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Wait, wtf is happening to Earth?

On June 29, the planet experienced the shortest day since scientists started keeping records in the 1960s. It completed a full rotation 1.59 milliseconds faster than usual. That doesn’t sound like much but here’s why it has left scientists entirely gobsmacked.

Why the Earth spins: 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.” And thanks to the laws of physics—“the tendency of the body that's rotating, to carry on rotating until something actively tries to stop it”—Earth has been merrily spinning on its axis ever since. 

But, but, but: That movement is not constant. The speed is affected by Earth’s own core and surface winds—and also the gravitational pull of nearby objects. Like the Moon. For billions of years it has been tugging at us, slowing us down, ever so slightly:

“The moon’s gravitational pull creates a slight bump in the solid surface of the Earth, near to, but not exactly underneath where the moon is. The disparity between the bump’s position and the moon’s pull creates a torque on both the Earth and moon with the end result that the Earth slows down gradually. That rotational energy is transferred to the moon, which is moving away from the Earth ever so slowly, at a rate of about an inch and a half every year.”

So it pulls while pushing itself further away.

The long slowdown: 1.4 billion years ago, a day was just 18.7 hours—and the moon was likely around 27,000 miles closer to us. The day lengthened to 22 hours around hundreds of millions of years ago. It hit 24 hours by the time humanity came along—and we’ve stayed around that mark ever since. Scientists expect our days to be even longer in the millennia to come.

The need for speed: Earth’s gain of momentum is, therefore, a bit astonishing. In fact, the planet has been spinning faster for a number of years. In 2020, we experienced 28 shortest days on record—and with each passing year, the planet has gained more speed. At least some scientists don’t expect the trend to reverse itself anytime soon.

So why is this happening?

There are plenty of theories but no proven explanation.

The Chandler Wobble: As Earth spins on its axis, it also wobbles—much like a spinning top. Scientists don’t know exactly why, but they can measure it like so:

“Imagine a gigantic ballpoint pen poked through the centre of the earth, entering at the South Pole and exiting at the North Pole. Imagine the pen is drawing on a scratch pad-equipped space station directly over the North Pole. After a day (one full rotation of the earth on its axis) the ballpoint pen draws a circular path, and not a dot, because of the ‘wobble’ in the earth's rotation on its axis.”

The wobble tends to increase the amount of time to make a rotation—i.e the length of the day. The problem is that the wobble has become less wobbly. It literally disappeared from 2017 to 2020—which is speeding up the spin. And the planet reached the historic minimum just as the days started to shorten.

The Earth’s core: The wobble itself may be affected by Earth’s core:

“Inside the Earth is a molten core; its surface is a mass of shifting continents, swelling oceans and vanishing glaciers. The entire planet is wrapped in a thick blanket of gases and it wobbles as it spins on its axis. All of these influence the Earth’s rotation, speeding it up or slowing it down.”

Basically, things on our planet “slosh” around quite a bit. And anything that moves all that mass to the centre speeds up the rotation. Think of an ice skater who tucks her arms in to spin very fast, like so:

Climate change: Well, is there anything climate change is not responsible for? In this case, scientists aren’t all that sure. Back in 2007, German scientists modelled the likely effects of global warming on Earth’s speed. They found that rising ocean temperatures will push mass toward the upper latitudes—causing the rotation to speed up. They predicted that by 2200 the length of a day would be reduced by 0.12 milliseconds. But it isn’t considered to be serious:

“While ‘it is not something you will see on your watch,’ says Landerer, it’s comparable to the influence of the tidal friction of the Earth-moon system, which slows our planet down by 2.3 milliseconds every 100 years.”

Rising temperatures can also melt polar caps—reducing weight on either end. Melting polar ice sheets also decrease pressure on the Earth’s mantle—which started moving toward the poles at the end of the last Ice Age. 

