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Earth's rotation rate is speeding up.
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What's up with that?
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[Music]
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Well,
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how long does it take Earth to rotate
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once on its axis? We call that a day. We
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count up all the seconds and the minutes
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and the minutes and the hours and the 24
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hours of the day and you get 86,400
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seconds. If Earth were your only measure
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of the rotation of the Earth,
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how would you know if it was changing?
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You need some other timekeeping device
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that's not only more accurate, but more
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precise than Earth itself to have any
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clue if anything is changing about
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Earth's rotation. That began in the
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1950s where we introduced the atomic
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clock. Atomic clock keeps time just by
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vibrations within the atom and it's
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unrelated to the sun, the moon, the
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stars, the earth. It's keeping its own
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time. And when we did that and we kept
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good records of the rate at which the
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atomic clock was passing time and the
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rate at which Earth's rotation was
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passing time, we found a difference. And
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beginning in the early 1970s,
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we started compensating for that
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difference. The difference in almost all
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cases was that Earth was slowing down.
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So you accumulate a second in the
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slowing down of our rotation and then
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you throw in a leapsec to compensate for
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it. The alternative would be to change
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the definition of a second so that the
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86,400
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seconds
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still fills the day. But that is a level
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of complication that modern society is
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not prepared to accept. The duration of
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a second is precisely defined and we'll
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just add and take them away as the earth
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deems necessary. When do we do this?
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Well, just by decree. It's in one of two
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places on the calendar. Either in the
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last minute of December 31st of that
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year. So the last minute would have 61
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seconds in it or the last minute of June
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30th midyear where that would be given a
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leapse second that last minute
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containing 61 seconds. So since 1972
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when we had good enough atomic data we
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have accumulated last I checked 27
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leapse seconds from the slowing of the
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rotation of the earth. This monitoring
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is done monthly, weekly, daily and you
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look at the accumulations in one
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direction or another. Earth could also
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speed up. We have mechanisms in place
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for that as well. If we speed up by a
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full second, then we take out a leap
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second in one of those two spots in the
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year. So the last minute would have 59
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seconds instead of 61 seconds. All this
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is happening under the hood. Your
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computer will know about it, but you
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don't have to because
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milliseconds on a day, a second every
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now and then added or subtracted from a
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calendar really makes no difference to I
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I I'm betting you don't live your life
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time to the millisecond. I'm just
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thinking just that's just me making
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assumptions about you. So, lots of
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things affect the rotation of the Earth.
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Foremost among them is the sloshing of
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oceanic tides on and off the continental
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shelves. These tides are primarily
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raised by the moon. And so the earth and
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the moon do this kind of ballet where
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the moon raises the tides. The tidal
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bulge of the earth actually speeds the
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moon up in its orbit forcing it to
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ascend in its distance from us. So the
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moon is actually spiraling away from us.
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Not by much, few centimeters a year. And
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how do we know that? Well, you can
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calculate what it would be given this
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dual dance that's going on. Or you can
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stick mirrors on the moon, beam a laser
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to it, time the return trip, and
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calculate how far away the moon is,
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which is exactly what we did with Apollo
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11. Back in 1969, Neil Armstrong and
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Buzz Aldrin laid down what are called
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corner reflectors. Light coming in from
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any direction will reflect back exactly
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parallel to the direction the light came
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in. And so you come back to Earth, beam
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a laser out of your telescope, check the
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flash on its way back, time it, and
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there you have it. We confirmed that in
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fact the moon is spiraling away from
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Earth, as it's been doing for billions
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of years. The moon used to be much
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closer and much bigger in the sky. All
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right. Well, that's one thing that slows
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down the rotation of the Earth. Other
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things can slow it down or speed it up.
