All of us, every day of our lives, are on the move.
And we don't mean the morning commute or taking the kids to school,
but a journey of epic proportions.
Even now as you are watching this, you're hurtling through space
at 100,000 kilometres an hour.
KATE: Every year our planet travels around the sun and we go with it.
I'm Kate Humble.
This is it, the sun is directly overhead.
My shadow is directly below me.
In this series, we'll follow the Earth's voyage through space
for one whole year
to witness the astonishing consequences this journey
has for us all.
HELEN: I'm Dr Helen Czerski,
and I study the physics of the natural world.
Wow, look at that.
I'll be investigating how our orbit powers the most spectacular weather
and how it's also shaped and reshaped our planet.
HELEN: As we travel through space,
the Earth orbits the sun at an angle of just over 23 degrees.
KATE: We're going to experience first hand
the dramatic effects of the Earth's tilt.
This is the moment we've been waiting for all day!
HELEN: Through wild weather.
It's really raining hard now!
KATE: And back in time.
All this here would have been covered in water.
Join us on the most remarkable journey of your life.
Since our journey began in July,
we've travelled over 700 million kilometres around the sun.
We've explored how our planet's orbit and spin
have a fundamental effect on how we live on Earth.
In this episode, we'll complete our year-long voyage and on the way,
discover how another aspect
of the Earth's relationship with the sun
has changed the course of history.
It's now March.
BIRDSONG
'And we start on a very special day...
'...at a very special place.'
This is the great pyramid in Chichen Itza -
the city is one of the world's great archaeological sites.
And it contains a remarkable insight
into our journey through space.
The ancient Maya had developed a deep understanding
of the Earth's movement around the sun,
and they built it into the very fabric of this city.
But it's something that can only be seen at two very precise
and magical times of the year.
One of those is today, March the 20th.
As afternoon approaches, the city fills
with followers of Mayan beliefs...
..and those curious to see a millennia-old wonder.
There is a unique and particular feature of our planet
as it orbits the sun
and it's encoded in the way that light and stone
interact at the great pyramid.
CHEERING
This is the moment that all these thousands
of people have been waiting for,
they've all stood up and there are hands raised to welcome in the sun,
and it's now aligned perfectly on the edge of the steps here,
creating this very specific pattern of light and shade
which resembles the body of a snake.
And that's no coincidence
because it joins up with the carved snake's head
at the bottom of the pyramid.
The Maya believed the snake, known as Kukulcan,
was a messenger between gods and man.
This is a remarkable display of Mayan architectural design.
The appearance of this snake isn't an accident,
they absolutely planned it
and it happens on the same day every year.
This is the spring equinox.
DRUMS BEAT
So, more than 1000 years ago,
the Maya recognised the equinox as a pivotal moment in the year.
CHEERING
And they were able to align this pyramid
with the sun's annual progress,
causing the snake to appear each equinox.
CHEERING
CRASHING WAVES
Here on Earth, there are a few moments that we all share,
because we're all on the same journey around the sun.
And one of those moments is the equinox,
when day and night are equal.
'It's a time of balance we can all experience,
'wherever we are on the planet.'
So whether you are here in Britain, amongst the fitful showers
and overcast skies,
'or in the bright spring sunshine of Mexico,'
on the March equinox you'll get 12 hours of daylight
and 12 hours of night time.
That's if the sun ever comes through the clouds!
But it's more than just a time of balance.
It's also a turning point in our year.
From the March equinox onwards,
the days get longer in the northern hemisphere,
'while in the southern hemisphere, the opposite occurs.
'This is because of a special feature of our planet
'as it journeys through space.'
Let's say this rock is the sun.
This is going to be our Earth,
and as the Earth travels around its orbit
spinning like this, it travels around on a flat plane.
So you would think that its axis would point upwards
but it isn't, it's tilted over at 23.4 degrees.
'This means that the North Pole, the stem of the apple,
'isn't vertical, it's at an angle.'
