All language subtitles for how.the.universe.works.s08e09.earths.death.orbit.480p.web.x264.rmteam

The Earth is taking us
on the ride of our lives,

hurtling through space
in ways we never imagined.

The Earth is extremely dynamic.

It is spinning on its axis.

It's whirling about the Sun.

It's corkscrewing
throughout this galaxy.

It's just never a dull moment.

It's like doing a waltz
on top of a carousel

that's on top
of a high-speed train.

Now we're unlocking the secrets

of our planet's voyage and
discovering that Earth's journey

affects us all.

We see evidence of this motion
everywhere we look,

not just in the motions
of objects in the sky,

but in the land
and the seas themselves.

Life on Earth
wouldn't be the same

if we didn't find ourselves

in this dramatic environment
in space.

But the ride
can be dangerous...

Plunging our planet
into the deep freeze...

Putting us in the path
of supernovas,

pulling Earth and the entire
galaxy toward the unknown.

The galaxy is traveling
through space.

Where is it going?

It's an area of really
cosmic mystery.

So strap in for
Earth's cosmic journey.

-- Captions by vitac --
www.vitac.com

captions paid for by
discovery communications

We are Earth's passengers
as our home planet

travels through the cosmos.

To us, everything seems calm.

Nothing could be further
from the truth.

Contrary to what you might think

just based on your
everyday experience,

the Earth is actually
hurtling through space

at amazing speeds
in a lot of different ways.

Even just the motion of the
stars through the sky at night

gives you a clue that the Earth
is not sitting still.

The Earth is spinning
at every moment,

and we can see this most clearly
in the fact

that we have day and night.

We might not think about it,

but our lives are tuned

to Earth's journey
as it spins through space.

There's really nothing
more basic to us

than that cycle of day
and night, that 24-hour cycle.

All of life on Earth evolved
with the day-night cycle,

so it's ingrained into
every organism on this planet.

Life evolved in lockstep

with Earth's spinning motion.

But in the ocean, corals
take things a step further

by keeping a record
of every planetary turn.

They have a daily cycle,
which creates a deposit

almost like a tree ring,

but instead of it being
once a year, it's once a day.

So, you look a bunch
of tree rings,

you can count the number
of years the tree was alive.

Corals record not just
the yearly cycle,

but the daily cycle
of night and day.

These are recorded
in these little growth bands

in the coral.

By counting
the corals' growth bands,

we can work out
the number of days in a year.

Sounds simple, but when we look
at ancient, fossilized coral,

we discover something strange.

We can look at fossils of corals

that are hundreds of millions
of years old,

and if we do that, we find that
the year is not 365 days long.

It's more like 420 days long.

When the
ancient corals were alive,

there were 420 days
in one Earth year,

meaning a day
was just 21 hours long.

To find out how
this was possible,

we need to go back to the start
of Earth's cosmic journey.

4.6 billion years ago,

our planet traveled
a dangerous path

through a chaotic
and violent solar system.

Collisions were frequent.

One giant impact set our planet
spinning rapidly...

And formed the moon.

Going all the way back to
the time that the moon formed,

the Earth may have had a day
as short as 2 1/2 hours.

As the Earth
continued on its path

through the early solar system,

our planet cooled,
and the surface became solid.

But the violence wasn't over.

The young Earth was bombarded

in the early days
of the solar system,

and when these rocks
hit the Earth,

they almost never hit
directly on.

They'd hit at an angle.

With each collision, it adds
a little bit more momentum

and a little bit more spin
to the Earth.

The added spin that you get is
a kind of like a merry-go-round.

You can imagine with each kid
that pushes and jumps

on the merry-go-round,
you have greater spin.

As our planet
journeyed on, asteroid impacts

set the young Earth

spinning 12 times faster
than it does today.

Our planet's rotational speed
has huge consequences for life.

On Earth, the spin of our planet

actually has an effect
on our weather.

With a shorter day,

one of the effects that might
have been apparent on Earth

at that time
was more storms developing.

A phenomenon
still in action today

drove these powerful
ancient storms.

We call it the Coriolis effect.

The Earth's spin
creates phenomena

in Earth's atmosphere
and oceans.

