How the Universe Works

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.

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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