If you haven’t accomplished as much this summer as you had hoped to, you can blame forces far beyond your control: a few of these dog days, by one measure, are among the shortest you’ve ever lived through.
For most of humanity’s history, we have measured time by the sun as it rises and sets—essentially, through Earth’s orientation to the cosmos surrounding us. But compare that technique with modern, superprecise timekeeping, and soon you’ll find that each day varies a bit in length at the scale of thousandths of a second. This summer a few factors are adding up to make a handful of Earth’s spins—those occurring on July 10, July 22 and August 5—more than a millisecond faster than the average of the past several decades.
Yes, there are scientists whose job is to track these things; no, they are not particularly concerned by these developments. “It’s a very small phenomenon,” says Christian Bizouard, an astronomer at the Paris Observatory and primary scientist at the International Earth Rotation and Reference Systems Service’s Earth Orientation Center. “There is nothing extraordinary [happening].”
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Bizouard has a point, of course—no one is going to notice the sun rising a millisecond earlier or later than we might otherwise expect. But tracking Earth’s rotation to this level of precision is vital because countless aspects of modern life rely on our ability to pinpoint locations to within a meter, and high-precision GPS navigation requires that satellites know exactly where they are compared with features on Earth’s surface.
So Bizouard and his colleagues track Earth’s orientation in space. To do so, they have enlisted astronomers all over the planet to monitor a collection of about 300 objects, he says, primarily the bright, very distant, supermassive-black-hole-powered objects known as quasars. All day, every day, pairs of distant observatories tuned to radio wavelengths of light check in on their specific object. By measuring the timing mismatch between light received at each station, scientists can calculate the precise location of the observatories and, in turn, the planet.
That’s how scientists know that the amount of time it takes Earth to complete one rotation varies slightly. But why does the planet’s speed vary? Even if you may never notice their loss, the missing milliseconds offer us a glimpse into the intricate oddities of the planet we live on—so let’s track them down.
Officially, time is defined by nine-billion-some vibrations of a cesium atom per second, 86,400 seconds per day. Inconveniently, Earth’s behavior isn’t governed by cesium atoms. Physics holds that, as a solid object moving in a vacuum, Earth ought to keep spinning at the same rate unless some outside force intervenes, says Duncan Agnew, a geophysicist at the Scripps Institution of Oceanography.
But Earth isn’t quite a simple solid object, and it has a rather large moon that can provide outside force. That means several different factors can affect Earth’s rotation speed.
Two of these factors—the core and the atmosphere—each affects Earth’s spin under a similar principle. The overall rotational speed of the Earth system must stay steady, so if a component’s movement changes, then the overall planet has to compensate. “The sum of all the rotations has to add up to the same thing,” Agnew says. “If part of the Earth is going slower, another part has to go faster.”
Take Earth’s core, for example, hiding below what we think of as the solid ground we walk on. Only the inner portion of the core is actually solid; the rest is fluid. “There’s this giant ball of molten iron about the size of the moon inside the Earth,” Agnew says. All that liquid metal (there’s a little nickel mixed in with the iron) is moving, creating the magnetic field that shields us from some of the many hazards of space.
The core’s activity is quite a mystery. The region isn’t actually all that far away—less than 2,000 miles from the surface, closer than New York City is from Los Angeles—but it cannot be directly accessed and is therefore very difficult to understand. In recent decades, for whatever reason, the core’s spin has been slowing, forcing the rest of Earth to speed up to compensate.
“The core is what changes how fast the Earth rotates on periods of 10 years to hundreds of years,” Agnew says. “The core has been slowing down for the last 50 years, and as a result, the Earth has been speeding up.” (This speed-up is part of why timekeepers have not implemented an artificial leap second—a tactic used annually during small stretches of the late 20th century—since 2016 and don’t expect to anytime soon.)
A similar phenomenon plays out in Earth’s atmosphere. Like the core, the atmosphere is a fluid mass—and although it’s a very complex one, scientists have much better insight into it than into the elusive core. The atmosphere changes with the seasons as the sun’s radiation falls disproportionately on different parts of the planet.
The Northern and Southern Hemispheres each have a primary polar jet stream, a river of strong wind flowing from west to east that wanders north and south as it carries weather around the planet. Because of Earth’s topography and the influence of ocean currents, the Southern Hemisphere’s jet stream is stronger overall than the Northern Hemisphere’s. And each jet stream is fastest during its hemisphere’s winter, slowing somewhat in local summer. Combine those factors and the Northern Hemisphere summer sees a small decrease in total speeds of westerly wind (those flowing west to east), Agnew says—forcing the solid Earth to spin a smidge more rapidly to compensate.
This atmospheric effect is why the rotation rate changes in an annual cycle, with the days when Earth rotates fastest tending to cluster in the Northern Hemisphere’s summer, particularly July and August.
To the extent that the core explains decadal changes and the atmosphere explains annual ones, the moon explains millennial and daily differences in Earth’s rotation rate.
At geologic timescales, Earth’s rotation is slowing down because of the moon’s tidal influences on the water that fills our planet’s oceans. The moon’s gravity sloshes water around, causing an infinitesimal friction between ocean and seafloor. “That’s been slowing the Earth down since the Earth had oceans,” Agnew says.
This trend doesn’t register to humans, but over time, the effect is quite noticeable. About 70 million years ago, shortly before the extinction of nonavian dinosaurs, a day was about half an hour shorter than it is today, for example. Wind the clock even further back, to 245 million years ago, when dinosaurs first came on the scene, and a day lasted a bit more than 22 and a half hours, scientists have calculated.
The moon causes a second phenomenon that affects Earth’s rotation on a human timescale. Beachgoers know full well that the moon’s gravity causes the seas’ daily high and low tides, and the solid Earth rises and falls a little bit in response to the moon as well, albeit not nearly as noticeably.
But the moon’s orbit doesn’t line up with Earth’s equator: our constant companion’s path is a bit tilted compared with Earth. Because of this, the tidal bulges wander north and south over the course of the moon’s loops around Earth. When the moon is right over the equator, the tidal bulges are, too, and therefore their mass is farther away from the planet’s spin axis; when the moon is the farthest north or south, the bulges move away from the equator, slightly closer to the planet’s spin axis. This taps into the same physics as a spinning ice skater with outstretched arms does when they hug their chest to speed up—Earth’s rotation rate speeds up just a hair when the moon is at the northernmost or southernmost point in its orbit, about every two weeks.
All these factors combine for the remarkably complicated state of Earth’s rotation rate: it is slowing over geologic time because of ocean friction but has been speeding up over recent decades because of the core, and its spin speed slightly increases every summer from the atmosphere and every two weeks from the moon’s north-south wandering.
The changes make such good sense in terms of physics that scientists like Bizouard are able to take variations in Earth’s rotation rate for granted. And scientists have some grasp of the annual and weekly changes in Earth’s spin rate, allowing them to expect the speedy summer days. But the mysteries of Earth’s core prevent these experts from confidently charting how Earth’s rotation will change into the future. “We are not able to predict anything,” Bizouard says.
Scientists put out predictions anyway, of course. As summer approached, they thought August 5 might be the shortest day of the year, a full 1.5 milliseconds shorter than usual. Current estimates still indicate that this day will be about that much shorter, and that August 18 may be another contender for the year’s fastest rotation. For comparison, the shortest rotation day in recent years was on July 5, 2024, when we lost 1.66 milliseconds.
Yes, you’ve probably now spent more time wrapping your mind around Earth’s quickest days than you’ve ever lost to the vagaries of our planet’s spin; I know I have. Let’s just call it another reason why we live on the most remarkable planet out there.