Earth will spin faster on July 22 to create 2nd-shortest day in history

(Image credit: joecicak/Getty Images)

Tuesday, July 22, will be one of the shortest days on record, with Earth completing a full revolution in a little less time than usual.

 You won't notice the difference, which is only 1.34 milliseconds shorter than the typical 24-hour day, but it's a part of an odd pattern in Earth's rotating behavior that has been developing recently.  Should it persist, atomic clocks could have to remove a second in 2029—a so-called negative leap second, which has never been done before.

The Earth's rotational speed is not constant.  In the past, a day was significantly shorter than the 24 hours (86,400 seconds) that we are used to now.  A 2023 study found that over a large portion of Earth's early history, a day lasted roughly 19 hours because of a balance between lunar ocean tides and solar atmospheric tides.  On the other hand, the length of a day on Earth has steadily increased over time.  It has been progressively moving away from Earth due to tidal friction from the moon, which has been the main reason.  The moon drains Earth's rotational energy as it moves away, slowing down the planet's rotation and lengthening its days.

According to Timeanddate.com, the shortest day ever recorded was 1.05 milliseconds shorter than 24 hours, starting from the beginning of records (with the development of the atomic clock) in 1973 and ending in 2020. However, Earth has beaten its own speed records several times since 2020. On July 5, 2024, Earth's rotation was completed 1.66 milliseconds faster than usual, making it the shortest day ever recorded.

According to scientists, the shortest days of the year may occur on July 9, July 22, and August 5 in 2025.  But according to fresh data, July 10 was the shortest day of 2025 thus far, measuring 1.36 milliseconds less than 24 hours.  Earth is predicted to finish its spin 1.34 milliseconds ahead of schedule on July 22, placing it in second place.  According to current projections, July 22 will be the second-shortest day of the year, with Aug. 5 being around 1.25 milliseconds shorter than normal.

Earth just had one of its shortest days ever, with more to come. (Image credit: Created in Canva by Daisy Dobrijevic)

There are indications that the acceleration might be slowing down. The underlying cause of the recent rotational adjustments is yet unknown, although the rate at which the duration of the days is decreasing seems to be slowing.

According to a 2024 study, Earth's spin might be impacted by the melting of polar ice and the rise in sea levels. However, this mass redistribution may be mitigating the acceleration rather than causing it. The slowing of Earth's liquid core, which may be shifting angular momentum to cause the mantle and crust to spin a little faster, is a more plausible culprit that is located far below our feet.

According to Leonid Zotov, a prominent expert in Earth rotation at Moscow State University, "the cause of this acceleration is not explained," Timeanddate.com reported.  "The majority of scientists think it originates from within the Earth.  This enormous acceleration cannot be explained by ocean and atmospheric models.

 According to Zotov, the Earth's rotation might soon slow down once more.  If he is correct, this abrupt acceleration might only be a brief exception from the planet's long-term trend toward longer days and slower spin.

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