Ever feel like there’s just not enough time in the day? As it turns out, you may be onto something. The Earth is rotating faster than it has in the last half-century, causing our days to be slightly shorter than we are used to. And, despite the fact that it is an infinitesimally small difference, it’s become a big headache for physicists, computer programmers, and even stockbrokers.
Why Earth rotates
Our solar system formed about 4.5 billion years ago, when a dense cloud of interstellar dust and gas started to collapse in on itself and began to spin. There are remnants of this original movement in our planet’s current rotation, thanks to angular momentum — essentially, “the tendency of the rotating body to continue rotating until something actively tries to stop it,” explains Peter Whibberley, a senior research scientist at the UK’s National Physical Laboratory.
Our planet has been spinning for billions of years due to angular momentum, and we experience night and day. However, it has not always spun at the same rate.
Earth made about 420 rotations in the time it took to orbit the Sun hundreds of millions of years ago; we can see evidence of how each year was jam-packed with extra days by examining growth lines on fossil corals. Although days have gradually grown longer over time (due, in part, to how the moon pulls at Earth’s oceans, which slows us down a bit), during humanity’s watch, we’ve remained consistent at about 24 hours for a full rotation — which translates to about 365 rotations per trip ’round the Sun.
However, as scientists have gotten better at observing Earth’s rotation and keeping track of time, they’ve discovered that there are only minor fluctuations in how long it takes to complete a full rotation.
A new way to track time
Scientists developed atomic clocks in the 1950s that kept time by observing how electrons in cesium atoms fell from a high-energy, excited state back to their normal ones. Because the periods of atomic clocks are generated by this unchanging atomic behavior, they are not affected by external changes such as temperature shifts in the same way that traditional clocks are.
Over the years, though, scientists spotted a problem: The unimpeachably steady atomic clocks were shifting slightly from the time that the rest of the world kept.
“As time goes on, there is a gradual divergence between the time of atomic clocks and the time measured by astronomy, that is, by the position of Earth or the moon and stars,” says Judah Levine, a physicist in the time and frequency division of the National Institute of Standards and Technology. Basically, a year as recorded by atomic clocks was a bit faster than that same year calculated from Earth’s movement. “In order to keep that divergence from getting too big, in 1972, the decision was made to periodically add leap seconds to atomic clocks,” Levine says.
Leap seconds function similarly to the leap days that we add to the end of February every four years to compensate for the fact that Earth orbits the Sun in 365.25 days. However, unlike leap years, which occur on a regular basis every four years, leap seconds are unpredictable.
The International Earth Rotation and Reference Systems Service monitors the speed of the planet’s rotation by sending laser beams to satellites to measure their movement, among other methods. When the time plotted by Earth’s movement approaches one second out of sync with the time measured by atomic clocks, scientists all over the world coordinate to stop atomic clocks for one second, at 11:59:59 pm on June 30 or December 31, to allow astronomical clocks to catch up. There you have it — a leap second.
Scientists have added leap seconds every few years since the first one was added in 1972. They are added in an erratic manner because the Earth’s rotation is erratic, with intermittent periods of speeding up and slowing down that disrupt the planet’s millions of years-long gradual slowdown.
“The rotation rate of Earth is a complicated business. It has to do with exchange of angular momentum between Earth and the atmosphere and the effects of the ocean and the effect of the moon,” Levine says. “You’re not able to predict what’s going to happen very far in the future.”
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.
“This lack of the need for leap seconds was not predicted,” Levine says. “The assumption was, in fact, that Earth would continue to slow down and leap seconds would continue to be needed. And so this effect, this result, is very surprising.”
The trouble with leap seconds
Scientists may have to take action depending on how much the Earth’s rotation speeds up and how long that trend continues. “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.”
A negative leap second, on the other hand, would present scientists with an entirely 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.
“The primary backbone of the internet is that time is continuous,” Levine says. Things fall apart when there isn’t a steady, continuous flow of information. Repeating or skipping a second confuses the system and can cause gaps in what is supposed to be a continuous stream of data. Leap seconds also pose a problem for the financial industry, where each transaction must have its own unique time stamp — a problem that could arise if that 23:59:59 second repeats itself.
Some companies, such as Google, have sought their own solutions to leap seconds. On a leap second day, instead of stopping the clock to allow Earth to catch up with atomic time, Google extends each second by a fraction of a second. “That’s a way of doing it,” Levine says, “but that doesn’t agree with the international standard for how time is defined.”
Time as a tool
In the grand scheme of things, however, we’re only talking about a fraction of a second every couple of years. You’ve probably lived through a slew of leap seconds without even realizing it. And, if we consider time to be a tool for measuring things we see in the world around us, such as the transition from one day to the next, there’s an argument to be made for following the time set by Earth’s movement rather than the electrons in an atomic clock — no matter how precise they may be.
Levine believes that leap seconds aren’t 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.
Nonetheless, he admits that, while time is just a construct, a decidedly human attempt to make sense of our experiences in a big, strange universe, “it’s also true that you have the idea that at 12 o’clock noon, the Sun is overhead.” So, even if you don’t think about it often, you have a connection to astronomical time.” Leap seconds are just a small, almost imperceptible way of keeping that link alive.