Core Movements Could Be Causing Tiny Shifts in Earth’s Spin Speed

Source: Geophysical Research Letters

One day on Earth is typically understood to last 24 hours, or 86,400 seconds. However, the amount of time it takes for our planet to complete a full rotation has fluctuated slightly over time.

Previous studies have documented a long-term trend of days lengthening by 1.72 ± 0.03 milliseconds per century since 720 BCE. These gradual changes are driven by the Moon’s gravitational pull on Earth slowing the planet’s spin. (The effect would be greater by about three quarters of a millisecond, but the rebounding of solid ground after the last ice age slightly lessens the effect.) However, there also exist decadal and millennial-scale perturbations in day length that cannot be explained by these forces.

Some researchers have hypothesized that these length-of-day fluctuations are caused by climatic changes, including the melting of ice sheets and movement of fresh water, or by magnetohydrodynamic movement within Earth’s molten, iron core. Using historical records of eclipses and lunar occultations to understand how day length has changed over time, along with machine learning to analyze these data, Kiani Shahvandi et al. tested both theories.

To understand the climate effects on day length, the researchers looked at historical data on barystatic mass variations of changing polar ice sheets, glaciers, and terrestrial waters. Using Bayesian physics-informed neural networks (BPINNs) along with data from independent archaeomagnetic and modern geomagnetic observations, the team calculated how both barystatic processes and magnetohydrodynamics may have influenced Earth’s spin and day length over the past 3,000 years.

The researchers found that barystatic processes’ influence on day length since 720 BCE was small and generally opposite to overall trends (e.g., these processes might have a slight shortening effect on day length over a given period, but other forces caused the day to lengthen much more, far outweighing the shortening effects). In contrast, they found that magnetohydrodynamic influences matched the observed length-of-day fluctuations seen in the long-term record.

The team notes that their work with this novel approach shows that core dynamics may explain length-of-day changes. (Geophysical Research Letters, https://doi.org/10.1029/2024GL111148, 2024)

—Sarah Derouin (@sarahderouin.bsky.social), Science Writer

Citation: Derouin, S. (2024), Core movements could be causing tiny shifts in Earth’s spin speed, Eos, 105, https://doi.org/10.1029/2024EO240563. Published on 9 December 2024.

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