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For example, earthquakes. Continental
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drift shifts placement of mass on
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Earth's surface. You say, "Well, so it's
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still just a sphere." But no, you can
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picture an ice skater. They go into sort
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of a slow spin with their arms extended
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and then they slowly bring their hands
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in and they speed up. What they've done
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is they've brought mass of their body
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closer to the rotation axis. When you do
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that, a fundamental law of physics kicks
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in. The conservation of angular
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momentum. And so if you shrink an
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object, bringing your mass closer to the
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axis of rotation, you will speed up.
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Conversely, if you spread the mass
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farther out, you will slow down. Other
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things, the migration of animals can
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affect the rotation of the Earth.
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Migrations typically occur north south.
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Well, if you're going south, you're
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taking mass that's otherwise on Earth's
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surface and moving it farther away from
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the rotation axis as you approach the
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equator. You'll come to the equator,
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Earth will slow down, and then you go
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into the southern hemisphere, Earth
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speeds up again. Let's see other things
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that can affect Earth's rotation.
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Definitely the melting of polar ice, not
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only in Antarctica, but in Greenland.
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This is miles of ice that as it melts
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during our episodes of climate change,
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that water flows back into the ocean,
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creating a different distribution of
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mass on Earth's surface. Anything that
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goes from the pole towards the equator
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will slow down Earth's rotation. So yes,
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climate change is another force that
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will slow down our rotation. And so too
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can our liquid core. Earth has a heat
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source left over from its formation and
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from the decay of radioactive materials
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that has liquefied the core and you have
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movement in the core. That's what gives
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us our magnetic field. The movement of
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conducting metals within the core of the
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earth that changes where the mass is.
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All of these phenomenon can either speed
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up the rotation of the earth and slow it
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down. We get asked by how much tiny
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amounts milliseconds
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millisecond is a thousandth of a second
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and often it's just fractions of a
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millisecond. Now some of the headlines
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I've seen are earth is speeding up and
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we don't know why and it's mysterious.
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Really?
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I'd call that clickbait. It's possible
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to not know why something is happening
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yet have it not be mysterious at all.
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For example, let's say you own three
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dogs and you go out for dinner with your
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loved one and you come back and you find
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pee in the kitchen floor. You don't know
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which dog did it. But it's not
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mysterious. Not knowing something is not
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the same thing as something being
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mysterious. Not knowing what is the
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smoking gun that's speeding Earth up
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one day versus another does not make it
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mysterious. It means let's go to all of
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the factors that we know of. Could there
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be a factor we haven't considered yet?
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Maybe, but unlikely because we've been
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in the business of doing this for 50
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years. The universe brims with
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mysteries, don't get me wrong, but the
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forces that are altering Earth's
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rotation are not among them. This is
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tracked daily. We compare the rotation
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of the Earth to the atomic clock daily.
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And we know these numbers and they're
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they're fractions of a millisecond. Are
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they accumulating in one direction or
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another? Are they just canceling each
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other out from day to day? Of course,
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there's going to be a day in the year
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that has more of a shift than other
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days. That's how data work. You're going
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to make a federal case out of it. You're
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gonna make a headline of it. Biggest
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shift of the year. Yeah. Every year has
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a biggest shift in each direction.
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There's a biggest shift. I think someone
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found out they can make clickbait out of
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it. And that's what we're in the middle
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of right now. So rest calmly tonight or
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any other night because the daily
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changes in the rotation of the Earth.
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First, as I said, we only really care
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about it when it's accumulated a second
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and then we throw in or take out a
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leapse second. But I'm pretty sure
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there's nothing in your life as there's
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nothing in my life that requires that I
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keep track of time to the millisecond.
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So just contemplate the fact that we are
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on a spinning planet in orbit around the
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sun like a pirouetting dancer in a
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cosmic ballet choreographed by the
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forces of gravity. Until next time, keep
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looking up.
00:09:48
New findings may be shaking up
00:09:50
everything we thought we knew. Data
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suggests dark energy might be weakening,
00:09:55
meaning the universe's expansion could
00:09:58
actually be slowing down. If true, it
00:10:01
would challenge one of the biggest
00:10:02
assumptions in modern physics. But
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