Original subtitles

You are speeding around the Sun at 67,000 miles an hour.

In the next year, you will travel 584 million miles to end up exactly where you started.

It's a journey the Earth has made more than four billion times since its formation.

As it orbits, our planet also spins, and is tilted over to the side.

These three things, orbit, tilt and spin, have created and shaped our world.

In this programme we'll see how spin plays a key role in controlling the planet we live on.

It gives us our day and night cycle.

It drives our most dramatic weather and controls our climate.

And has even steered the course of human history.

Spin has created the world we know today.

Every clay and at almost every location on our planet, nature puts on one of her most majestic displays.

But we usually ignore it.

The rise of the sun.

The vast burning ball that tracks across our skies used to be considered a god.

But today we take it for granted.

It's a beautiful sight but it also reminds us of something it's all too easy to forget.

The Earth is spinning as it flies though space, and we're spinning with it.

Every day our planet spins once about its axis.

This is what makes the sun rise and set.

This daily cycle of light and dark dominates life on Earth.

With day comes warmth and light.

Plants use the daytime to grow, converting the light into energy through photosynthesis.

And reptiles warm their cold blood in the sun.

As day turns to night,

sea creatures of all kinds rise from the depths into the shallower waters.

In the darkness they can feed and avoid predators, retreating again as the sun rises.

It's the largest migration in the world, and it happens every day.

And the blackness of night also brings with it creatures...

...that have evolved to live without the sun's light.

Some have developed huge eyes to spot prey in the dark,

while others navigate the darkness using, not sight, but sound.

Spin sets the rhythm of life on our planet.

But does far more than give us the cycle of night and day.

The power of spin actually shapes the world around us,

because our rotating Earth creates an effect that can stir the atmosphere and move the oceans.

You can really see the spin of the Earth at work in late August.

In the northern hemisphere, summer is coming to an end.

But it has a sting in its tail.

Hurricanes, cyclones , typhoons.

Around the world, these giant rotating storms...

...can cause billions of dollars of damage and claim thousands of lives.

And they only exist because our planet spins.

Josh Wurman is an atmospheric scientist.

He's investigating the formation of these gigantic storms.

A hurricane is a huge rotating column of air, hundreds of kilometres across,

that roars across the tropical oceans...

...and occasionally makes landfall on populated areas,

causing some of the most destructive weather that we have on the planet.

It's the afternoon of the 20th of August 2011.

Hurricane forecasters are keeping an eye on an area of low pressure making its way across the Atlantic.

The spin of the Earth would soon turn this low pressure system...

...into one of the most talked-about weather events in recent history.

Her name: Irene.

Irene, like many hurricanes, formed in the eastern and central Atlantic,

and was forecast to be very strong, very violent, perhaps a category three hurricane.

In Florida, MikeTheiss was also watching lrene's growth very closely.

Not to avoid her path, but to put himself right in it.

He's a storm chaser.

I put myself in a hurricane where the worst winds are going to be.

I want people to look at one of my videos and just be completely awe-inspired...

...and say like, 'how did that guy survive to - to get that video?'

There we are, it's crazy.

The violent hurricane winds Mike puts himself in are created by the Earth's spin.

Because the Earth's rotation can twist surface winds into these huge spiralling storms.

But before Irene could become a hurricane, she needed to grow.

Fortunately for her, she was born at the right time.

Throughout the late summer and early fall the ocean has absorbed a lot of sun, all summer.

And the ocean has heated up.

It's about 3O degrees Celsius out there...

...and that's a huge amount of energy for thunderstorm clusters to develop...

...and eventually turn into tropical storms and hurricanes.

Heat rises, the warm ocean heats the moist air above it, forcing it upwards.

At the ocean's surface, more air rushes in to replace it, and it too is heated and rises.

A powerful cycle is set in motion,

with more and more air being sucked in towards the low pressure centre.

But to become a hurricane, it needs to rotate, and for that we need one more vital ingredient.

The spin of the earth.

Even though we can't feel it,

our spinning world creates an effect that has a global impact.

Strange things happen when you spin.

We're used to objects travelling though the air in a straight line.

