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My favorite second of the year is  December 31 at 11:59 and 59 seconds,   the moment right before a new year starts. So much anticipation, so much potential!

But the speed of the Earth’s rotation over the  last year has some astronomers and computer   scientists thinking that someday we might need  an unprecedented adjustment: deleting a second. And if we do, the 11:59:59 second  would be the one that gets the ax. Unacceptable!

So, there are generally two ways we keep time. One is just by looking up at the sky. If you  call when the Sun reaches its highest point   in the sky “noon,” and then you wait until the  Earth rotates around and then it’s “noon” again,   that’s exactly one solar day.

Then, there’s Coordinated Universal Time, or UTC. That has nothing to do with  what’s going on in the sky,   it relies on certain properties of atoms that  change both very consistently and very fast. For example, the electrons  around cesium atoms will   jump between two configurations more  than nine billion times per second.

Computer scientists figured out exactly how many  of those oscillations fit into a solar second,   and defined the second based on that number. So in theory, the two methods should agree  with each other. Except, they don’t, because   the Earth’s rotation is subject to all sorts of  changing forces that mess with astronomical time.

For example, the Moon’s  gravitational pull can slow us down. But there are also more unexpected,  unpredictable variables, like wind. You might have heard of the  El Niño or La Niña systems,   which are years-long trends in  the winds in the Pacific Ocean.

We usually hear them talked about in  regards to how they affect weather.   But they also tend to change Earth’s rotation  for reasons that are not totally intuitive. Like, a 2014 study in Geophysical Research  Letters looked at this with El Niño, specifically. One thing they studied was the Eastern  Pacific El Niños, which are the ones   that can lead to the biggest change in  Earth’s rotation — nearly a millisecond.

And according to this study, that might  have to do with the way they create   pressure systems around the  Earth’s biggest mountain ranges. These El Niños make big high-pressure  systems on the eastern sides of mountain   ranges that ring the Pacific — like the  Rockies, the Andes, and the Himalayas. Because the Earth rotates west to east,   those high-pressure systems, and the  low pressures on the western sides,   essentially use the mountains as levers against  the direction of rotation and they slow us down.

On the flip side, during La Niña  times, the opposite can happen,   almost like the wind is pushing the Earth  along its rotation by flicking its mountains. Another factor that affects the Earth’s rotation  speed is the amount of water in the oceans. Think of this like a figure skater doing a spin.   The closer they pull their arms  in, the faster they will turn.

That’s conservation of angular momentum at work. So when a lot of water is locked up in ice on land  near the poles, like on Greenland or Antarctica,   it’s like the Earth’s arms are  tucked way in, and it spins faster. But if that ice melts and  it flows into the oceans,   it’s like the arms are flinging  out the sides, and we slow down.

Every day, all of these effects — and more! —  combine to give us a day of a certain length. But the computer scientists’ atomic clocks   don’t care about how fast the Earth  spins. They keep ticking along regardless.

So to make sure we’re all on the same page,  whenever astronomical and coordinated universal   time disagree by more than a second, everyone  agrees to make an adjustment, a leap second. On average over the past 50 years  or so, we’ve only been slowing down,   so we’ve only ever had to add leap seconds. But last year, we were pretty fast.  We had 28 of the fastest ever days.

All summer, Earth seemed to have a little  extra pep in its step, and July 19,   2020 clocked in at a whopping 1.46  milliseconds faster than normal. Now it’s hard to know for  sure exactly why this is,   but it’s likely a combination  of the processes we mentioned. Like, the wind patterns in the Pacific are  in a La Niña phase, which should speed us up.

And maybe the distribution of mass in the ocean  was more figure skater-y than in previous years. In general, as climate change causes  more ice to melt at the poles and   flow toward the equator, that should slow us down. But in some cases, when a current near the  poles flows more slowly than normal and more   mass gets concentrated at the ends of the  planet, the Earth’s rotation will speed up.

That’s actually something that happened in  2009. So maybe it was true of 2020 as well. Beyond this, there are also  other factors that play a role,   like interactions inside the  Earth and the flow of the mantle.

Regardless, scientists expect  2021 to continue this trend,   and we might accumulate up to 19  milliseconds over the next year. That’s still 981 milliseconds away from a  full second, so even if the Earth keeps up   this pace it may take a while before we  need to delete a second from existence. But this change in trend is definitely unusual.

So researchers will continue to keep track of  how soon the sun comes up tomorrow — and why. And who knows? In a few years’ time, they might  be coming for 11:59 and 59 seconds on December 31.

And they can pry it from my cold… not dead hands.   I’m not going to hold onto  it that hard. It’s fine. Learning about the Earth’s rotation  involves a kind of surprising variety   of scientific fields, but that’s part  of what makes it such a good puzzle.

And if that’s the sort of thing you enjoy,   you might want to try out  Brilliant’s Daily Challenges. They’re questions about all kinds of science,  and are a great way to learn something new   every day. And with every challenge, Brilliant  also gives you all the context you need.

If you want to try today’s Daily Challenges  for free, go to Brilliant.org/SciShow.   And if you decide to sign up there, you’ll also  get 20% off an annual Premium subscription. {♫Outro♫}.