Scientists have identified a significant factor explaining why Earth's rotation gradually speeds up and slows down: a gravitational competition deep within the planet. Researchers propose that the Earth's inner core, which rotates at a different rate to the rest of the planet, creates a misalignment that generates gravitational forces affecting the planet's spin over decades. This discovery helps explain long-term variations in day length that have puzzled geologists for years.

The study, published in Nature, focuses on the interaction between the inner core and the surrounding mantle layers. The researchers constructed a model using seismic data from 1964 to 2019 and found that the gravitational forces attempting to realign the misaligned inner core closely matched the observed timing and magnitude of actual changes in Earth's rotation. Whilst other factors such as atmospheric fluctuations, ocean currents, earthquakes and melting ice sheets also influence day length, this gravitational mechanism accounts for a major portion of the longer-term variation, with gravity ultimately "winning" this internal tug-of-war.