The Earth is spinning faster today than 50 years ago

Have you ever felt like there just wasn’t enough time in the day? Turns out you might be onto something. The Earth is spinning faster than it has in the last half century, which means our days are a bit shorter than we are used to. And although this is an infinitesimally small difference, it has become a big headache for physicists, computer programmers and even stockbrokers.


Why the Earth Rotates

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 remnants of this original motion in our planet’s current rotation, thanks to angular momentum – essentially, “the tendency of the spinning body, to keep spinning until something actively tries to stop it” , explains Peter Whibberley, Principal Investigator. at the UK’s National Physical Laboratory.

Thanks to this angular moment, our planet has been rotating for billions of years and we live night and day. But it hasn’t always turned at the same pace.

Hundreds of millions of years ago, the Earth completed about 420 rotations in the time it took to orbit the Sun; we can see evidence of how each year was filled with extra days by examining the growth lines on fossil corals. Although the days have gradually gotten longer over time (partly due to the way the moon tugs on Earth’s oceans, which slows us down a bit), during humanity’s watch we have remained stable at about 24 hours for a full rotation – which translates to about 365 rotations per revolution of the Sun.

As scientists improved their ability to observe Earth’s rotation and keep track of time, they realized that we experience little fluctuation in the time it takes to complete a full rotation.


A new way to track time

In the 1950s, scientists developed atomic clocks that kept time based on how electrons in cesium atoms fall back from a high-energy excited state to their normal state. Since the periods of atomic clocks are generated by this immutable atomic behavior, they are not disturbed by external changes such as temperature changes as traditional clocks can.

Over the years, however, scientists spotted a problem: Impeccably stable atomic clocks drifted slightly off the time the rest of the world kept.

“Over time, there is a gradual divergence between the time of atomic clocks and the time measured by astronomy, that is, by the position of the Earth or the moon and the stars,” explains Judah Levine, physicist in the time and frequency domain. 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 the motion of the Earth. “In order to prevent this discrepancy from becoming too great, in 1972 the decision was made to periodically add leap seconds to atomic clocks,” says Levine.

Leap seconds work much like the leap days we add at the end of February every four years to compensate for the fact that it actually takes about 365.25 days for the Earth to orbit the Sun. But unlike leap years, which regularly occur every four years, leap seconds are unpredictable.

The International Earth Rotation and Reference Systems Service monitors the rotational speed of the planet by sending laser beams to satellites to measure their motion, along with other techniques. When the time traced by the movement of the Earth approaches one second off the time measured by the atomic clocks, scientists around the world coordinate to stop the atomic clocks for exactly one second, at 11:59 p.m. 59 s on June 30 or December 31, to allow astronomical clocks to catch up. Voila – a leap second.


Unexpected change

Since the first leap second was added in 1972, scientists have added leap seconds every few years. They are added irregularly because Earth’s rotation is irregular, with intermittent periods of speeding up and slowing down that interrupt the gradual slowing down of the planet that has been going on for millions of years.

“The Earth’s rotation rate is a complicated matter. It has to do with the exchange of angular momentum between the Earth and the atmosphere and the effects of the ocean and the effect of the moon,” says Levine. “You are not able to predict what will happen very far in the future.”

But over the past decade, the slowing of the Earth’s rotation 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 don’t know why.

“This lack of need for leap seconds was not intended,” says Levine. “The assumption was, in fact, that the Earth would continue to slow down and that leap seconds would continue to be needed. And so this effect, this result, is very surprising.


The problem with leap seconds

Depending on the acceleration of Earth’s rotations and the duration of this trend, scientists may need to take action. “There’s this concern right now that if the Earth’s rotation rate increases further, we might need to have what’s called a negative leap second,” Whibberley said. “In other words, instead of inserting an extra second to allow Earth to catch up, we need to remove a second from the atomic time scale to put it back together 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 problem is that the software that should handle that has never been operationally tested before,” adds Whibberley.

Whether it’s a regular leap second or a negative leap second, in fact, these tiny changes can be a huge headache for industries ranging from telecommunications to navigation systems. This is because leap seconds interfere with time in ways that computers are not prepared to handle.

“The main backbone of the Internet is that time is continuous,” says Levine. When there is not a steady and continuous flow of information, things fall apart. Repeating a second or skipping it triggers the whole system and can cause gaps in what is supposed to be a steady stream of data. Leap seconds also present a challenge for the financial industry, where each transaction must have its own unique timestamp – a potential problem when that second 23:59:59 repeats.

Some companies have looked for their own solutions to skip seconds, like Google’s swipe. Instead of stopping the clock to let Earth catch up with atomic time, Google lengthens each second a little bit on a second leap day. “It’s one way,” Levine says, “but it doesn’t fit the international standard for how time is defined.”


Time as a tool

In the grand scheme of things, however, we’re talking about very small amounts of time – just a second every two years. You’ve experienced many leap seconds and you probably weren’t even aware of it. And if we think of time as a tool to measure the 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 tracking time set by the movement of the Earth rather than the electrons in an atomic clock – however accurate.

Levine says he thinks leap seconds might not be worth the trouble they cause: “My personal 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 deviate even a minute from the “true” time recorded by atomic clocks.

Yet 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 vast and strange universe, “it’s also true that you get the idea that at noon the sun is overhead. And so you, even if you don’t often think about it, have a connection to astronomical time. Leap seconds are just a tiny, almost invisible way of keep this link alive.

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