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Thanks to Brilliant for supporting this episode of SciShow.
Go to Brilliant.org/SciShow to learn more. {♫Intro♫} 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♫}
