Across the Northern Hemisphere, people are soaking up the long hours of summer sunlight, with no idea that they’re experiencing some of the shortest days since modern timekeeping began—by a millisecond or so.
July 9 was the shortest day of the year because Earth spun faster—1.34 milliseconds quicker than the standard 24-hour rotation, according to the International Earth Rotation and Reference Systems Service (IERS) and U.S. Naval Observatory. This summer it’s predicted we’ll see two more historically short days—July 22 and August 5.
Fluctuations in Earth‘s spin rate aren’t uncommon, but scientists are mulling the reasons behind a recent surge in faster rotations.
“Over the past decade, the average length of day has mostly been shrinking. This has been especially notable over the past five years or so, sometimes clocking in at less than 24 hours,” says Nicholas Stamatakos, an astronomer at the U.S. Naval Observatory’s Earth Orientation Department.
Here’s the science behind why Earth is spinning faster as it orbits the sun, if scientists think it will last, and why even the odd millisecond might impact critical technologies.
Why is Earth spinning faster?
Variations of Earth’s rotation are due to a complicated mix of factors but scientists say a few stand out.
The moon affects Earth differently during its orbit. When it’s positioned close to the equator twice a month, gravitational pull slows Earth’s spin a bit. But the moon also moves nearer the poles twice a month, and at those times the moon’s gravity helps Earth spin a bit faster.
Our planet and atmosphere rotate in sync, sharing a momentum that can’t be changed by only one of the two. Our atmosphere spins more slowly during the summer due to variations in the jet stream. This means the planet must speed up, to compensate and maintain the system’s angular momentum at a constant level.
Inside the planet, something weird is also happening that physicists can’t totally explain. For some reason, Earth’s core has been slowing down for the last 50 years, and to maintain angular momentum, the solid Earth above has been spinning a bit faster to compensate. “We don’t know why, or what the core might do in the future,” says Duncan Agnew, a geophysicist at Scripps Institution of Oceanography.
(Earth’s inner core is doing something weird.)
Are these the shortest days in history?
The past few years have seen the shortest days since atomic timekeeping began in 1955, but they aren’t even close to the shortest days in Earth’s history.
On average Earth’s days have been growing longer for several billion years. Since the moon was formed, its gravity and influence on ocean tides incrementally slows Earth’s rate of rotation.
(Can the moon influence human health? Surprising research suggests it might.)
Analysis of ancient mollusk shells from the dinosaurs’ sunset era reveals that 70 million years ago the year’s 372 days were only 23.5 hours long. Fossilized corals from 430 million years ago suggest that far more frequent sunrises and sunsets once framed a 21-hour day.
Even as the long-term trend lengthens days, short-term fluctuations have produced plenty of short-day periods. “The days were all shorter than the upcoming ones in the 1820s and from about 1865 to about 1880,” says Agnew. “According to the IERS there were actually shorter days in June/August 2022 and June 2024.”
Will you be able to notice sped-up days?
Days of long daylight are obvious, like those around the summer solstice. So are the long, cold nights before and after December 21.
But if Earth’s 24-hour cycle was shortened by the blink of an eye you likely wouldn’t notice: A blink takes 100 to 400 milliseconds, while the difference in day length is only about 1 millisecond.
“A single, isolated shorter day is important only for astronomers and other specialists,” says Judah Levine, a physicist and Fellow of the National Institute of Standards and Technology. Astronomers need extremely accurate timekeeping to understand the positions and movements of celestial objects and events—tiny discrepancies can create big errors.
When time lost to shortened days begins to add up, timekeepers add an extra second to the running clock, bringing astronomical time back in sync with the atomic time it had lagged behind. This is called a leap second, and it’s been implemented about every year and a half since 1972—most people don’t notice that either.
Will a faster-spinning Earth affect technology or timekeeping?
Slight changes to Earth’s rotation didn’t matter much until atomic timekeeping arrived in 1955. Atomic time is constant, so Earth’s varying spin creates a gap between astronomical and atomic time.
To let astronomical time catch up, leap seconds have been added 27 times since 1972. Now that Earth is speeding up timekeepers may implement a first-ever negative leap second by 2029 to slow astronomical time.
Leap seconds have plagued computers, GPS, and telecommunications, most notably in 2012 when Linux and other systems hiccuped at the adjustment. An unprecedented negative leap second could prove more problematic since software systems may assume that time always moves in the same direction. “It could have a devastating effect on the software relying on timers or schedulers,” Meta engineers warn.
How climate change impacts Earth’s rotation
Studies suggest that factors like melting ice, rising seas, and depleted groundwater are changing Earth’s mass and slowing its spin. This slows its rotation in the same way an ice skater lowers her arms from overhead to stall her spin.
(The world’s glaciers are melting faster than we thought.)
Historically such variations have been the result of natural cycles but a pair of NASA-funded studies suggests that time itself may be feeling climate change’s impact on the Earth.
“Our analysis shows that in the past century, modern climate change alone has contributed to length of day by about 0.6 to 0.7 milliseconds, and that rate will likely double in this century,” says Surendra Adhikari, an earth systems scientist at the Jet Propulsion Laboratory.
Such a slowdown could be at odds with the factors recently working to speed Earth’s spin, highlighting the complexity of the planet’s rotation.
(Take a look inside Earth’s interior.)
Stamatakos stresses that while we’ve seen Earth’s spin vary for centuries, the complicated interplay of its different causes still presents a challenge for scientists—especially when looking forward.
“Accurate predictions of length of day beyond a six-month time frame are not possible,” he warns, “due to the incomplete understanding of all these geodetic phenomena.”
So, while the sun will keep coming up tomorrow, it’s hard to say exactly how long each day will last.
