Key Takeaways:

  1. Earth’s rotation has sped up in the last 50 years, resulting in slightly shorter days, which poses challenges for various industries.
  2. The fluctuating rotation speed of Earth has led to the periodic addition of leap seconds to atomic clocks to keep them in sync with astronomical time.
  3. Scientists track Earth’s rotation using techniques like measuring laser beams sent to satellites, but the reasons behind the recent acceleration remain unclear.
  4. The possibility of a “negative leap second” presents new challenges for industries reliant on precise timekeeping, such as telecommunications and finance.
  5. While leap seconds might seem insignificant, they highlight the intricate relationship between human-defined time and astronomical phenomena.

Ever feel like there’s just not enough time in the day? Turns out, you might be onto something. Earth is rotating faster than it has in the last half-century, resulting in our days being ever-so-slightly shorter than we’re used to. And while it’s an infinitesimally small difference, it’s become a big headache for physicists, computer programmers, and even stockbrokers.

Our solar system formed about 4.5 billion years ago, when a dense cloud of interstellar dust and gas collapsed in on itself and began to spin. There are vestiges of this original movement in our planet’s current rotation, thanks to angular momentum — essentially, “the tendency of the body that’s rotating, to carry on rotating until something actively tries to stop it,” explains Peter Whibberley, a senior research scientist at the UK’s National Physical Laboratory.

As scientists have improved at observing Earth’s rotation and keeping track of time, however, they’ve realized that we experience little fluctuations in how long it takes to make a full rotation.

In the 1950s, scientists developed atomic clocks that kept time based on how electrons in cesium atoms fall from a high-energy, excited state back to their normal ones. Since atomic clocks’ periods are generated by this unchanging atomic behavior, they don’t get thrown off by external changes like temperature shifts the way that traditional clocks can.

Leap seconds work a little like the leap days that we tack on to the end of February every four years to make up for the fact that it really takes around 365.25 days for Earth to orbit the Sun. But unlike leap years, which come steadily every four years, leap seconds are unpredictable.

Since the first leap second was added in 1972, scientists have added leap seconds every few years. They’re added irregularly because Earth’s rotation is erratic, with intermittent periods of speeding up and slowing down that interrupt the planet’s millions-of-years-long gradual slowdown.

But in the past decade or so, Earth’s rotational slowdown has … well, slowed down. There hasn’t been a leap second added since 2016, and our planet is currently spinning faster than it has in half a century. Scientists aren’t sure why.

Depending on how much Earth’s rotations speed up and how long that trend continues, scientists might have to take action. “There is this concern at the moment that if Earth’s rotation rate increases further that we might need to have what’s called a negative leap second,” Whibberley says. “In other words, instead of inserting an extra second to allow Earth to catch up, we have to take out a second from the atomic timescale to bring it back into state with Earth.”

But a negative leap second would present scientists with a whole new set of challenges. “There’s never been a negative leap second before and the concern is that software that would have to handle that has never been tested operationally before,” Whibberley adds.

Whether a regular leap second or a negative leap second is called for, in fact, these tiny changes can be a massive headache for industries ranging from telecommunications to navigation systems. That’s because leap seconds meddle with time in a way that computers aren’t prepared to handle.

In the grand scheme of things, though, we’re talking about very tiny amounts of time — just one second every couple of years. You’ve lived through plenty of leap seconds and probably weren’t even aware of them. And if we view time as a tool to measure things we see in the world around us, like the transition from one day to the next, then there’s an argument to be made for following the time set by the movement of Earth rather than the electrons in an atomic clock — no matter how precise they might be.

Levine says he thinks that leap seconds might not be worth the trouble they cause: “My private opinion is that the cure is worse than the disease.” If we stopped adjusting our clocks to account for leap seconds, it could take a century to get even a minute off from the “true” time recorded by atomic clocks.

Still, he concedes that while it’s true that time is just a construct, a decidedly human attempt to make sense of our experiences in a big, weird universe, “it’s also true that you have the idea that at 12 o’clock noon, the Sun is overhead. And so you, although you don’t think about it often, do have a link to astronomical time.” Leap seconds are just a tiny, nearly invisible way of keeping that link alive.

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