And that tilt stays pointing in the same direction
as the Earth travels around on its orbit.
Because of this tilt for part of our orbit,
the hemisphere north of the equator leans towards the sun.
This brings with it extra solar energy,
which fuels spring and then summer.
Six months later, the situation is reversed.
The southern hemisphere now leans towards the sun,
while the northern hemisphere experiences declining energy,
ushering in winter.
Tilt creates the Earth's seasons.
But there's a moment, twice a year as we orbit,
when the sun favours neither hemisphere.
At this point, both experience 12 hours of daylight and night time.
This is the equinox.
If the Earth wasn't tilted,
every day would be like the equinox,
with the 24 hours equally split between day and night.
And that would mean no seasons.
'his time we're following the Earth's journey from the spring equinox'
to the point when the tilt of the Earth gives us
our longest day of the year -
June 21st, the summer solstice.
Over this three-month period, seasonal warming sets in motion
the greatest planetary transformations of the year.
Winter has covered a great swathe of the northern hemisphere in snow.
But now it's melting, receding to the edge of the Arctic Circle...
..where spring is about to arrive in dramatic fashion.
BIRDSONG
This is the Hay River,
which meanders north for 700 kilometres
through the Canadian tundra.
Here in the north,
the river is still in the grip of winter ice.
But upstream to the south, the ice
has been cracked by the spring warmth.
By the end of April, the broken ice is on the move.
At this point in its journey north,
the river tumbles over a 35-metre drop,
giving us this spectacular sight.
This is Alexandra Falls.
And you can see that the central flow is flowing strongly
and does all winter
but the majority of the falls are still frozen solid.
For six months, hardly anything at these falls has changed.
Now, in the space of just a few hours,
a transformation has begun...
..as the ice armada approaches from the warmer south.
ICE BOULDERS GRIND
But there is still not enough water in the river
to force the ice over the falls...
..and it piles up in a great ice dam.
ICE GRINDS
But eventually,
it gives way.
ROARING AND CRACKING
This is the moment we've been waiting for all day.
All this broken ice
has been backing up behind the waterfall,
and what needed to happen to shift it
was for the water level just to come up.
And it's literally just happened and as you can see,
it is pouring and pouring
over the waterfall, in this great dramatic jumble
of mud and broken ice.
ROARING
It's just mesmerising to watch.
RUSHING WATER
Look at this huge piece now, falling off.
And you know at home when we talk about the arrival of spring,
and we talk about the snowdrops coming and the first birds tweeting,
well, this is spring, Hay River-style!
There's nothing gentle or quiet about it.
It's violent, it's noisy
and it's entirely speeded up by these warm meltwaters
that have come from the south.
By April, increasing warmth from the spring sun is transforming
the landscape of the northern hemisphere.
And it's turning green.
Using photosynthesis, plants convert the sun's energy
into the fuel needed for them to grow and flower.
CUBS BARK
Everywhere, nature is responding to these changing conditions.
CHICKS SQUAWK
North of the Hay River, the caribou are on the move,
heading towards the newly revealed pastures.
The first to arrive are the pregnant females.
Within a couple of weeks, the rest of the herd gathers
until as many as 150,000 animals are together.
The Arctic has come to life.
On the other side of the planet, in the southern hemisphere,
the opposite seasonal change is underway.
Temperatures are plummeting with the shorter days of autumn.
The average temperature at this time of year
is minus 40 degrees Celsius.
Animals, rather sensibly, abandon the continent,
with one notable exception.
The Emperor Penguin.
They choose these short, freezing days to mate,
because the sea ice has re-formed,
and is now strong enough to support their vast breeding colonies.
All over our planet, the natural world reacts to the shifting energy
we receive from the sun.
HELEN: As our planet's orbit takes us towards June,
the Earth's tilt powers great seasonal change.
And this gives rise to some dramatic weather phenomena
that are concentrated at this time of year.
The most extreme occurs over the Midwest of the United States.