But start to rotate, and it's a very different story.

No matter where they throw the ball from,

and no matter who they throw it to, the ball follows a curved path.

And because they're spinning counter-clockwise,

to the left, from the point of view of the thrower it's always curved to the right.

This cun/ing of objects caused by rotation is called the Coriolis effect.

And it's the Coriolis effect that makes hurricanes spin.

Because here on Earth we're spinning like the merry-go-round.

So just as the balls curve, the winds on our rotating planet curve too.

There's very low pressure at the centre of a hurricane.

And it's surrounded by higher pressure.

And normally air would approach that low directly from the high pressure surrounding it.

But the Coriolis effect causes that air to be deflected towards the right,

so the air coming in from all directions is reflected towards the right...

...and it spirals around the low instead of going directly in.

And that causes there to be a strong rotation, a strong spiralling of air, around the hurricane.

The deeper then that the low is, the more violently the wind spins around that hurricane.

By late August, the Coriolis effect had created a monster.

Irene was now a powerful hurricane, and was right on top of the Bahamas.

And Mike Theiss was there to film it all.

Here we are in Nassau, Bahamas on Paradise Island.

Hurricane Irene has made her presence felt.

The devastating winds Mike witnessed were the Earth's spin in action.

As you see behind me, the trees are whipping really good in the wind here. Ow!

Just look at that sea.

That says hurricane, right there.

Whoa. Wow.

But there was more still to come,

because as Irene moved on from the Bahamas she was headed straight for the east coast of the US.

Houston, this storm is now stretching from Cuba to the Carolinas,

and that is one scary big storm.

If you are in the projected path of this hurricane, you have to take precautions now.

Don't wait, don't delay.

Irene tracked up towards New York.

There was wide-scale flooding.

And the city's transport systems shut down.

In all, Irene was responsible for up to seven billion dollars worth of damage...

...and claimed at least 49 lives.

And all of this because our planet spins.

Hurricanes are dramatic examples of the Coriolis effect in action.

But it impacts on many more areas of our lives.

Today's planes travel far enough and fast enough that they need to compensate for the Coriolis effect...

...to avoid their routes curving off course.

If you're thinking of firing a long range missile and you don't factor in the Coriolis effect,

you may end up bombing somewhere many miles from your intended target.

However, one thing it does not do is affect your bathroom sink.

The popular myth that household water spirals in the opposite direction in different hemispheres...

...is simply not true.

The domestic sink is nowhere near big enough for it to be a factor.

But even if it has no effect on our drains, the Coriolis effect has a huge impact on life on Earth.

Because it doesn't just give us hurricanes.

The curving air the Coriolis effect creates actually controls our global climate.

To understand how it does this,

we need to look closer at the place where spin has its greatest impact.

In a field a few miles south of San Francisco,

a group of engineers are preparing to make a journey 100,000 feet up into the sky.

This isn't NASA, but with a few hundred dollars worth of equipment,

they're about to go on a voyage that normally only astronauts can take.

With them is Dr Helen Czerski.

What we have here is a helium balloon, and it's going to float up through the atmosphere...

...and it will give us a really good insight into all this stuff above our heads.

Balloons are a fabulous way to look at the atmosphere,

because they can look at parts of the atmosphere that it would be very difficult for a human to look at.

Attached to the balloon is a GPS transmitter, to track its journey.

And four cameras will record everything the balloon sees.

Its journey will reveal something you might not expect.

The movement of the atmosphere is not nearly as random as you might think.

Here we go.

So our balloon has just disappeared up into the atmosphere.

I can still see a tiny little white clot.

And now we're going to track it, using GPS and follow it on its journey through the sky.

To begin with, the balloon goes pretty well straight up.

This is what you'd expect, because the atmosphere spins with the planet.

But then the balloon starts to move sideways.

With the team chasing it, the balloon is being carried away.

And this tells us a crucial fact about the atmosphere:

Although it spins with the Eart, the atmosphere isn't completely locked to the surface.

Okay, it looks like it's just crossing the freeway at Morgan Hill to the east.

While the surface of the Earth is spinning steadily,

the atmosphere is free to move in different directions, and at different speeds.

