The days are getting shorter, and not just because summer is ending in the Northern Hemisphere. On Tuesday, August 5, Earth's solar day will be slightly shorter than the usual 24 hours, according to Timeanddate.com, making it not only one of the shortest days of 2025, but also since records began.
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At just 1.25 milliseconds below the 86,400-second mark, the difference won't be noticeable, but it's part of a mysterious trend that's baffling scientists: Earth is spinning faster. After decades of slowing down, our planet's rotation has been speeding up in recent years — and timekeepers have no clear explanation.
To understand what's going on, it helps to define what a day is. Earth's actual rotational period — a complete 360-degree rotation relative to the background stars — takes 23 hours, 56 minutes and 4.1 seconds, according to EarthSky. That's the sidereal day — and it explains why stars and planets appear to rise about four minutes earlier each day and why the night sky changes with the seasons. After all, Earth moves in its orbit around the sun as it rotates.
However, the 24-hour day we follow is the solar day, measured not in relation to the stars, but to the sun. A day is defined from noon to the next noon and is equal to 24 hours or 86,400 seconds. This measurement seems to be mysteriously shorter than it should be.
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There are three dates in 2025 when scientists have predicted that Earth's solar day will be shorter than 24 hours — July 9 (by 1.23 milliseconds), July 22 (1.36 milliseconds), and August 5. However, the shortest recorded solar day was July 5, 2024, when it lasted 1.66 milliseconds less than 24 hours.
What allows scientists to pinpoint the dates of July 9, July 22, and August 5 is the precise position of the Moon. On these three days, changes in the Moon's position relative to the Earth's equator — and specifically its ecliptic deviation — can affect tidal forces that in turn subtly affect the Earth's rotation rate.
Although the Moon can cause short-term fluctuations in the Earth's rotation speed, the underlying causes of the long-term acceleration of rotation in recent years remain less understood. It has been suggested that global warming may play a role, but the most likely explanation seems to be a slowing of the rotation of the Earth's liquid core, which forces the rest of the planet to spin faster.
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You won't notice any change in Earth's rotation on August 5, but if the phenomenon continues until 2029, a so-called negative second may need to be added — for the first time in history.
Source: space.com
