Scientists Trace Earth's Shifting Day Length to a Gravitational Tug Inside the Planet

The length of an Earth day is not perfectly constant — it varies by small amounts, sometimes just milliseconds, and researchers have tracked these shifts for decades. Some of the more sudden changes come from atmospheric and ocean fluctuations, major earthquakes redistributing mass, and melting ice adding water to the oceans. But variations that unfold over several decades appear to originate much deeper inside the planet, in the outer core — a roughly 1,400-mile-thick layer of hot liquid metal beneath the rocky mantle that is constantly in motion.

Until now, it was unclear how motion in this deep layer could influence the rotation of the entire planet, since friction between the core and mantle is too weak to account for it. In a study published this week in Nature, researchers propose a different mechanism: gravity generated by the inner core.

A tug-of-war inside the planet

According to Mathieu Dumberry, a geophysicist at the University of Alberta involved in the research, Earth's solid inner core rotates at a slightly different rate than the rest of the planet, which causes it to drift out of alignment with mass irregularities in the mantle over time. Because the core "wants to be aligned," gravity continually pulls it back toward a more harmonious position, and that pull subtly affects the rotation speed of the planet's surface.

The team reconstructed this interaction for the period between 1964 and 2019, combining seismic estimates of the inner core's rotation with models of liquid metal flow in the outer core. The resulting gravitational model closely matched the timing and magnitude of the gradual changes in day length that had actually been observed.

The researchers acknowledge that other, still-unidentified forces likely contribute, and their model does not account for every force at play. Even so, it produced a compelling match with real-world observations. As Dumberry put it, the process may resemble a tug-of-war between competing forces, but "gravity still wins."