Earth’s day is not exactly 24 hours, and the inner core keeps changing it

Earth takes a few milliseconds longer or shorter to turn on some days than on others, and the differences build up over decades rather than averaging out. University of Alberta physicist Mathieu Dumberry and his PhD student Huifeng Zhang traced a large part of that wobble to the solid inner core, the iron ball at the planet’s center, in a paper in Nature.

The calendar treats a day as 24 hours of 3,600 seconds each. The planet itself keeps no such schedule, and the gap between the two is small enough to ignore in daily life and large enough to force adjustments to the world’s clocks.

A day defined as 86,400 seconds

The 24-hour day has a definition behind it. According to the Paris Observatory’s explanation of leap seconds, the second was once defined as the 86,400th fraction of the mean solar day, and that stayed the standard until 1967, when the second was redefined using the vibrations of cesium-133 atoms in atomic clocks.

Once atomic clocks existed, the planet could be measured against something steadier than itself. Earth’s rotation turned out to drift. The Moon’s tides slow it gradually, and the observatory also lists atmospheric winds, which cause fluctuations over one to 30 years, and movements of the liquid core, which cause irregular accelerations. An international agreement in 1972 requires that atomic time and Earth-based time never differ by more than one second, and a leap second is added whenever the planet falls behind. The observatory notes that, after the leap second of January 1, 2017, the gap between International Atomic Time and UTC stood at 37 seconds, a difference maintained by the leap-second system that began in 1972 and tied to the planet’s irregular spin rather than to any fixed schedule of corrections.

Gravitational torque from a lumpy inner core

Scientists have known for roughly 30 years that Earth’s liquid outer core rotates at varying speeds, and the mantle above it responds in the opposite direction, because the total angular momentum of the system has to stay constant. The University of Alberta’s account describes this core-mantle exchange as a 30-year-old picture that still left open how the inner core takes part in it, and presents the paper as a resolution of that puzzle.

Zhang and Dumberry propose a missing link. The inner core is not perfectly spherical, so when it rotates slightly out of alignment with the mantle, gravity pulls the two back toward each other. That gravitational torque speeds up or slows the mantle, and with it the rotation of the whole planet as seen from the surface. Opposing it is a second torque at the core-mantle boundary, where friction and electromagnetic drag resist the change. The study, summarized by the university’s release on ScienceDaily, says the shifting balance between those two forces produces the observed change in day length.

Gizmodo’s report adds that three couplings were weighed in the Nature paper: gravitational, electromagnetic and topographic, the last involving irregularities along the core-mantle boundary. The gravitational coupling proved the most influential, and the other two largely counterbalance it over multidecadal timescales.

A solid core that flows

The finding carries a surprise about the material itself. To make the model match the record, the inner core has to deform viscously, which the researchers say happens on a timescale of roughly ten years. A ball of iron under enormous pressure does not look like something that flows, and The Brighter Side’s coverage frames the result as evidence that the solid core is more dynamic than its composition suggests, deforming over a few decades or less.

The record the model reproduces is modest in size. Day length wandered by several milliseconds between the early 1970s and 2021, according to Gizmodo’s summary of the paper, and the same observed changes run in cycles of roughly 10 to 70 years across the interval from 1964 to 2019, as the Brighter Side describes it. No person would notice a few thousandths of a second added to a day, since the change is far below anything the senses register, but instruments tracking Earth’s orientation register it, and the leap-second system exists because those drifts accumulate.

Earth’s inner core is one contributor, not the only one. Tidal drag, the atmosphere and the oceans still shape rotation on their own timescales, which is why Gizmodo points out that the decade-scale variation addressed by Zhang and Dumberry sits apart from short atmospheric changes and from the million-year slowing caused by the Moon. Zhang said the team is investigating whether the same forces also drive a six-year oscillation in day length. In his words, these different pieces of information can come together to provide a more coherent picture of Earth’s deep interior.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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