Earth’s Rotation Is Speeding Up — Here’s What It Means for Timekeeping

For most of human history, our ancestors kept time by the sun—the interval between one sunrise and the next marked the passage of what we call a day. But then, in the 1950s, scientists invented atomic clocks capable of far more precise measurements. A second is now formally defined as roughly 9 billion vibrations of a cesium atom, and a day consists of 86,400 such seconds, according to the International Bureau of Weights and Measures (BIPM).

Besides allowing people all over the world to coordinate in ways that were once unimaginable, atomic clocks also revealed that Earth’s rotation isn’t as consistent as it seems: a day doesn’t always last 86,400 atomic seconds. For various reasons, the time it takes our planet to complete one revolution on its axis changes from century to century, from month to month, and even from day to day.

The overall trend is toward longer days — that is, slower rotation — but in recent years, Earth has been speeding up. According to Time & Date, July 5, 2024, was the shortest day on record, and several days last summer came close to stealing the title. That said, Duncan Agnew, professor emeritus of geophysics at the Scripps Institution of Oceanography, notes that “on record” simply means since the birth of atomic clocks.

“If you go back more than a century,” he told Discover, “all the days before that were shorter.”

These changes are infinitesimal from a human perspective; July 5, 2024, was a mere 1.66 milliseconds shy of atomic clock time, and the average day during the Roman Empire was only about 40 milliseconds shorter than it is today, according to Agnew. Still, even these minute differences must be accounted for to keep our deeply interconnected global society running smoothly. And over longer timescales, milliseconds really add up — in the 66 million years since dinosaurs went extinct, the day has lengthened by half an hour, according to a 2020 study in Paleoceanography and Paleoclimatology.

Read More: Climate Change May Be Making Our Days A Little Longer — Here’s How We Know

Why The Planet Speeds Up And Slows Down

To understand why Earth’s rotation can speed up or slow down, consider a spinning figure skater: when she extends her arms and legs, spreading her mass farther away from her rotational axis, she spins slower; when she pulls them closer to her rotational axis, she spins faster. This basic physics principle is known as the conservation of angular momentum.

Earth, like a figure skater, must obey the law of physics. So when some of its mass shifts from one place to another, its overall speed adjusts to compensate. One of the biggest effects comes from changes deep within the planet’s molten interior, which can exchange angular momentum with the rest of the planet. For unknown reasons, Agnew said, “the core decided to go more slowly, and the rest of the Earth sped up as a consequence.”

That phenomenon is largely responsible for the unusually short days in recent years

But lots of other factors play a role in Earth’s speed, too. In a 2024 paper in Nature, Agnew showed that climate change has to some extent counteracted the core’s influence — as the polar ice caps shrink, meltwater circulates throughout the oceans and slows the entire planet. Any redistribution of mass can slightly alter Earth’s speed one way or the other: seasonal weather patterns, animal migrations, and even tree leaves — each spring they raise biomass high above the ground, then drop it back down in the fall, according to Auburn University.

How the Moon Slows the Earth

When you look at Earth’s history in full, though, it becomes clear that melting ice and core fluctuations don’t amount to much. Their effects are dwarfed by one steady force: the moon.

Its gravitational pull sends ocean tides crashing against the continental shelf day in and day out, and that relentless tidal friction has been slowing our planet’s rotation since the moon formed some 4.5 billion years ago.

If it weren’t for the fact that the sun will engulf the Earth in approximately 5 billion years, it would eventually become tidally locked to the moon and cease to rotate.

For decades, scientists believed that the gradual winding down would continue uninterrupted. “The assumption was that Earth would rotate more and more slowly,” Agnew told Discover. “The days would just get longer and longer.”

Thanks to the whims of our planet’s outer core, however, we’re now moving in the other direction, toward shorter days. That trend won’t last forever — in the long run, tidal friction will win out — but it’s already causing trouble for global timekeeping.

Will We Need A Negative Leap Second?

The National Institute of Standards and Technology established the leap second system in 1972 to offset the difference between Earth’s actual daily rotations and the length of a “day” as measured by atomic clocks: whenever the two fall far enough out of alignment, the International Earth Rotation and Reference Systems Service announces that it’s time to insert an extra second.

In the decades since, we’ve only ever added seconds, and nobody gave much thought to the prospect of subtracting them. But the rule-making body for such things, the General Conference on Weights and Measures, requires that Coordinated Universal Time (as measured by atomic clocks) match Earth’s rotation rate to within a second. If the planet keeps speeding up, the only remedy will be a negative leap second.

Among some members of the timekeeping community, that inspires a fear akin to the Y2K panic: what if our digital systems aren’t prepared when the time comes, and widespread infrastructural failure ensues?

“If something goes wrong,” Agnew said, “it will be something we haven’t thought of because if we thought of it, we would have dealt with it in advance.”

To avoid this, it’s likely the relevant authorities will do away with leap seconds altogether, positive and negative. According to the BIPM, as early as 2027, timekeeping regulations may be adjusted to allow a full hour of mismatch between Coordinated Universal Time and Earth’s rotation — which, as Agnew put it, “effectively removes the problem into the quite distant future.”

However this temporal snafu gets resolved, the world will keep turning — sometimes faster, sometimes slower, but never quite as predictably as a clock.

Read More: Earth Is Spinning Faster Now Than It Was 50 Years Ago

Article Sources

Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article: