Study ties changes in Earth’s day length to forces deep inside the planet
A model of inner-core gravity and opposing core–mantle interactions closely matched gradual changes in Earth’s spin from 1964 to 2019. Researchers say other forces are also at work.
Earth’s day length can vary by a few milliseconds, changes too small to notice in daily life. A Nature study proposes that forces deep inside the planet help explain gradual shifts in its rotation over decades.
The researchers modelled gravity pulling the inner core towards alignment with mass irregularities in the mantle, alongside interactions between the core and mantle that oppose that pull. As the competing forces act over time, they could slightly speed up or slow down Earth’s spin.
Using seismic estimates of inner-core rotation and models of liquid-metal flows in the outer core, the team reconstructed the interaction’s effect from 1964 to 2019. Its model closely reproduced the timing and magnitude of observed gradual changes in Earth’s spin.
The findings offer a proposed explanation for how forces within Earth can influence its surface rotation, where friction between the core and mantle is thought to be too weak to transmit some effects.
The study does not account for all forces, and the researchers say unknown factors are likely involved. Atmospheric and oceanic fluctuations, earthquakes and melting ice can also affect Earth’s rotation.
