On July 22, 2025, Earth will complete its rotation 1.34 milliseconds faster than the usual 24 hours, making it one of the shortest days on record. This slight shift, while imperceptible to humans, is part of a broader and puzzling trend where Earth’s spin has been accelerating since 2020. If the trend persists, scientists warn that a negative leap second may need to be added to atomic clocks in 2029—an unprecedented event, explains Space.
Earth’s rotation has not been constant over time. Millions of years ago, a day was much shorter than it is today. However, Earth’s rotation has gradually slowed, primarily due to the Moon’s gravitational pull. The recent trend toward shorter days has raised questions about the causes of this acceleration, with potential implications for timekeeping and our understanding of Earth’s dynamics.
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Earth’s Rotation: A Long History of Slowing
In the distant past, Earth’s days were far shorter than the 24 hours we experience today. Research published in 2023 indicated that for a large part of Earth’s early history, a day lasted only around 19 hours. This was due to the balance between solar atmospheric tides and lunar ocean tides. Over time, Earth’s day has lengthened, primarily because of tidal friction, a process in which the Moon’s gravitational force gradually slows Earth’s rotation.
As the Moon moves farther away from Earth, this friction continues to sap Earth’s rotational energy. This has led to a gradual increase in the length of a day. For most of Earth’s history, this process has been relatively steady, but the recent acceleration of Earth’s rotation has broken with this established trend.
The Acceleration of Earth’s Spin: Records Are Being Shattered
Since 2020, Earth’s rotation has been increasingly faster, setting new records for the shortest days. In the years leading up to this shift, the shortest day on record was 1.05 milliseconds shorter than the standard 24 hours.
However, since 2020, Earth has broken its own speed records. The shortest day ever recorded occurred on July 5, 2024, when Earth completed a full rotation 1.66 milliseconds faster than usual. Following this, July 22, 2025, is expected to be the second-shortest day, with a 1.34-millisecond reduction.
These changes raise concerns for timekeeping systems. As Earth’s spin continues to fluctuate, experts suggest that a “negative leap second” might be required to keep atomic clocks synchronized with Earth’s time. This would be a first, as a leap second has only ever been added to account for Earth’s slowing rotation.
The Mystery Behind Earth’s Accelerating Rotation: Theories and Explanations
The precise cause of this acceleration is still unknown. One theory, explored in a 2024 study, suggests that melting polar ice and rising sea levels might be redistributing Earth’s mass in a way that influences the planet’s spin. However, scientists believe that this is more likely to moderate the acceleration rather than drive it.
The leading theory points to the slowing of Earth’s liquid core. Researchers suggest that changes in the movement of Earth’s inner layers may be redistributing angular momentum, causing the mantle and crust to spin faster.
Leonid Zotov, an expert on Earth’s rotation at Moscow State University, explained that most scientists now suspect the acceleration originates deep within the planet. Ocean and atmospheric models do not adequately explain the rapid increase in rotation speed. While Zotov has predicted that Earth’s rotation may eventually slow again, for now, this unexpected acceleration remains a mystery.