Every spring day we experience the interaction
between Earth's orbit and its tilt.
At its most simple, the days get longer and the land gets warmer.
But it also affects the atmosphere,
with important consequences.
It's the driving force behind the most significant weather events
on our planet.
WIND WHIRLS FEROCIOUSLY
A tornado is the most volatile of these seasonal weather events.
They occur most frequently in the spring
and especially in the Midwest of America -
a region known as Tornado Alley.
MAN: 'Did you see that? The whole house came apart!
'Oh, my God!'
HELEN: But despite its violence,
at the core of a tornado is a very simple process.
This goes on like a backpack.
To experience it, I'm taking to the air,
over the Midwestern state of Colorado.
One, two, three, go. Run!
Paragliding pilots like Honza Rejmanek,
love this time of year.
Spring provides the perfect conditions for soaring...
..because the increasing temperatures generate thermals.
So right now we are in a thermal.
These are basically almost like invisible smokestacks of rising air.
Right now we've found one, I'm going to take a turn in it
and circle around and try to gain height.
'Thermals form when the sun warms the ground,
'and the ground, in turn, warms the air above it.'
What I'm experiencing
is one of the most fundamental principles of atmospheric physics -
warmer air rises.
'When air warms, it expands and becomes less dense.
'So this air has a lower atmospheric pressure
'than the cooler air that surrounds it.'
So it floats upwards, forming this rising thermal column.
The atmosphere tries to even out differences
in air temperature and pressure,
attempting to return to equilibrium.
So the rising thermal will mix with the cooler air above.
This basic process of moving towards equilibrium
lies at the heart of every significant weather event
on the planet.
'But in the springtime air over Tornado Alley,
'there's a regional anomaly
'that intensifies this basic atmospheric process.
'The result is that here,
'particularly powerful storms can develop.'
There's a stable layer of dry air that acts as a barrier
between the warm air down below and the cooler air higher up.
So the warm air is trapped,
and what's more, the ground keeps heating it as the day goes on.
WIND WHISTLES
THUNDERCLAP
The thermals get more and more powerful until, by late afternoon,
they finally punch through the barrier layer at colossal speed.
These rapid updraughts of less dense, lower pressure air
are so strong that they generate huge thunderstorms.
THUNDER RUMBLES
It's from these thunderstorms that, in certain conditions,
tornadoes can emerge.
'I'm going to investigate how this happens...'
Not as bad as north of us.
...with the help of atmospheric scientist, Josh Wurman.
I don't know what to make of these stringy little features.
The first step in our quest for a tornado
is locating a promising storm.
After a couple of days on the road, we manage to intercept
one moving north through Colorado.
So what's happening behind me is the storm is building
and in the middle of that storm over there,
there's an updraught with low pressure at the centre of it.
And all the air around the outside has higher pressure,
and that high pressure is pushing air into the centre
and up into the storm, and that's what building the storm.
The atmosphere tries to even out
the extreme differences in temperature
that have been generated.
So the air movements at the core of the storm
become exceptionally powerful.
'Hail is one characteristic product of this atmospheric violence.'
'The hail formed when an updraught cooled rapidly,
'so that water condensed out of the air, and turned immediately to ice.'
SHOUTING: This is what was carried from the south,
and it was pushed up into the storm
and it gave the storm its energy.
And now it's falling back down on me!
GIGGLES
Wow!
CAMERAMAN: That's it. Let's get inside. This is too hard now.
And even though this is chaotic and messy,
what this is, is a demonstration that the atmosphere is an unstable place,
and there are all these differences in temperatures and pressures.
And this is what happens when the atmosphere moves around
to even everything out, and make it all the same.
It's not looking very peaceful at the moment
but that's what it's trying to get back to.
THUNDER RUMBLES
When tornadoes do form, they are often preceded by hail.
'But this time, there's no twister.
'So we're back on the road,
'still trying to see a storm spawn a tornado.
'After a week of tracking promising storms without success,
'Josh's specialist radar detects one
'which shows a revealing swirl of clouds.
