This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: You might think the length of a day never varies—exactly 24 hours. You'd be wrong, at least if you count days in milliseconds, as physicists do.
For about 30 years, scientists have known that the rotation of Earth's liquid core—detected through changes in its magnetic field—speeds up over a few decades, then slows down over a few more. Meanwhile, the mantle—the rocky shell that includes the crust and is 3,000 kilometers (1,860 miles) thick—slows down, then speeds up to compensate, because the planet's angular momentum must remain constant. That change in the mantle's rotational speed can shorten or lengthen a day by a few milliseconds over time.
How the core and mantle exchange angular momentum has so far eluded scientists. But in a new study published in Nature, University of Alberta physics Ph.D. student Huifeng Zhang and professor Mathieu Dumberry show that small changes in the rotational speed of the inner core—the solid, central part of the core, which is not exactly spherical—exert what's called a "gravitational torque" on mass anomalies within the mantle, causing a slight fluctuation in the length of a day. Another form of torque at the boundary of the core and mantle creates friction and electromagnetic drag—called core-mantle boundary torque—that resist the gravitational tug and limit changes in the length of a day.
Subtle shifts in the balance between gravitational and core-mantle torque are ultimately responsible for the small changes observed in the length of a day. In addition to solving this long-standing mystery, the research shines a spotlight on Earth's depths, including the inner core, which "deforms viscously," Zhang and Dumberry say, on a time scale of about 10 years. Huifeng Zhang et al, Gravitational torque drives multidecadal variations in length of day, Nature (2026).
DOI: 10.1038/s41586-026-10999-2 Swati Mestri holds a bachelor's degree in Electronics Engineering and has worked as a content editor since 2019. She has experience editing research documents across technology, health care, and materials science, and has a particular interest in technology and space. Full profile → Bachelor's in mathematical biology, Master's in creative writing.
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