2019. Their results point to a persistent eastward misalignment of the inner core that creates competing torques on the mantle, though seismic and modeling limits prevent precise short-term predictions. Improved observations and models would sharpen our knowledge of Earth's deep interior.
Something Deep Inside Earth Is Slowly Changing Time — Days Are Getting a Few Milliseconds Longer
Something Deep Inside Earth Is Shifting Timebpawesome - Getty Images
Though a day is defined as 24 hours, that interval is not fixed to the millisecond. Earth's rotation speeds up and slows down on decadal timescales, producing tiny variations in day length measured in milliseconds. New research suggests the source of some of these long-term changes lies far below the surface: interactions between the inner core, the liquid outer core and the lowermost mantle.
Surface Forces Aren't the Whole Story. Large earthquakes, melting ice sheets, shifting oceans and powerful storms can all change Earth's spin by redistributing mass or exerting torque. But those surface drivers do not fully explain decadal fluctuations in the planet's rotation.
Core–Mantle Interactions Provide a Missing Piece. Viscous drag at the core–mantle boundary (CMB) appears too weak to account for the observed torques on the mantle. If the lowermost mantle conducts electricity, however, flows in the conducting outer core could produce electromagnetic stresses across the CMB and couple the core's motion to the mantle. That coupling would allow deep flows to influence Earth's rotation.
What the Study Did
Geophysicists Huifeng Zhang and Mathieu Dumberry (University of Alberta) combined seismic estimates of the inner core's rotation with numerical models of outer-core flow to reconstruct how core dynamics contributed to changes in the length of day from 1964 through 2019. Their reconstruction appears in Nature and aims to quantify how angular momentum is exchanged among the inner core, outer core and lowermost mantle.
"We have known for more than 30 years ... that the decadal changes in day length are caused by core–mantle interactions," the authors write. "Yet we still lack a successful [length of day] prediction based on a modeling of the torque on the mantle."
Key Findings
The models point to a small but persistent eastward misalignment of the inner core relative to the mantle. Because the inner core currently rotates slightly faster than the rest of the planet, gravity exerts a steady eastward torque on the mantle as it tries to restore alignment. That eastward gravitational torque competes with westward torques generated at the CMB (which may include electromagnetic or topographic components). Any imbalance between these opposing torques shows up as a tiny change in day length.
Timescales and Limits: The team could reconstruct multi-decade trends reliably but could not resolve short, interannual fluctuations. Seismic estimates of inner-core rotation lack the precision to capture rapid changes, and core-flow models have limited resolution and may omit some relevant forces.
Why It Matters
Understanding how the deep Earth exchanges angular momentum helps refine models of the planet's internal structure, material properties and dynamics. Although the millisecond-scale changes will not be noticed in daily life, they are important for precise timekeeping, satellite navigation and geophysical theory.
Bottom line: Subtle, persistent interactions thousands of kilometers beneath our feet are nudging the length of a day by milliseconds. Better seismic data and higher-resolution core-flow models are needed to turn this understanding into reliable short-term predictions.
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