Earth’s rotation has unexpectedly sped up, shortening our days and leaving scientists puzzled. While the difference is almost imperceptible, it’s creating significant challenges in timekeeping and technology. Atomic clocks now struggle to stay in sync with Earth’s rotation, and scientists may need to remove a “negative leap second” to compensate. This change in Earth’s spin has implications for industries that depend on precise time, like finance and telecommunications. Researchers are still uncertain about why Earth’s slowdown has paused after decades of adding leap seconds to adjust time.

Ever feel like there’s not enough time in the day? It turns out you may be right. Earth has been spinning faster than it has in the past fifty years, slightly shortening our days. While this difference is almost imperceptible, it has caused complications for physicists, computer programmers, and even stockbrokers.

Our solar system began forming about 4.5 billion years ago when a dense cloud of interstellar gas and dust collapsed and began to rotate. This original spin is still affecting Earth today through angular momentum—“the tendency of a spinning object to keep spinning unless something forces it to stop,” says Peter Whibberley, a senior research scientist at the UK’s National Physical Laboratory.

This angular momentum has kept Earth spinning for billions of years, giving us night and day. However, Earth hasn’t always spun at the same rate.

Hundreds of millions of years ago, Earth completed about 420 rotations during its orbit around the Sun. Evidence of these extra days can be found in fossil corals. Over time, days have gotten longer, partly due to the moon’s gravitational pull on Earth’s oceans, which slows the planet down. However, for most of human history, Earth’s rotation has remained stable at

 around 24 hours, resulting in roughly 365 rotations during each orbit of the Sun.

As our ability to observe Earth’s rotation has improved, scientists have noticed small variations in how long it takes for a full rotation.

In the 1950s, scientists developed atomic clocks, which measure time by observing how electrons in cesium atoms move between energy states. Since atomic clocks don’t rely on external factors like temperature, they are more accurate than traditional clocks.

However, scientists noticed a discrepancy: atomic clocks were gradually falling out of sync with the time measured by Earth’s rotation.

“As time goes on, atomic clocks and time based on astronomical observations, such as the position of the Earth, moon, and stars, begin to diverge,” says Judah Levine, a physicist at the National Institute of Standards and Technology. Essentially, a year according to atomic clocks is slightly faster than a year calculated by Earth’s rotation. To correct this, in 1972, scientists began adding leap seconds to atomic clocks periodically, says Levine.

Leap seconds function similarly to leap years, which account for the fact that Earth’s orbit around the Sun actually takes about 365.25 days. However, unlike leap years, leap seconds are unpredictable.

The International Earth Rotation and Reference Systems Service monitors Earth’s rotation by using lasers to track satellites and other techniques. When Earth’s rotation is nearly one second out of sync with atomic time, scientists worldwide stop atomic clocks for one second at either 11:59:59 pm on June 30 or December 31, allowing astronomical time to catch up—thus creating a leap second.

Since the introduction of leap seconds in 1972, they’ve been added irregularly as Earth’s rotation speeds up and slows down. These fluctuations interrupt the planet’s long-term trend of gradually slowing down.

“Earth’s rotation rate is influenced by complex factors like the interaction between Earth and the atmosphere, ocean effects, and the influence of the moon,” Levine explains. “It’s hard to predict what will happen in the future.”

But over the past decade, Earth’s slowdown has unexpectedly paused. The last leap second was added in 2016, and Earth is currently spinning faster than it has in the past 50 years. Scientists aren’t sure why.

“This lack of need for leap seconds wasn’t anticipated,” says Levine. “It was assumed Earth would keep slowing down, so this result is surprising.”

If Earth’s rotation continues to speed up, scientists may need to take new measures. “There’s concern that if Earth’s rotation rate keeps increasing, we might need to implement a negative leap second,” Whibberley explains. “Instead of adding a second, we’d need to remove one from the atomic clock to stay in sync with Earth.”

However, a negative leap second would create new problems. “No one has ever implemented a negative leap second before, and there’s concern that software systems might not be able to handle it,” Whibberley adds.

Whether positive or negative, leap seconds create significant issues for industries like telecommunications and navigation, where accurate and continuous time is crucial.

“The internet relies on continuous time,” says Levine. When time isn’t steady, it can disrupt data transmission, causing errors. The financial sector, which relies on precise timestamps for transactions, also faces challenges when seconds repeat or are skipped

Some companies, like Google, have found workarounds. Instead of stopping time for a leap second, Google gradually extends each second on leap second days. “That’s one solution,” Levine notes, “but it doesn’t match the international standard for timekeeping.”

Although leap seconds adjust only small amounts of time—one second every few years—most people don’t notice them. If we think of time as a way to track the world around us, like the transition from day to night, there’s an argument for aligning time with Earth’s rotation, even if atomic clocks are more precise.

Levine personally questions whether leap seconds are worth the trouble: “I think the fix causes more problems than the issue itself.” If we stopped adjusting for leap seconds, it could take over a century for atomic clocks to be a minute ahead of astronomical time.

However, he acknowledges that while time is a human construct meant to help us understand our experience in the universe, “there’s still the idea that at noon, the Sun is overhead. Even if you don’t think about it, you’re connected to astronomical time.” Leap seconds help preserve that connection.