The Earth’s Core Is Slowing Down: Why Your Days Might Get Slightly Longer

For most people, a day feels constant and predictable, lasting exactly 24 hours. However, Earth’s rotation is not perfectly uniform, and precise measurements show that the length of a day changes by tiny fractions of a second over time. Recent research indicates that Earth’s solid inner core may now be rotating slightly more slowly relative to the mantle and crust, and that this subtle shift could influence day length in measurable but extremely small ways. Although these changes are not readily apparent in daily life, they reveal important details about how the Earth works deep beneath the surface.

Understanding Earth’s Layered Interior

Earth is composed of concentric layers that behave very differently from one another. The outermost crust rests on the mantle, a thick layer of hot, slowly deforming rock. Beneath the mantle lies the liquid outer core, and within that lies the solid inner core, a dense sphere composed primarily of iron and nickel about 1,220 kilometres in radius and located roughly 5,000 kilometres below the surface.

Because the inner core is surrounded by a fluid outer core, it is not rigidly locked to the rest of the planet. Instead, it can rotate at a slightly different rate than the mantle and crust above it. Scientists refer to this as differential rotation, or superrotation, when the inner core spins faster than the surface, and subrotation when it spins more slowly.

What the New Research Shows

In 2024, Wei Wang and John E. Vidale published a study in Nature analysing seismic waves from 121 repeating earthquakes recorded between 1991 and 2023. These earthquakes produced nearly identical seismic signals that propagated through Earth’s inner core and were recorded by seismic stations worldwide. By comparing subtle changes in the waveforms over time, the researchers were able to infer how the inner core’s orientation had shifted relative to the mantle.

Their results suggest that the inner core rotated slightly faster than the rest of the planet in the early 2000s but began slowing around 2010. According to their analysis, the inner core is now rotating slightly more slowly than the mantle. This change is small, but it represents one of the clearest pieces of evidence yet that the inner core’s rotation rate is not constant. John Vidale explained in media coverage that the effect on the length of a day would be extremely small, on the order of a thousandth of a second, and would be difficult to separate from other influences such as atmospheric and oceanic changes. Nevertheless, the seismic evidence indicates that the inner core’s motion has reversed direction relative to earlier decades.

Why the Core’s Motion Can Affect Day Length

Earth’s rotation follows the principle of conservation of angular momentum. When mass within a rotating system changes position or speed, other parts of the system must adjust slightly to compensate. If the inner core slows relative to the mantle, tiny adjustments in rotation can occur at the surface. These adjustments are extremely small. Variations in the length of a day are typically measured in milliseconds or fractions of milliseconds. According to long-term geophysical monitoring, day length fluctuates continuously due to winds, ocean currents, seasonal mass redistribution, and gravitational interactions with the Moon. Core dynamics are one additional factor within this complex system.

Earlier research published in Nature in 2013 by Richard Holme and Olivier de Viron identified a roughly 5.9-year oscillation in day length that appears to correlate with magnetic field variations generated in the core. Their findings provided independent evidence that deep Earth processes contribute to short-term rotational changes.

The Role of the Geodynamo

The liquid outer core generates Earth’s magnetic field through a process known as the geodynamo. Convection of electrically conductive molten iron produces magnetic forces that can couple with the solid inner core. Gravitational interactions between the inner core and the mantle also play a role. These combined forces create a dynamic system in which angular momentum is exchanged between layers.

Scientists believe that these interactions explain why the inner core does not rotate at a constant rate. The slowing observed in recent years may represent part of a longer oscillatory cycle rather than a permanent trend.

What This Means for Everyday Life

The change in day length caused by inner-core motion is too small to affect human schedules or biological rhythms. However, extremely precise timekeeping systems, such as Coordinated Universal Time, must account for minute irregularities in Earth’s rotation. When necessary, leap seconds are added to keep atomic time aligned with Earth’s actual spin.

From a scientific perspective, the new findings improve understanding of Earth’s deep interior and refine models of mass and momentum exchange within the planet. They also demonstrate how seismic monitoring can detect changes occurring thousands of kilometres below the surface. Although your day will still feel like 24 hours, research shows that Earth’s rotation is subtly dynamic. Beneath the crust, the inner core is engaged in a slow and complex dance with the outer core and mantle, and that motion, though imperceptible to us, slightly reshapes the rhythm of our planet.