The Earth's rotation is not a constant clock
In short: We treat 24 hours as an absolute, but our planet’s rotation is actually in flux. While the changes are measured in mere milliseconds, they reveal a violent and dynamic tug-of-war happening thousands of kilometers beneath our feet, where the core and mantle trade momentum.
While we experience a standard day as 24 hours, Earth's rotation is not perfectly constant. Scientists have observed that the length of a day can shift by several milliseconds over decades. This phenomenon is driven by an exchange of angular momentum between the planet's liquid core and its mantle—the roughly 3,000-kilometer-thick rocky layer including the crust. Because Earth's total angular momentum must remain constant, when the core speeds up, the mantle slows down, and vice versa. These fluctuations have been detectable by physicists for roughly three decades. Changes in our planet's magnetic field provided the initial evidence of these core movements. This dynamic behavior reveals a world that is surprisingly active deep underground.
Gravitational torque is proposed as a mechanism for this process, according to September 2023 research from Huifeng Zhang and Mathieu Dumberry in Nature. Irregularities within the mantle's mass distribution react with shifting inner core rotation speeds due to its non-spherical shape. This creates a gravitational tug that alters the mantle's rotation. However, this isn't the only force at play; 'core-mantle boundary torque' creates friction and electromagnetic drag that opposes this pull, acting as a stabilizer for our planet's rotation. This research was published specifically in the journal Nature. It addresses a long-standing mystery regarding why day lengths vary. The study highlights how the inner core's motion interacts with uneven mass distributions.
The findings suggest that Earth's deepest interior is far more active than its solid composition suggests. The inner core appears to deform viscously on a timescale of about 10 years. This means the very center of our world is constantly reshaping itself in response to internal forces, subtly dictating the rhythm of time on the surface. The researchers are from the University of Alberta. Their work provides new insights into the planet's deepest interior. This suggests that Earth's center responds to forces far more dynamically than its solid state might imply.
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Sources
- Summary by Intellect · source: Why Some Days Aren’t 24 Hours
- Also: Earth’s day isn’t exactly 24 hours. The reason may lie deep inside the planet
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