OTOH: There are scientists who point out that climate change ought to lengthen our day—not shorten it: 

“When glaciers melt into the ocean, the shape of the Earth changes slightly, becoming flatter at the poles and bulging at the equator. But… this effect can’t explain why the planet suddenly would spin faster because melting glaciers should have the opposite effect: The planet’s moment of inertia would increase, which would slow us down.”

Earthquakes: A big earthquake can change the speed of the entire planet. The 2004 tsunami shortened the length of the day by around 3 microseconds, while the Great Tōhoku Earthquake of 2011 in Japan sped up Earth's rotation by a relatively tiny 1.8 microseconds. But these changes are very tiny—and can’t explain what is happening now. FYI: ocean currents can also achieve the same effect:

“In November 2009, events in the Southern Ocean made the Earth spin ever-so-slightly faster, shortening half of the days in the month by 0.1 milliseconds each. It turned out the powerful ocean current that rings the continent—the Antarctic Circumpolar Current—was to blame.”

Interesting, but why does this matter to me?

Ah, this is where pesky ‘leap seconds’ and universal time become important. And why tech companies are very, very cross at scientists.

The universal atomic clock: In the 1950s, scientists developed atomic clocks to more accurately measure time (a description of how they work here). But they realised that there was a growing lag between their clock time and astronomical time: 

“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.”

As we noted before, per astronomical time, Earth is getting slower—which meant atomic clocks were speeding ahead. So in 1972, scientists decided to periodically add a ‘leap second’ to keep the two in sync. 

The leap second: We add a leap day every four years to ensure we only have 365 days in any given year—even though it takes Earth 365.25 days to actually orbit the sun. The leap second is a teeny version of the same. Except, since Earth’s rotation is more erratic, there is no regular schedule for leap seconds. Here’s how they are added:

“[Scientists keep] tabs on how quickly the planet spins by sending laser beams to satellites to measure their movement, along with other techniques. When the time plotted by Earth’s movement approaches one second out of sync with the time measured by atomic clocks, scientists around the world coordinate to stop atomic clocks for exactly one second, at 11:59:59 pm on June 30 or December 31, to allow astronomical clocks to catch up. Voila—a leap second.”

So they literally stop the clock—it goes from 23:59:59 to 23:59:60 and then on to 00:00:00. So that day actually has 86,401 seconds. Since its introduction, we have added 27 leap seconds to date.

A negative leap second: If Earth is speeding up—and carries on until it is a second ahead of the atomic clock—then we will have to do the opposite: “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.” So the clock would jump from 23:59:58 to 00:00:00. Or it would stay at 23:59:59 for an extra second. Neither of which we’ve done before.

A tech nightmare: As you can imagine, even an added new second is extremely confusing for any tech device. The leap second created havoc in 2012—triggering a massive outage at servers owned by companies like Reddit and Mozilla. Back in July, Meta and other big tech companies like Google and Amazon teamed up to campaign against adding leap seconds—arguing that a scientific norm invented in the 70s is impractical in a digital world. 

Subtraction is worse: A negative leap second may be even more damaging, claims Meta: "The impact of a negative leap second has never been tested on a large scale; it could have a devastating effect on the software relying on timers or schedulers."  The internet is simply not built for it:

“‘The primary backbone of the internet is that time is continuous,’ [physicist Judah] Levine says. When there’s not a steady, continuous feed of information, things fall apart. Repeating a second or skipping over it trips up the whole system and can cause gaps in what’s 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 time stamp—a potential problem when that 23:59:59 second repeats itself.”

The bottomline: The happy news is that we likely won’t need to deduct a second for another seven to eight years. And Earth may—in its whimsical fashion—–slow back down instead… just because.

Reading list

Oddly enough, Daily Mail has the most detailed piece on the subject (who knew?). But if you object to getting information from tabloids, check out Inverse or CBS News. Discover Magazine is excellent on the leap second. Read Meta’s detailed blog post on why it hates all leap seconds.