This determines patterns
of circulation

in combination with
the heat energy from the Sun.

The rotation of the Earth
matched with solar heating,

especially at the equator,
causes air to rise up

and then also sort of to move
sideways and sets up spin.

As the young Earth
continued its journey,

the planet's rotation whipped up
ferocious, planet-wide storms.

The fast spin would have been
disastrous for any life.

The storms would have
been so big,

it's hard to say if life
would have evolved at all.

Fortunately
for humankind,

Earth has a
traveling companion --

the moon, and it helped
slow our planet's spin.

What happened next was kind of
a wonderful gravitational dance

between these two bodies.

As they were spinning,
they were also interacting

with each other.

The moon's gravity
pulled on Earth's oceans,

generating tidal bulges.

Materials moving in and out of
the tidal bulges as they spun,

and this creates friction
and a kind of drag

that actually slowed down
the rotation.

The tides also
helped create life.

Giant tides swept nutrients
from the land

into the oceans
for the first time.

A primordial soup began brewing.

And life arose.

As Earth's spin continued
to slow down,

life spread across the planet.

But our planet's spin is just
one part of our cosmic ride.

Realizing how complicated
our larger environment

in the universe is
is a wonderful thing.

There's so many things that
affect the orbit of the Earth,

the tilt of the Earth,
things that affect our climate.

Clues to Earth's
space voyage are hidden

all across the world.

Could our planet's
wild ride explain

how one of the driest places
on the planet was once wet?

The Sahara desert --
dry, dusty, desolate.

But hidden deep in a desert cave

is a clue that
thousands of years ago,

the Sahara was a lush,
green paradise.

Archaeologists have
unearthed rock art

which clearly depicts humans
and animals swimming in lakes,

and by looking
at satellite images,

we can trace out the outlines
of ancient river valleys.

The Earth's fast
rotation influenced

our planet's ancient
weather patterns.

Could another motion
have changed the desert climate?

To unravel the mystery,
we need to rewind the clock

4.6 billion years

to when the infant solar system

was a planetary
shooting gallery

And the Earth spun through the
solar system with a slight tilt.

Earlier on, before
the formation of the moon,

the Earth didn't have
much of a tilt,

and the impact knocked us
completely out of whack

to about 80 degrees.

Our planet might have
continued its journey

tilted right over,
but over billions of years,

the moon's gravity pulled
the Earth upright,

just not completely.

The moon is not quite in the
equatorial plane of the Earth.

It's above the equatorial plane,

and it pulls the Earth's axis
into a 23 1/2-degree tilt.

If we didn't have
the tilt to about 23 degrees,

then we wouldn't have
the seasons,

and these seasons drive a lot of
the crops and the growth

and the ability to survive
all across the globe.

But the ancient
greening of the Sahara

can't be explained
by the changing seasons.

Another planetary motion
must be in play.

A clue is found in the
night sky --

the north star.

Right now, the Earth's axis
is pointed towards a star

in the sky called
the north star -- Polaris.

We've actually named it
after the fact

that the north pole of the Earth
points toward it in the sky.

But Polaris hasn't
always been the north star.

5,000 years ago,
it was a totally different star.

It was Thuban,
which is in Draco,

a different constellation.

Sometime in the future,
it'll be Vega,

the brightest star
in the constellation Lyra,

so the north star
actually changes

because the north pole's
position in the sky changes.

The changing pole star
is evidence

that Earth is wobbling
through space.

It's a process called
precession,

something that also affects
spinning tops.

If you take a top
and let it spin really rapidly

and poke it,

the axis of rotation
will spin around like this,

and it's much slower than
the actual spin of the object.

That is precession, this circle
that the axis is making.

This happens to the Earth,
as well.

A cosmic poke caused
Earth's space wobble,

but what has the power
to poke a planet?

Once again, the key is gravity,

this time Earth's
gravitational interactions

with the Moon and Sun.

These forces are that poke
on the spinning Earth,

and that's what makes the
Earth's axis spin in precession,

and it takes about 20,000 years
to make a complete circle once.

As the Earth continues
its journey around the Sun,

this precession changes the
planet's climate dramatically.

When the Earth's north pole
leans toward the Sun,

northern hemisphere
summers are sunnier...

