While commonly understood as a fixed 24 hours, Earth’s rotation exhibits variations measurable in milliseconds. These fluctuations result from complex interactions between different layers of the planet, according to research presented in a study released Sept. 23 in Nature.
Scientists have documented for approximately three decades that Earth’s liquid core does not maintain a constant rotation rate. Observations of the planet’s magnetic field have shown that the core gradually accelerates over periods spanning several decades, then decelerates during subsequent decades. Simultaneously, the mantle—the rocky layer roughly 3,000 kilometers thick that encompasses the crust—responds with opposite motion. When the core accelerates, the mantle slows marginally, and when the core decelerates, the mantle accelerates. This reciprocal relationship maintains Earth’s overall angular momentum at a constant level. Even minor variations in the mantle’s rotational speed can shift the length of a day by several milliseconds.
Researchers at the University of Alberta have proposed an explanation for how the core and mantle exchange this rotational momentum. According to their findings, variations in the inner core’s rotational speed—the solid region at Earth’s center—generate what is termed a gravitational torque. Because the inner core is not perfectly spherical, changes in its motion can interact gravitationally with irregularly distributed mass within the mantle. This interaction can subtly modify the mantle’s rotation rate, producing measurable changes in day length.
A competing force operates at the boundary between the core and mantle, known as core mantle boundary torque. This force creates friction and electromagnetic resistance that counteracts the gravitational pull, thereby constraining the magnitude of potential day length changes. The researchers attribute observed variations in day length to a delicate equilibrium between these two opposing forces.
Beyond resolving this longstanding question about Earth’s changing rotation, the findings reveal that the inner core may be more dynamic than previously assumed. Research suggests the inner core undergoes viscous deformation on a timescale of approximately 10 years, indicating that Earth’s deepest interior may undergo shape changes and respond to forces more dynamically than its solid composition might suggest.
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