'We have to move fast - tornadoes form and vanish very quickly.'
JOSH: Going out ahead, this big dark area's the core.
So we're basically going to penetrate through the core
and see what's interesting.
'Tornadoes form when powerful rotating cylinders of air
'within the storm
'get caught by an updraught and are knocked on their side.'
Right now, we're kind of in the centre of the coiled part of this...
'When that column of rotating air touches the ground,
'a tornado is born.'
INDISTINCT RADIO CONVERSATION
At the tornado's core is an area of intense low pressure,
which draws high pressure air towards it.
The dust and debris picked up by the tornado
reveal the swirling pattern of winds.
So, this is it.
The high pressure is swirling inwards and up that funnel.
And it's enormous!
I had no idea it would look that big!
That's just amazing!
And here it's almost calm.
But over there, those winds are going at hundreds of miles an hour,
pushing stuff right up into the heart of the storm.
I just... I can't stop looking at it, it's incredible.
Just 15 minutes after it first touched down, the tornado dissipates.
There's still so much that we don't understand about storms.
We don't understand when they're going to produce hail,
when they're going to produce rain,
when they are going to produce tornadoes.
But what we do understand is that a storm like this
is a manifestation of something happening round us all the time.
Our planet's atmosphere is a mosaic of warmer and cooler air masses,
constantly in motion.
The air is rising, falling and swirling around
as it seeks to balance differences in temperature and pressure
and return to equilibrium.
During April and May,
the effect of the Earth's tilt is to enhance those differences
by increasing surface temperatures, which in turn heat the air.
So all over the northern hemisphere,
spring is the season for volatile storms.
Tornadoes are only one consequence.
The heavy and sudden downpours from storms can result in flash floods...
..like the one that hit the town of Barranquilla in Colombia
in May 2011.
These occur when the rain inundates densely saturated ground.
The water isn't fully absorbed, but instead flows rapidly downhill
in a near-instantaneous torrent.
Thunderstorms can also give birth to an unexpected phenomenon...
..massive dust storms called haboobs.
This one blew into Phoenix, Arizona in 2011.
Haboobs are produced in normally arid regions,
when the leading edge of a storm collapses,
generating a super-fast downdraught
that kicks up a wall of dust and sand in front of it.
As May turns to June, the volatility in our atmosphere
drives the biggest single weather event on the planet.
An event centred on the Indian subcontinent.
TRAFFIC HUMS
CAR HORNS TOOT
This is the city of Udaipur in Rajasthan.
It's in the northwestern corner of India.
Since March, temperatures here
have been steadily rising as the Earth's tilt
has warmed the northern hemisphere.
But by June, everything is on the brink of an exhilarating change.
I'm here at the time of an epic weather event of huge importance
not just to Rajasthan but to the whole subcontinent
and the over billion people who live here.
'There's a wonderful place to appreciate the event's significance,
'on one of the hills that overlook the city,
'here, at this cliff-top palace.'
It was built at the end of the 19th century
by the 72nd Maharana of Udaipur
and it's known as Sajjan Garh.
'Although now abandoned, Sajjan Garh's halls
'and courtyards still have an evocative, fading grandeur.'
The palace was designed with a whole series of balconies and verandas
and you do get the most staggering view of the city from up here.
But that's not what the Maharana was interested in.
He built this palace to get a pure, unadulterated view of the sky
and the clouds that start to build at this time of year.
Sajjan Garh is the monsoon palace.
When the rains do eventually arrive,
they'll be an essential relief from the heat of the Indian summer.
But what's intriguing is that the monsoon is actually a consequence
of the rising seasonal temperatures that precede it.
To reveal why this is, we need to travel 2,000 kilometres...
..south.
I'm in the coastal state where the monsoon first arrives in India -
Kerala.
The key to understanding the monsoon is here, on the beach.
The monsoon is powered by a simple,
but incredibly significant difference -
the difference between land and sea.
