The length of a day on Earth is not constant. For several decades, scientists have been recording slight fluctuations in the rotation speed of our planet. Usually, these fluctuations are only a few milliseconds, so people do not notice these changes; however, they are of great scientific interest to geophysicists. Previously, researchers have established that the speed of Earth's rotation is influenced by processes in the atmosphere and oceans, major earthquakes that redistribute the planet's mass, and the melting of continental glaciers that alters the distribution of water in the world's oceans. However, these factors explain only a part of the observed changes. Over longer time intervals, the causes appear to be hidden deep within the Earth. Beneath the rocky mantle lies the outer core—a layer of molten metal approximately 2,250 kilometers thick. Its constant movement can slightly change the planet's rotation speed, but until now, scientists did not understand how this influence is transmitted outward. ## Gravity of the Inner Core The authors of the new study proposed a mechanism that may explain this connection. They believe that the inner core rotates at a different speed than the rest of the Earth. As a result, it is slightly offset relative to the inhomogeneities in the mass distribution in the mantle. Gravity seeks to return the core to a more stable position, while other processes within the planet counteract this. This peculiar "tug-of-war" between various forces leads to small accelerations and decelerations of Earth's rotation. > "In the end, gravity still wins," noted one of the study's authors, geophysicist Mathieu Dumberry from the University of Alberta. ## Model Matched Observations The researchers reconstructed changes in Earth's rotation speed from 1964 to 2019. To do this, they used seismic observation data on the rotation of the inner core and models of the movement of liquid metal in the outer core. The resulting model accurately reproduced both the timing and magnitude of the changes in the length of a day that have been recorded over several decades. ## Work Continues The authors emphasize that the proposed mechanism is likely not the only factor affecting Earth's rotation. Other processes within the planet may also be at work that have not yet been studied. Nevertheless, the results of the study help explain one of the main reasons for long-term changes in the length of a day and contribute to a better understanding of the processes occurring in the Earth's interior. The study is published in the journal [Nature](https://www.nature.com/articles/d41586-026-03018-x).