This free movement of air around the planet...

...is why the Coriolis effect has such a big impact on the atmosphere.

We don't feel the spin because we're too small and we're attached to the ground, effectively.

But the atmosphere, because it's fluid and moving around the planet,

does feel the influence of the Earth's spin.

The team is travelling at about 5O miles an hour,

yet the balloon is continuing to race ahead of them, blown by the consistently moving air.

The atmosphere is actually full of these regular winds,

constantly moving air around the planet.

What's passing over the top might seem quite disordered, quite complex, there's no pattern to it.

But if you look on a bigger scale,

there are patterns in the weather that's passing over each part of the planet.

The balloon has now travelled over 8O miles from its starting point...

...and it's about15 miles above the earth's surface, twice as high as airliners fly.

But it can't go much higher.

I think the balloon right now is up near the top of its trajectory,

so it's about 80,000 feet above the surface of the Earth.

And up where it is there's very little, very, very low air pressure.

So about 95 per cent of the atmosphere is below where this balloon is,

and because the pressure's so low,

the balloon will have expanded to about three times its initial diameter.

But that is about as much as it can take.

As it falls back through the atmosphere,

the burst balloon is reaching speeds of around 140 miles an hour.

Its journey ends in a field 100 miles from its starting point.

That's just shredded balloon, that's amazing.

Look at that.

That's just a fantastic picture.

We can see the Earth and we can see black outer space outside it.

And in between the two there's this fuzzy blue line which is the atmosphere.

On this massive planet, only a really thin layer right at the top is this fluid,

but this dominates our lives.

Fantastic.

The thin layer of air around our planet may look random and chaotic,

but it's actually highly structured, large scale consistent winds blow within it.

These winds are all steered by the Coriolis effect,

and this ordering of the atmosphere has a huge impact on the land below.

When you look at the Earth, you can see some fairly obvious bands.

White snow at the poles, yellow deserts and bands of green vegetation in between.

Each reflects dramatically different climate zones, and each has its own weather.

This division of the Earth into distinctive bands...

...is actually a product of the Coriolis effect acting on our moving atmosphere.

To see how the Earth's spin can create these climate bands,

we need to go to the place where this global movement of the atmosphere begins.

This is the Equator Monument in Ecuador, and we're here on a particularly significant day.

It's the 23rd of September, the equinox.

Today at midday, the sun passes directly overhead here.

It's the perfect day to show why the equatorial zone is so important to our global climate system.

The equinox happens because the Earth spins at an angle.

It is tilted over to the side.

This means, over a year, first the northern hemisphere,

then the southern, points towards the Sun.

The midday sun appears to travel back and forth across the surface of our planet,

and twice a year, at the equinoxes, the Sun is directly overhead at the equator.

This means the equator,

being at the midpoint of the Sun's path, received more heat than anywhere else.

This has a profound effect on our planet.

With the help of the Earth's spin, it creates a global system of winds.

These winds will go on to dictate climate around the world.

The first impact of this vast movement of air is easy to find.

We're still on the equator, but we're in the heart of the jungle.

This is the Amazon rainforest.

Sean McCracken is investigating how the atmosphere creates the vast rainforest.

The answer to that will show us not just why it's so lush here,

but also why this is the birthplace of our global climate system.

And it's all down to the rainforest's location beneath our sun.

The sun here at the equator is just travelling directly over the top of us.

It really drives all the energy that these vast number of plants and trees...

...need that we're surrounded by here.

To really see how the atmosphere creates the rainforest, you need to leave the forest floor.

Rainforests are actually quite surprising places.

With so much heat concentrated on this area, you might not expect it to be so wet and lush.

But far from drying the rainforest out,

the sun is creating a regular movement of air that actually feeds the jungle with moisture.

The sun is constantly feeding down on the equator.

Heat causes the air to rise, as the air rises from the equator it takes water vapour with it.

Hot moist air is forced to rise by the sun's heat.

This is the start of our planetary wind system.

And it is also the key to the rainforest climate zone.

As it rises, it condenses, it cools down.

As it cools, those water droplets coalesce into large rain droplets returning to the forest as rain.