With unexpected consequences
for the Sahara.

It turns out
that at times when

the climate in the Sahara
gets more sunshine,

that warms the climate up,

and the winds come in
from the ocean.

It's called the monsoon effect
and brings water --

rain into the Sahara,
where now it's dry.

This pattern of
wet and dry climates

that produce this
20,000-year cycle

is what we see in the Sahara,

where at times
of strong monsoon,

the Sahara is wetter,
a green Sahara.

10,000 years ago,

as the Earth wobbled
around the Sun,

monsoons were unleashed,
turning the desert green.

Humans migrated to the
newly lush Sahara

and created the cave paintings
we see today.

But this wasn't the first time
humans crossed a green Sahara.

A dry Sahara
would have presented

an insurmountable barrier

to our ancestors
for migrating out of Africa,

but during one of these cycles
when the Sahara was green,

it would have been a corridor

that we could have migrated
and dispersed on.

As the Earth
traveled the cosmos,

our planet's precession
changed the Sahara

and the fate of humankind,
but there are more cosmic forces

affecting our journey
through the universe.

The Earth's orbit around the Sun
is another thing

that seems very, very solid.

We understand how it works,
but everything in the universe

is a delicately tuned
dance of gravity,

and things can change even
from tiny little influences.

As our planet
hurtles through space,

other worlds influence our path.

Could a planetary bully push
Earth's climate to the extreme?

Earth carries us on a
wild journey through the cosmos.

And clues to the effects
of this trip

are hiding in our own backyard.

In New York City, amongst the
buildings and traffic,

we find moraines,

rocks left behind
by retreating glaciers.

18,000 years ago,

a sheet of ice taller than any
skyscraper covered Manhattan.

Ice ages have struck regularly
throughout Earth's history,

putting our planet
in a deep freeze.

There was a period
in Earth's history

several hundred
million years ago,

the snowball Earth period,

when we went through
a very extreme glaciation,

if you will,
a very extreme ice age

where we think perhaps
the entire Earth

was covered in an ice sheet.

The trigger?

Earth's orbital dance
around the Sun.

We tend to think of ourselves

sitting relatively stationary
on the Earth.

It's pretty comforting,
actually,

but we're orbiting the Sun
at about 66,000 miles per hour.

Every day, Earth
travels over 1.6 million miles

on its journey
around the Sun.

This orbit isn't always
completely round.

Earth is generally going
around the Sun

in more or less
a circular orbit,

but over time, the massaging
of this orbit from the Sun,

from the moon
on the Earth's orbit

causes the orbit of the Earth
to change

so that sometimes
it's an ellipse.

Sometimes
it's more of a circle.

Right now on the Earth,

we're in kind of the most
circular time in the orbit,

so that means the summers
are relatively mild,

and the winters
are relatively mild,

but imagine not that long ago
in the past,

it could have been really
dramatically different.

When the Earth is a little bit
closer to the Sun,

maybe you have
a really severe summer,

and then on the other side
of the orbit,

you're a little farther away
from the Sun than normal,

so you have
a really severe winter.

Our environment is very,
very sensitive to these things,

and when the Earth's orbit
is stretched out,

that can actually
trigger an ice age.

Our planet's
100,000-year orbital cycle

caused the ice age
that buried New York.

And ice ages have had a big
effect on human history.

15,000 years ago,

plunging temperatures
locked water away

in glaciers and ice caps.

Sea levels dropped, creating
land bridges between continents.

Humans migrated from Asia
to America by foot,

and for the first time,
America was inhabited.

May 2018 -- scientists revealed
a whole new dynamic

to Earth's journey.

Every 405,000 years,

our planet's orbital voyage
stretches to the extreme,

and Earth's planetary neighbors
are to blame.

Because Jupiter is the most
massive planet

in our solar system,

it is in many ways the bully
on the playground, right?

Its dynamics, its gravity
sculpts a lot of the dynamics

of the solar system.

It actually tugs
and pulls on the orbit

of the Earth itself.

It's responsible for some of
the very changes

that drive our climatic cycle
here on our planet.

Jupiter isn't the only
bully in the playground.

