Today is a scientifically short day

57 5 minutes read

Ample sunshine, ice cream, and afternoons at the beach might leave you wishing that summer would never end. Unfortunately, this year the season is speeding by a smidge faster than normal. Through July and August of 2025, slight fluctuations in Earth’s rotation mean many of the days have been or will be measurably (though not conspicuously) shorter.

Today, July 22, is projected to be about 1.34 milliseconds under the typical 24-hours (86,400 seconds, where a second is defined by an atomic clock), according to a report from timeanddate.com. It’s the latest in a recent string of slightly abridged days. On July 10, the shortest of 2025 so far, Earth completed its daily spin 1.38 milliseconds faster than usual, per data from the International Earth Rotation and Reference Systems Service (IERS). At the beginning of August, multiple days in a row are forecast to all be more than a millisecond less than 24-hours.

Earlier projections predicted some of these days might be quick enough to be a record since atomic timekeeping began in the 1950s. But revised estimates suggest not. The current standing record for shortest day remains July 5, 2024, which was 1.66 milliseconds below 86,400 seconds.

The variation is all part of a trend in recent years where Earth’s rotation is speeding up. And though it may sound alarming, the acceleration it’s nothing to worry about, says Duncan Agnew, a geophysicist at Scripps Institution of Oceanography at UC San Diego. “It’s not something you’d notice,” he tells Popular Science.

Our planet’s day length has never been stagnant. Tidal friction from the moon, glacial melt, atmospheric winds and weather, and the motion of the molten core all have some sway on how fast Earth spins on its axis as it makes its annual orbit around the sun. Some of these forces act against each other, and all act on different timescales.

Shifting the Spin

Over the longest-term, the cumulative effect of the moon pulling on Earth, adding friction to its rotation, is slowing us down. About 70 million years ago, during the late Cretaceous, a dinosaur’s day on Earth would have been about 30-minutes shorter, as revealed by a detailed analysis of fossilized mollusk shells. In millions more years, we’d expect additional minutes tacked onto each spin cycle. Given enough time (about 50 billion years), the moon would eventually drag Earth’s rotation to a crawl, matching the speed of its own orbit around Earth through tidal locking. At this point, a day on Earth would last about a month–equal to one lunar cycle. Though we’ll likely never reach this point, as our sun is projected to expand into a red giant about 5 billion years from now, fully engulfing both Earth and the moon long before we’re set to stop spinning.

Then, there’s the movement of Earth’s core, which is countering that longest-term trend on the scale of decades. Our planet’s molten core also rotates (though more turbulently than the planet’s surface). This core movement is why and how Earth has a magnetic field: spinning iron soup generates electrical currents that add up to an electromagnetic effect. But in recent years, the core has slowed. Scientists aren’t exactly certain why, Agnew notes. Despite the fact that each U.S. coast is closer to the center of the Earth than it is to the other, “we really don’t understand very much about the core.” he says. Predicting its movement “is like trying to predict the weather without actually being able to see anything.”

What is certain though is that Earth’s surface, core, and atmosphere comprise a closed physical system. The law of conservation of angular momentum means that a spinning system must maintain its total rotational momentum, unless acted on by outside torque, explains Agnew. As the core movement slows, that lost speed must be picked up by another part of the system. So, counterintuitively, the surface starts to rotate faster. (It’s difficult to imagine in the abstract, but easy enough to demonstrate.) This core pattern explains the bulk of why days have been trending shorter for the last 50 years.

Similarly, the motion of our atmosphere also dictates surface movement. Atmospheric currents like the jetstream vary seasonally. In the summer, these perpetual winds weaken and slow. Again, thanks to angular momentum conservation, that motion is transferred to the surface instead, causing Earth to speed up. This is why, year-to-year, there are clear and predictable seasonal peaks and valleys in Earth’s length of day. Smaller fluctuations in weather and global wind speeds have an additional influence, Clark Wilson, a geophysicist and geodesist at the University of Texas Austin, tells Popular Science. As climate change alters weather patterns and redistributes water across the globe, it, too, is affecting the planet’s spin. Earth’s surface rotates slightly slower than it would otherwise because of ice cap melt, though the effect isn’t enough to overcome the impact of the core.

Finally, the moon becomes important again periodically, as its relative position to Earth in its elliptical orbit shifts how much friction it exerts. Twice a month, the moon is overhead directly at the equator, and twice a month it reaches a maximum north or south of the equator. At these maximum, off-center points, its pull is lessened, and Earth speeds up a little bit, says Agnew.

Minuscule Measurements

Though milliseconds of delay can add up to leap seconds over time, they don’t generally disrupt life on Earth. The reason multiple international bodies monitor and measure Earth’s tilt and rotation so closely is because the numbers do matter for precise Global Positioning Systems (GPS), like the type militaries rely on.

“The modern day interest in these variations is really for navigation purposes,” Wilson says. GPS depends on satellites orbiting Earth. When our planet changes speed (or the axis tilt varies), the relationship between those satellites and ourselves changes too. In order to triangulate near-exact locations on Earth, scientists need to know where the planet is in relation to GPS satellites. A one millisecond error in spin speed would translate to about 1.5 feet discrepancy in location, Wilson notes.

satellite above united states — Illustration of a Global Positioning System (GPS) Satellite. *Image: NASA*

To make such precise speed predictions, down to the hundredth of a millisecond, the IERS and other bodies depend on predictable effects like the moon position, tides, and seasonal atmospheric shifts, Agnew explains. They also incorporate global weather data, notes Wilson. Then, when it comes to actually measuring spin speed, an array of radio telescopes assesses our planet’s movement in relation to a very distant, functionally static celestial reference frame.

Predictions can be slightly off, so we won’t know exactly how fast today went until it’s over. But it’s a good bet that shorter days are yet to come. All the more reason to make the most of the summertime you’ve got left.