Hurry up! Scientists predict today will be (slightly) shorter than normal

If today feels like it's flying by, you can blame it on our spinning planet: A group of scientists tracking Earth's rotation predicts that the day will be a fraction of a second shorter than normal.

The International Earth Rotation and Reference Systems Service, which measures the Earth's rotation and forecasts the length of the day, has predicted that July 22 could come out about a millisecond short of a standard 24-hour spin.

Variations in the Earth's rotation are not uncommon: On July 9, for example, the Earth's quick spin shortened the day by nearly 1.4 milliseconds. But recent headlines from all over the world have been hyping "freakishly short" days in July and August.

Scientific experts who constantly monitor the Earth's rotation seem a bit baffled and amused that a few slightly more-speedy-than-normal spins this summer have captured the public's imagination.

"We've known about the rotation of the Earth being variable for about a hundred years," says Dennis McCarthy, the former director of time at the U.S. Naval Observatory. "This is just one of those little variations that comes along."

McCarthy, who's literally written the book on time and Earth's rotation, says the speed of the planet's spin gets influenced by all kinds of factors: the gravitational influence of the moon, the effect of winds and the atmosphere, plus the movements of the Earth's liquid core.

Round and round

Good data on the Earth's rotation goes back thousands of years, thanks to observations of solar eclipses in China. These days, researchers around the world track the spinning of the globe by pointing radio telescopes at quasars, incredibly luminous cores of distant galaxies that are so far away, they act as fixed points in space.

That kind of data, plus information from GPS systems, all gets sent to the International Earth Rotation and Reference Systems Service, which uses it to forecast the day length. Predictions are also made by others, such as the U.S. Naval Observatory.

Generally speaking, over millions of years, the Earth's rotation has been slowing down, a trend that's expected to continue into the future, says McCarthy. The main reason is the moon. The moon's gravitational pull on the Earth creates ocean tides, and a process called "tidal braking" gradually slows the Earth's spin.

"We know it's slowing down, because we have a real good idea of the effect of the moon," says McCarthy. "Although that slowing down is continuing, there are departures from that general pattern."

Specifically, over the last decade or so, there's been a bumper crop of days that are somewhat short.

Last year's shortest day, July 5, 2024, was 1.65 milliseconds shorter than the usual 86,400 seconds, says Thomas Herring, a geophysicist with MIT.

He says that was the shortest day since the 1950s, which is when researchers started comparing the rotation of the Earth to the very accurate time standard provided by atomic clocks that measure the regular oscillations of atoms.

In the past, the world's timekeepers have occasionally resorted to adding in some "leap seconds" — these extra seconds keep atomic time in sync with Earth's rotation as it slows. The last time an extra second was added to the clock was in 2016.

The recent speedy spins of the Earth, however, raise the possibility of instituting "negative leap seconds," or basically taking away a second rather than adding one, which is something that's never been done before.

"I think it will be unlikely that a negative leap second will be invoked," says Herring, given that regular old leap seconds seem to be falling out of favor.

While he and others say that the recent short-duration days are of little significance in and of themselves, understanding and being able to predict the precise rotation of the Earth is important for all kinds of applications—everything from launching missiles to navigation to high-tech farming.

At the equator, in one second, the Earth will rotate the length of four football fields, says Nick Stamatakos, head of the Earth orientation department at the U.S. Naval Observatory. "The Earth's moving pretty fast," he says. "So any little variations will accumulate, and it's an issue."

By looking at the recent rotation history along with information about weather systems and long-term patterns, researchers can make predictions about how the Earth will spin on any given day. These predictions generally get less accurate the farther out they go.

Climate changes the day

While the moon has long been the major player in controlling the speed of the Earth's rotation, one study found that humans are playing a role.

Climate change is melting ice at the poles and sending water down to the equator, slowing the rotation down. The researchers believe that this effect is already lengthening the days.

Some have calculated that in a worst-case scenario, climate change could eventually redistribute water in a way that would make the days milliseconds longer. That would make humans, rather than the moon, the dominant driver of changes to day length.

"We can really become the dominant drivers of Earth's rotation, due to human-induced climate change," says lead researcher Benedikt Soja of ETH Zürich, a university in Switzerland. "That was really surprising, and really an interesting revelation to us."
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