How Earth's Inner Core Creates Tiny Fluctuations in Day Length

Earth's Rotation Varies in Milliseconds

Earth's day is not a fixed 24 hours. While the variation is imperceptible to human experience, physicists can measure shifts in day length down to the millisecond level. These fluctuations stem from complex interactions deep within the planet, where different layers exchange rotational momentum to maintain Earth's total angular momentum. Over decades, these subtle exchanges accumulate enough to noticeably alter how long a day actually is.

The Core-Mantle Momentum Exchange

For approximately 30 years, scientists have recognized that Earth's liquid outer core does not rotate at a constant rate. Magnetic field observations reveal that this core gradually accelerates over periods spanning several decades, then decelerates during subsequent decades. The mantle—the roughly 3,000-kilometer-thick rocky layer that includes Earth's crust—responds in the opposite direction, slowing when the core speeds up and accelerating when the core slows down. This inverse relationship preserves Earth's overall angular momentum, a fundamental principle of physics that prevents the planet from spinning chaotically. Even minute changes in the mantle's rotation are sufficient to shift day length by a few milliseconds.

Gravitational Torque: The Missing Mechanism

While scientists understood this momentum exchange occurred, they struggled for decades to explain the physical mechanism enabling it. A study published in Nature on September 23 by researchers at the University of Alberta provides a compelling explanation. Physics PhD student Huifeng Zhang and professor Mathieu Dumberry demonstrated that small variations in the rotational speed of Earth's solid inner core—the planet's deepest region—generate what physicists call a gravitational torque. Because the inner core is not perfectly spherical, changes in its motion interact gravitationally with irregularly distributed mass in the mantle above it. These gravitational interactions subtly affect how rapidly the mantle rotates, producing measurable changes in day length.

Competing Forces Keep Changes in Balance

The gravitational torque from the inner core does not operate unopposed. Another force exists at the boundary between the core and mantle, known as core-mantle boundary torque, which generates friction and electromagnetic drag that opposes the gravitational pull. This resistance prevents day-length variations from becoming dramatic, limiting the cumulative effect of the inner core's influence. The researchers propose that observed changes in day length arise from delicate fluctuations in the balance between these two competing forces—gravitational torque pulling in one direction and frictional torque resisting in the other.

Earth's Interior May Be More Dynamic Than Previously Thought

Beyond solving a longstanding geophysical puzzle, the findings suggest that Earth's deepest interior behaves more dynamically than its solid composition might suggest. Zhang and Dumberry indicate that the inner core appears to deform viscously on a timescale of approximately 10 years, meaning it changes shape and responds to forces at a pace faster than would be expected from a rigid solid. This discovery implies that Earth's deepest layers are far more fluid and responsive to physical stresses than conventional understanding indicated, opening new questions about the nature of matter under extreme pressure and temperature at the planet's center.