Earth’s recent spell of unusually rapid rotation may have peaked, with the shortest day of the year set for July 26, marking a shift after years of record-breaking short days. Scientists attribute the change to lunar cycles, polar ice melt, and core dynamics, though the exact cause remains debated.
The Earth’s rotation is slowing again after several years of record-breaking short days, with the shortest day of 2026 expected on Sunday, July 26, according to data from the International Earth Rotation and Reference Systems Service (IERS). This follows a period of accelerated rotation that saw days as short as -1.37 milliseconds below the standard 24-hour mark in 2025. The shift has sparked scientific debate over whether this marks a temporary fluctuation or a long-term trend.
A Shift in Earth’s Rotation
The latest mid-July predictions from IERS suggest Earth’s rotation will slow after peaking, when the planet completed its spin -1.37 milliseconds below the standard 24 hours. This acceleration, which surprised scientists after decades of expectations that the planet’s rotation would continue its gradual long-term slowdown, was linked to the Moon’s position relative to the equator, as well as internal geophysical factors. However, the latest figures suggest the period of faster rotation may now have peaked, with the shortest day of 2026 expected to measure around 0.65 milliseconds shorter than the standard 24 hours.
Leonid Zotov, a geodesy specialist at Moscow State University, told TimeAndDate.com that Earth’s rotation has started to decelerate,
noting that the deepest minimum over the past decade has been passed: Earth has passed its highest velocity.
This aligns with findings from NASA, which highlighted the role of polar ice melt in redistributing mass toward the equator, slowing the planet’s spin. The accelerated melting of glaciers in Greenland and Antarctica releases massive amounts of water into the oceans, displacing mass towards the equator and slowing the planet’s rotation,
a NASA-funded researcher explained.
The Role of the Moon and Core Dynamics
Scientists have long understood that tidal forces gradually slow the planet over centuries and millennia, but recent fluctuations suggest other forces are at play. Graham Jones, an astronomer at TimeAndDate.com, noted that Earth’s spin is influenced by the Moon’s monthly orbit: We spin more slowly when the moon is above the equator, and more quickly when the moon is to the north or south of the equator.
This phenomenon may explain some of the observed variations.
Meanwhile, the Earth’s core dynamics add another layer of complexity. Researchers at the University of Vienna and ETH Zürich found that shifts in the planet’s rotation are gradually slowing as climate change melts glaciers and polar ice sheets, redistributing water across the planet and subtly lengthening the day. The fluid movements of the outer core redistribute rotational energy,
one source noted. These elements, combined with the loss of groundwater and the rise of sea levels, affect the planet’s mass distribution.
Implications for Timekeeping and Science
The variability in Earth’s rotation has practical implications for global timekeeping. Time on Earth is based on Coordinated Universal Time, which closely follows the planet’s rotation. As Earth’s rate of rotation changes, “leap seconds” are added or taken away every so often. So far, leap seconds have only ever been added, with a slight slowing of Earth’s rotation observed by atomic clocks. The recent acceleration raised the possibility of the world’s first negative leap second.
For industries reliant on precise timekeeping—such as satellite navigation and telecommunications—the changes underscore the need for adaptive systems. If this trend persists, the implementation of a negative leap second might be considered to adjust clocks to the new astronomical conditions,
a source noted. However, the IERS has not yet confirmed such a move, citing the need for further data.
Uncertainties and Future Research
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