The jungle here can receive over a hundred inches of rain every year.

The high volume of water that drops here in the form of rain...

...really is what drives the high biodiversity that we find here,

creating these vast stretches of rainforest within a small area around the equator on the planet.

But this is not the end of the story.

Because the rising air that created the rainforest doesn't stay at the equator.

With the Earth's spin, it's about to create yet another huge climate band around the Earth.

The air rises high into the atmosphere, to around 1O miles up.

It has nowhere to go but out,

and spreads north and south, drawn towards the cooler poles.

But thanks to the Coriolis effect, it follows a curved path.

In the northern hemisphere bending to the right.

This huge river in the sky is now circling from west to east.

Dr Czerski is on her way to meet that air as it changes direction once again.

As the air travels northwards it gets bent to the right,

and as it's bent around to the right it starts to - to cool down,

to become more dense, and to sink.

And l'm going to join that airstream by going out of this door.

Okay, here we go.

The air that left the equator has now cooled.

And just as the hot air rises, so too cold air descends,

dramatically changing the landscape below.

Dr Czerski is now 10,000 feet above Arizona, travelling with that air.

So we're falling now together, along with all that air, this waterfall of sky.

Ah, glad to be back on the ground.

So l've landed in a really dry, dusty place with just these scrubby little bushes around.

This is the Sonoran Desert in North America.

It's two thousand miles north of the equator.

In the summer, it can reach 5O degrees centigrade.

But it's not heat that defines a desert.

It's the lack of water.

Here it can rain as little as three inches in a year,

so it's no surprise that the wildlife is very different from the rainforest.

This is a classic dry environment.

You can see there's no water here.

Stones, sand on the ground, and the plants - look at this:

spiky, very unfriendly plant holding onto the moisture it's got.

The Coriolis effect caused the equatorial air to descend here.

It's still the same air that created the lush rainforest, so why is now so dry?

So when that air rose at the equator, it took lots and lots of moisture with it.

But as it rose up, all of that moisture rained out back down into the tropics.

And so as it started its journey away from the tropics towards us here,

it was very, very dry air.

By bending the dry equatorial air,

the spin of the Earth has created a desert two thousand miles north of the equator.

And it doesn'tjust happen in North America.

All around the planet, where that dry air is descending,

you get deserts, both north and south of the equator;

there are two bands of deserts going all the way around the planet.

There's the Thar Desert in India, the Arabian, and of course there's the Sahara in Africa.

They're all created by falling dry air that originated close to the equator.

These deserts form the second great climate zone round our plant.

If it wasn't for the spin of the Earth, it wouldn't exist.

But the air that falls in the desert doesn't stay here.

It has one final journey to make.

And arguably for us, it's been the most important of them all.

The air that's arriving here now will get sucked back towards the equator,

and as it goes it's bent to the right by the Coriolis effect, so you get close to the equator,

big winds going round towards the west, and we call those the trade winds.

The trade winds form a band of consistently moving westward air,

and from the 15th century onwards,

European sailors increasingly exploited these winds to explore the world.

They played a central role in the colonisation of the Americas,

and world exploration during the age of sail.

The trade winds were the freeways of the ocean.

The pattern of our planet, the rainforest at the equator,

the deserts at about 3O degrees of latitude,

even the course of human history itself,

all those would be completely different if our planet didn't spin.

When the trade winds arrive back at the equator, they close the loop.

The resulting pattern of wind is called a circulation cell.

But it doesn't exist in isolation.

The pattern repeats, covering our entire planet.

With three cells in the northern hemisphere and three in the southern,

the Earth's spin brings order to the atmosphere,

creating the climate zones we're so familiar with today.

The atmosphere is not the only place where the Coriolis effect makes its presence felt.

It can move OCGGHS tOO.

Two thirds of the Earth's surface is covered with water.

Just like the atmosphere, it too is free to move around the planet.

This means the Earth's spin can control the oceans.

And this has a huge impact on the world's climate.

Professor Meric Srokosz is an oceanographer.

He's investigating the movement of water in the North Atlantic.

So we look at the action.

