Scientists have long known that a day isn't exactly 24 hours when measured with extreme precision, such as in milliseconds. Over the past 30 years, researchers have observed that Earth's liquid core, which is the innermost part of the planet, sometimes spins faster over a few decades and then slows down again. At the same time, the mantle—the thick, rocky layer that includes Earth's crust and extends about 3,000 kilometers (1,860 miles) deep—slows down and then speeds up to balance the changes. These movements help maintain Earth’s angular momentum, but they can cause the length of a day to change slightly over time, by a few milliseconds. In a new study published in the journal Nature, researchers from the University of Alberta, including Ph.D. student Huifeng Zhang and professor Mathieu Dumberry, propose that small changes in the rotational speed of Earth’s inner core—its solid, central part, which is not perfectly round—create what is known as a "gravitational torque." This torque affects mass anomalies within the mantle, causing slight fluctuations in the length of a day. Additionally, another type of force, called "core-mantle boundary torque," occurs at the interface between the core and mantle. This force creates friction and electromagnetic drag, which counteract the gravitational pull and limit how much the day's length can change. The balance between these two types of torque—gravitational and core-mantle boundary—determines the small variations in the length of a day. These changes are not immediately noticeable in everyday life but are detectable using precise instruments that measure Earth’s rotation. The study also sheds light on the dynamic nature of Earth’s interior, particularly the inner core, which can deform slowly over time. Zhang and Dumberry note that this deformation occurs on a timescale of about 10 years, highlighting how Earth's deep layers are constantly changing, even if those changes are subtle and take place over long periods.