Venus is
a fairly big planet

about the size of the Earth

and also comes closest
to us in its orbit,

so these two planets put just
a little tiny elongation

onto our Earth's orbit,

and as the cycle continues,
the more extreme it gets,

we can actually notice
a temperature difference

that happens about once
every 405,000 years.

Jupiter and Venus gang
up on Earth gravitationally,

pulling Earth's orbit
into an even greater ellipse.

Our planet's hot weather
becomes hotter,

and its cold weather
gets much colder.

Today, we're in a moderate
part of the cycle,

but in just
60,000 years time,

we could plunge
into another deep freeze.

It's a little bit like
a cosmic butterfly effect.

I mean, even
the smallest effects

can have, you know,
a big influence over time.

Earth's orbit around the Sun

is just part of our
far larger cosmic journey.

The entire solar system is
hurtling around the milky way,

taking us places
we don't want to be.

Sometimes our planet
might wander

into what's essentially
a bad neighborhood.

What dangers await us?

And could these neighborhoods
spell disaster

for life on Earth?

Over the last
3.7 billion years,

a series of extinction events
wiped out

almost 95 percent
of all species on Earth.

Now research suggests
our planet's orbit

could be partly to blame,

but not the Earth's orbit
around the Sun,

our planet's larger and longer
journey around the milky way.

Our solar system and our Sun
is shooting through the galaxy

at about 530,000 miles per hour
around the center of our galaxy.

And that center of the galaxy is
about 26,000 light-years away,

so it should take the Sun
about 230 million years

to trace out one full orbit
around the center of the galaxy.

Despite racing
around the milky way

at 1/2 million miles an hour,

Earth has completed less than
20 laps of the galaxy

in our planet's entire history,

and it turns out
this galactic ride

is more complicated
than it seems.

Now, if you look at the Earth
going around the Sun,

it defines an ellipse,

but that's a flat figure,
and you'd think,

"well, the Sun probably goes
around in a plane, as well,"

and it turns out
not that simple.

Most of the mass
of the solar system

is concentrated in the Sun,

so Earth and the other planets
smoothly orbit our star.

But the mass of the milky way
is spread out unevenly.

That changes the gravity
of the galaxy,

and so it changes how things
move in it, and in fact,

if you give something a little
bit of an up or down motion,

it'll bob up and down
as it goes around.

Riding the Earth is
almost like riding a carousel.

As the Sun and the Earth
go around the galaxy,

the Sun also goes up and down

like you're on one of
those horses with the pole,

and so what this can do

is take us into different
galactic environments.

This bobbing motion
takes Earth and the solar system

on a 60,000-year journey

up and down through
the milky way's galactic plane.

Our orbit also takes us

through different
galactic neighborhoods.

Today, we're traveling through
a calm suburb of the galaxy.

But sometimes things
get a little bumpy.

Sometimes our Sun and our planet
might wander

into what's essentially
a bad neighborhood.

You know, maybe it's an area

where there's a lot of
star formation going on,

where there's a lot of young
stars that are very active,

or maybe it's a location
where there are dying stars,

and things are about
to get really hot.

The baddest
neighborhoods in the galaxy

may be the spiral arms.

These gas-filled regions orbit
the galactic center

more slowly than the Earth,

so our planet
passes through an arm

roughly every 150 million years.

These arms are where
gas clouds tend to hang out,

and if they get compressed,
they form a lot of stars.

When they form a lot of stars,
they make bright, blue stars.

And they don't last long,
and they blow up as supernovae.

So it's possible that as we're
passing through these regions,

these are places you might not
want to be in,

so in the distant past,

this may have
affected the Earth.

When giant stars
go supernova,

the stars' outer layers
blast into space

along with a shock wave

traveling at
20,000 miles a second.

Supernovas also release
cosmic rays --

space bullets
that shoot across the galaxy

at close to the speed of light.

And you don't want to be
exposed to too many of them,

but in high enough doses,

these things penetrate our cells
and damage our DNA

and over the long-term can cause
really bad damage

to human bodies.

As we ride through space,

Earth's magnetic field protects
us from most cosmic rays.

But in 2018,
we discovered evidence

that a hail of space bullets

overwhelmed our planet's
magnetic field in the past.

