A Gravitational Battle Within the Earth Is Changing the Length of Days

The length of your day can subtly change due to gravitational forces deep within the Earth.

Scientists have identified a new mechanism influencing the Earth's rotation speed, which changes the length of days. While atmospheric, oceanic, and seismic events cause shorter-term fluctuations, a decades-long variation appears to stem from the Earth's outer core, a layer of liquid metal. Researchers propose that gravitational forces exerted by the inner core push rotation in one direction, while opposing interactions between the outer core and mantle resist this motion. This gravitational tug-of-war, based on the inner core's differential rotation and misalignment with mantle irregularities, influences the planet's spin. A study published in Nature used seismic data and fluid dynamics models to reconstruct these interactions from 1964 to 2019, finding that this gravitational mechanism accurately reproduces observed gradual changes in Earth's rotation over that period.

This research uncovers a previously unquantified gravitational interaction between the Earth's inner core and mantle as a significant driver of long-term rotational variations. It suggests that the planet's spin is not solely a product of external or near-surface phenomena, but is also subject to internal gravitational dynamics. The study's success in modeling observed rotational changes implies that this core-mantle gravitational exchange is a substantial, though previously mysterious, factor in our planet's rotational stability.

The findings imply a more complex interplay of forces governing Earth's rotation than previously understood, potentially impacting fields that rely on precise timing, such as satellite navigation and astronomical observations. It also highlights the ongoing mystery of how forces are transmitted from the deep interior to the planet's surface, suggesting further investigation into geophysical mechanisms is warranted.