Earth marked July 9, 2025, as its shortest day ever recorded, and scientists say two more similarly brief days are coming up on July 22 and August 5. This rare occurrence is due to the Moon’s current position, which is subtly increasing Earth’s spin, making days slightly shorter than the usual 24 hours.
A report by Live Science notes that the Moon’s location is expected to shorten these days by 1.3 to 1.51 milliseconds. A New York Post article, citing the International Earth Rotation and Reference Systems Service (IERS), confirms the trend and adds that the next two short days are likely to follow a similar pattern.
In response to these changes, IERS — the global authority on timekeeping — is expected to introduce a “negative leap second” in 2029 to balance global time systems. This would mark the first time such a correction has been implemented.
“This is an unprecedented situation and a big deal,” NY Post quoted Duncan Agnew, a geophysicist at the Scripps Institution of Oceanography, University of California, San Diego, as saying.
“It’s not a huge change in the Earth’s rotation that’s going to lead to some catastrophe or anything, but it is something notable,” he added.
Why Earth’s Rotation Isn’t Always the Same
A day on Earth is defined by the time it takes to complete one full spin on its axis — about 86,400 seconds or 24 hours. But this isn’t fixed. Factors like the positions of the Sun and Moon, magnetic shifts, and how Earth’s mass is distributed all play a role in slightly changing the rotation speed.
In the past, Earth’s rotation was much faster. Scientists believe that 1 to 2 billion years ago, a day lasted just 19 hours, likely due to the Moon being closer and exerting stronger gravitational force.
As the Moon has moved farther away, Earth’s rotation has generally slowed, lengthening days. But this trend hasn’t been consistent. For example, in 2020, scientists observed that Earth was spinning faster than at any time since precise records began in the 1970s.
On July 5, 2024, Earth recorded its fastest rotation ever, completing a full spin 1.66 milliseconds quicker than 24 hours, as reported by timeanddate.com.
What Happened on July 9 and What’s Ahead?
On July 9, Earth experienced a faster spin due to the Moon being farther from the equator and closer to the poles. This altered its gravitational pull on the planet’s axis—similar to spinning a top faster when pushed from the edge rather than the center.
The same celestial alignment is expected again on July 22 and August 5, which will likely result in two more days that are fractionally shorter than normal.
While the difference is too small to be felt in daily life, it’s enough to matter for scientific measurements and timekeeping systems — and may soon require historic adjustments to how we track time.