In many ways,
we really take for granted

how the Earth
protects us from space,

but there are records
that we've actually bounced

a little too close
to exploding stars.

Sediments in the ocean
show that

about 2 million years ago,

iron-60 was deposited
in our oceans.

Iron-60 is a
radioactive isotope of iron,

and there's only way we know of
it being made in the universe,

and that is exploding stars.

Around 2.8 million years ago,

a supernova exploded
just 150 light-years from Earth.

A few hundred years later,

a blizzard of cosmic rays
slammed into the Earth,

tearing through
our magnetic field.

These space bullets are
prime suspect in the extinction

of over 1/3 of
all coastal marine species.

Now, this is not
necessarily tied to the Earth

passing through one
of these spiral arms,

but it shows you
that being close to a supernova

is not necessarily a thing
you want to do.

Millions of years in the future,

Earth will pass through
another spiral arm

as our planet continues its
journey through the milky way.

And we will wander
into harm's way once again.

But there's another part of our
journey through space

that remains a mystery.

The Earth is spinning like a top
on its axis.

It's orbiting around the Sun.

the Sun itself is orbiting
around the center

of the milky way,
but that's far from it.

The milky way is
speeding through the universe,

and we are being dragged
along for the ride.

The problem is we can't see
where we're going.

Could Earth be headed
for a galactic crash?

Like the craziest
of theme park rides,

we are riding the Earth on a
wild journey through the cosmos.

Our planet spins, tilts, and
wobbles around the solar system

while bobbing like a carousel
through the milky way.

The dynamics of the Earth
moving in the cosmic void is,

like, the most unbelievable
journey you could ever imagine,

and as you get to larger scales,

the motions only become grander
and larger and more dynamic.

In 1977,
we tried to work out

the largest motion of all --

the movement of our galaxy
through the universe.

Clues hide in the
cosmic microwave background,

a remnant from the birth
of the universe.

The cosmic microwave
background is the radiation

that's left over
from the hot big bang.

The cosmic microwave background
is shining in all directions

as sort of this fixed thing,

and if we're moving through it
with some speed,

we will see that
in the light itself.

You can measure that,
and by measuring that,

you can get a sense of how fast
we're moving through that space.

By tracking our
movement through the universe

against a fixed point,

we can work out the milky way's
speed and direction.

But in 1977, telescopes weren't
the best tool for the job.

During the cold war, 1977,

you have the newly declassified
u-2 spy plane,

this very high-altitude
reconnaissance aircraft.

NASA retrofitted one of them
with an upward-facing window,

and with very
sensitive receivers,

this spy plane became
the first experiment

to for the first time
definitively measure the motion

of the galaxy
through the universe.

As the u-2 spy plane
soared above the Earth,

it measured the
cosmic microwave background

in unprecedented detail.

The data revealed
that our galaxy races

through the universe
at 370 miles a second.

That's over 1 million miles
an hour.

Think of how big
a galaxy is.

Hundreds of millions of stars,

and we're moving at
hundreds of miles per second.

That's just a tiny bit
mind-blowing.

The milky way's speed
isn't the most alarming part

of our galactic journey.

From Earth, we can't even see
in the direction our galaxy

is taking us.

We're flying blind.

Our view is blocked
by the milky way itself.

So if you're actually looking
through the plane of the galaxy,

through the plane
of the pancake,

your view of the galaxy
is obscured

by curtains of dust and gas
that envelop our solar system.

It's very bright.
There's a lot of stars.

And our view is blocked
by the galaxy itself,

so there's a zone
directly behind the galaxy

as we look towards
the galactic center

that's really an unknown area.

It's a zone we call
the zone of avoidance,

and it's an area of really
cosmic mystery.

Thanks to this
mysterious zone,

we thought we could be headed
for a galactic car crash.

Then we had a breakthrough.

We used powerful
radio telescopes

to look through
the zone of avoidance

for the first time.

One of the nice things
about using light that's

in the radio is that this kind
of light actually can go through

pretty dense kinds
of gas and stars

and allows us
to sort of look through things.

Using these radio telescopes
has given us a glimpse

into what lies beyond
the zone of avoidance.

Peering through
the zone of avoidance,

we found that the milky way's
path is clear,

but that's not all.