You can see the surface waves, and it looks very dynamic,

but underneath the surface there's lots of dynamic stuff going on as well.

Beneath the waves, there's large scale currents,

and they're moving water round in the ocean a bit like the winds blowing in the atmosphere.

And just like the winds in the atmosphere, the Coriolis effect comes into play.

Rotation is important for the ocean, just like it is for the atmosphere.

Because the planet is rotating,

the rotation causes the currents to veer,

and that has a big effect in terms of the way the currents flow in the ocean.

Like the winds above, the Coriolis effect is bending the currents below.

This helps create huge circular currents in our planet's oceans.

In the northern hemisphere they rotate clockwise, and in the southern they rotate counter-clockwise.

They're called ocean gyres, and surprisingly, they play a crucial role in the world's climate.

Because it's not just water they're moving around the ocean.

They're moving heat too.

L'll give you a shout when the first buoys are over.

Since 2001, Meric and his team have been gathering data at various sites in the North Atlantic.

Our research in the Atlantic is looking at the effect of the currents in the Atlantic...

...in terms of bringing heat northwards.

Essentially the sun heats the earth at the equator more than it does at the pole.

The ocean currents are moving a large amount of water.

Associated with that water is heat, and that helps to balance the global climate system.

The currents in our oceans are transporting vast amounts of energy away from the equator.

In the North Atlantic alone,

the equivalent to one million power stations worth of energy is being transported northwards.

This energy helps warm temperate zones far north of the equator.

The ocean gyres are moderating the Earth's climate and it's our planet's spin that makes them turn.

These vast ocean currents have local effects too.

The current moving north along the west coast of Chile...

...brings up cold nutrient-rich water from the depths.

This creates one of the most fertile fishing grounds on Earth.

2O per cent of the world's fish is caught from less than one per cent of the ocean.

The heat transported in the North Atlantic gyre is swept east by the prevailing winds.

This means, even in December you can go swimming off the coast of the UK.

Whilst at the same latitude in Eastern Canada, the landscape can be frozen solid.

Far from simply giving us day and night, the Earth's spin shapes our planet.

Through the Coriolis effect, it orders the atmosphere, giving us our global climate structure.

And it controls the movement of ocean currents, transporting the sun's energy around the world.

But the Earth's spin plays another crucial role in our world.

A role that not only has a huge daily impact on the planet;

it has dramatically altered the Earth's history.

And the best place to see it in action is on the Eastern coast of Canada.

Dr Graham Dayborn has come here to witness one of nature's great events.

Well, we're here in the upper end of the Bay of Fundy.

It is, as you can see, a spectacular setting.

But the fascination about this is the enormous tide.

The Bay of Fundy has the largest tides in the world.

Every day the water drains away.

This is the sea floor.

And these land masses are actually islands left behind by the receding tides.

The water has travelled out three miles from the coastline.

Right now, the tide is coming into the Bay of Fundy.

This is driven in from the Atlantic,

but as it enters the Bay of Fundy it gets into shallower and shallower water,

and as a result, it is forced to rise higher and higher.

There is about a hundred billion tonnes of water that come in over six hours.

That is as much as all the water in the rivers in the world flowing out at the same time.

The bay's unique shape and location mean the tides here are the biggest in the world.

This huge movement of water only happens because the Earth doesn't travel around the sun alone.

It has a companion - the moon.

So here we are, having witnessed this enormous rise of water over the last six hours.

All of that is entirely due to the fact that...

...the moon is attracting that water and pulling it away from the Earth, if you like.

We experience and see that as a bulge and a - a flood of water.

But the moon and the tidal bulge...

...it creates are not enough to give us the daily cycle of tides that we're so familiar with.

We need spin too.

We can almost think of the water as being static and the Earth spinning underneath it,

and that's why you see the tide coming in, the tide going out,

but it's really because the Earth itself has rotated underneath that envelope of water.

The moon is overhead, the Bay of Fundy is now in the middle of this huge bulge.

Thanks to the Earth's spin,

the rock formations Dr Dayborn walked past have become islands once again.

In the last six hours, the water in this basin here, which is, as you can see, very, very large,

has raised about 4O feet, 4O to 45 feet.

If you took a five-storey building and put it at the low tide mark,

right now you would not be able to see it.

The highest tides happen when the gravity of the moon works together with the gravity of the sun.

But in late September, something special happens.

Then both the sun and the moon are tracking along the equator.

As they pass over the centre of our planet,

we spin right through the middle of a mammoth tidal bulge.

We have our biggest tides.

This leads to strange events happening around the world.

In certain river valleys, tidal waves race upstream.

These are tidal bores.

In Britain, there's the river Severn.

In South America there's the Amazon bore.

The biggest of them all is on the Quiantang river in China.

They call it the Silver Dragon,

and in late September surfers from across the world head to the city of Hangzhou...

...for one of the more unusual surfing events of the calendar.

Our tides can be spectacular sights,

but they do much more than move water around the planet.

Off the coast of Bermuda,

Dr Judith Nagel-Myers is looking for a particular sort of sea creature.

One that can tell us about the extraordinary role tides have played in the history of the Earth's spin.

The sea creature is coral.

Corals are little animals that you can find in the warm seas of the tropics,

and it's amazing that these little animals can build those colourful,

various shapes and forms on the ocean floor.

But corals do far more than make the sea floor look pretty.

Encoded within them is a record of a very significant feature of our planet's rotation.

Corals need a lot of sunlight.

At day they grow very much faster than at night.

So when you look at the skeleton of a coral,

you can see day layers and night layers, just similar to tree rings.

Every day the coral puts down a very thin layer of limestone.

This means coral skeletons effectively count how many days there are in a year.

And this reveals something very strange about the Earth's past.

This is a modern coral,

which we actually cut so we can see exactly how those animals grow.

And if you look close you'll see growth bands.

You see dark and bright growth bands.

Those actually represent one year of coral growth.

If you would come even closer and look under a microscope,

you can see very fine layers which represent one day of coral growth.

And obviously in modern corals you can count 365 rings for this one year.

But corals have been around for millions of years,

and when you look at a much older piece of coral, it tells a very different story.

You can find fossilised corals all over the world.

This is a coral that is 400 million years old.

If you look close you can see the same growth lines that you can find on a modern coral.

But if you count those daily growth rings, you don't get 365, as you might expect.

Surprisingly, you find that there's actually 410 growth rings per year.

That tells us something really interesting about the Earth's history.

These fossil corals tell us that the Earth spun much quicker back 400 million years ago.

400 million years ago, a year lasted not 365 days, but 410.

And the only explanation for this is that the Earth was spinning faster.

In fact, a day wouldn't have lasted 24 hours, it would have lasted just 21.

To find out why, you have to go back to the earliest days of Earth's history.

Four and a half billion years ago, as the solar system was still forming,

the young Earth was hit by another fledgling planet, roughly the size of Mars.

Our world only just survived.

Debris from the impact then coalesced creating the moon,

which would have been much closer to the Earth than it is today.

It also set the Earth spinning much faster.

When the first oceans formed,

that would have led to tides hundreds of metres high crashing into the coastlines all around the world.

But the effect of the moon's gravity pulling on these huge tides,

has actually slowed down the Earth's spin.

The moon creates tidal bulges on our Earth, and the Earth is trying to spin underneath them,

and that causes friction that actually slows - slows the rotation down.

Over millions of years,

the tides have actually decreased the speed of the Earth's spin,

until the days became the 24 hours we are so familiar with now.

And as our spin has slowed, so the moon has drifted away,

and its effects have decreased.

But even today, the speed at which the Earth spins is still slowing down.

Our days are still getting longer, but only by about 2.3 milliseconds every century.

We've seen how the Earth's spin plays a critical role in our life on Earth.

Far from just creating our familiar day/night cycle, it defines weather across the planet.

It creates hurricanes.

It moves oceans and causes the tides.

And spin gives us our deserts and rainforests,

a global pattern of climate zones that can be seen from space.

But we've also seen how our spin is not constant.

As it has changed, so too has life on our planet.

The story of spin is not over.

It will continue to shape and reshape our world for millions of years to come.