We also discovered we are just
one tiny part

of a vast cluster of galaxies

sailing through
the cosmos together.

We think that our galaxy,
the milky way,

is part of a much grander
cosmic flow of galaxies,

and it is part of what might be

the Laniakea supercluster
of galaxies,

this giant, giant
cosmic structure

filled with potentially
thousands of galaxies

that is moving together
in this beautiful,

like, slow-moving river

that is sculpted and dictated
by gravity itself.

Earth voyages through
space along gravitational rivers

hundreds of millions
of light-years long

along with a fleet
of 100,000 other galaxies,

all moving toward
a single point in space,

a gravitational drain
known as the great attractor.

The great attractor is
the local region of gravity,

of strong gravity
in this patch of the universe.

It's where all the galaxies
in this chunk of the universe

are flowing towards.

At the location of the
great attractor

is a bunch of material --

a bunch of gas,
a bunch of galaxies,

an extremely massive cluster,

and over time,
more and more galaxies

add themselves to this cluster

as they continue to join
the great attractor.

It's unlikely Earth
will ever reach

the great attractor.

It seems the milky way is headed
for a collision after all.

Is this how our
cosmic journey ends?

Earth's voyage
across the cosmos

has lasted
for 4.5 billion years.

The big question -- how will our
journey finally end?

A clue comes from
the milky way's

traveling companion --
another galaxy, Andromeda,

filled with up
to a trillion stars

that's charting
a similar path to us.

The Andromeda galaxy is another
big spiral galaxy,

a lot like our milky way,

and it turns out the two of us
are heading for each other.

Andromeda
and the milky way

are currently 2.5 million
light-years apart,

but they're hurtling
towards each other

at over 250,000 miles an hour.

A collision is inevitable.

Galaxies colliding
with each other

sounds like science fiction,

but the collision of
the Andromeda galaxy

and the milky way galaxy
will be a spectacular event.

When the Andromeda and
milky way galaxies collide,

it's gonna be
a ridiculous light show.

The stars don't collide,
but the giant clouds of gas do,

and that's gonna trigger
star formation,

so we're gonna have what's
known as a Starburst galaxy.

When Andromeda finally merges
with the milky way,

all bets are off.

All of a sudden, a system
of hundreds of billions of stars

will be added to our own,
and there will be mass chaos,

but one thing
you can guarantee --

there is going to be
celestial fireworks.

This could be the
biggest light show in history,

resulting in the creation
of a brand-new galaxy.

We will have become
one giant galaxy.

Call it Milkdromeda,
if you will,

so we will look very different.

Our grand-design, spiral
milky way galaxy

will have probably
transformed itself

into an entirely
different shape.

The merging galaxies
could create

a giant,
elliptical-shaped galaxy,

but Earth might not
be around to see it.

Billions of stars are gonna
come careening into our galaxy,

very, very easily disrupting
the orbit of the Earth.

the Sun could be thrown
out of the galaxy entirely.

As stars, dust, and
gas swirl around each other,

gravitational interactions
could slingshot our solar system

out into intergalactic space.

We'll still orbit the Sun,
and everything will be fine,

kind of, but it just means

we'll see something
very different in our sky

in 4.6 billion years
than we do now.

Our planet could be sent

on a whole new
intergalactic ride,

shot out into the cosmos, away
from the new Milkdromeda galaxy.

There's no way of knowing
exactly what's going to happen,

and in that sense,
journey's end...

We'll just have to wait and see.

We might not know
the final destination

for Earth's cosmic journey,
but we do know this --

so far, it has been
an incredible ride.

We've had a fantastic journey

over the history
of Earth's existence.

We may not be immediately aware
of our motions

through the universe, but that
doesn't mean they're not there.

Many people think of the Earth

as this blue, calm marble
in space.

But in reality, it's violently
ripping around the Sun,

and the Sun is ripping
about the galaxy.

There's a lot of dynamics
happening.

It boggles the mind
just to think

of what the Earth
will have done.

The Earth will have
actually orbited the Sun

10 billion times.

It will have spun on its axis,
like, a trillion times

in a galaxy
that's moving through space.

Riding the Earth is anything
but a boring trip.