And in particular, the differing ways in which they respond to the sun.
Take this sand as an example.
The sun's energy is heating all of this surface,
but if I dig down just a little way...
..the sand underneath is quite cool, and that's quite familiar,
we see that on sunny beaches all the time.
And here, where it gets really hot,
the surface can reach 40 degrees Celsius.
Just 15 centimetres down into the sand,
it can be only 7 degrees Celsius.
So, all the sun's energy is going into a really thin surface layer,
and that layer heats up really, really, quickly.
The sun is also beating down on the ocean,
and that responds very, very differently.
This water is much warmer than the sea at home
but it's much cooler than the beach,
and the reason for that
is that the ocean takes much more of the sun's energy to heat it up.
So a kilogram of water will take three times as much energy
as a kilogram of sand to heat by one degree.
The ocean is also relatively cool because to heat the surface
you have to heat much more than just a thin layer.
What happens is that winds that blow across the surface of the ocean
generate turbulence which mixes that top layer.
So as soon as some water's been heated at the top,
it gets mixed down below.
'This means that, unlike the land, the ocean warms up only very slowly,
'as the sun's energy is absorbed.
'So as we enter summer, the land heats up quickly,
'while the ocean lags further and further behind.'
This increasing temperature difference is critical,
because both land and sea heat the air above them.
As the sun has baked the Indian subcontinent,
the land has warmed the air above it.
The warmer air is less dense, so it rises.
This draws in the cooler air from the ocean.
Because of India's particular geography,
this process is magnified.
It's a triangular peninsula, with wide, hot plains
and, crucially, a very long coastline.
This combination sets up a powerful
and sustained movement of cooler ocean air -
the monsoon wind.
Of course when most of us think of a monsoon
we think not of seasonal winds, but of rain.
'By setting up a time-lapse camera,
'I'm hoping to watch the rain clouds forming.'
THUNDERCLAP
Wow!
There is an enormous process on the go here.
When the sun shines down on the ocean surface,
some of the water at the surface will evaporate,
so water and energy are carried up into the atmosphere.
And as the monsoon winds come inland
and they carry that water vapour with them,
the heated land makes that moist air rise,
goes up into the clouds and there droplets condense -
the water condenses out, becomes visible, we see clouds.
When those droplets join together to form droplets which are large enough,
we get rain like this.
And it's really raining hard now!
So, what we are seeing now is a thin layer of the ocean
that's been lifted up,
shifted over here and is now being dumped on top of me.
'None of this would be happening if it wasn't for the Earth's tilt.
'It's the seasonal heating is what widens the gap in temperature
'between the land and the sea', and this drives everything.
And this massive system of rain and wind rushes inland
and that's the monsoon.
I'm wet! So drenched!
I feel like I've been in a shower for about ten minutes!
I suppose I have(!)
80% of all India's rains
arrive in this seasonal deluge.
It's not just the volume of the monsoon rains which is impressive.
It's the distance they travel.
As summer progresses in India,
the difference in temperature between land and ocean actually increases.
This makes the whole monsoon system more powerful,
drawing this moisture-laden air further and further inland.
From when the monsoon first arrives on the Kerala coast
around June the 1st,
it spreads more than 2,000 kilometres
until it eventually reaches the far north of the country.
Including Rajasthan.
VOICES CLAMOUR
CAR HORNS TOOT
It's remarkable that the moisture-laden winds
that originate many hundreds of kilometres to the south from here
are still capable of delivering rain in Rajasthan.
The rains aren't nearly as heavy here as they are in Kerala.
They tend to fall in short bursts
and sometimes there are several days between downpours.
And sometimes the monsoon fails altogether.
So effective systems of storing rainwater are critical.
This is Lake Pichola, and for the tourists that flock to Udaipur
in their thousands, it's a must see on their itinerary.
But the jewel is this, the Lake Palace,
which looks like it's almost floating on the surface
and is entirely surrounded by water.
But what's truly surprising is that this lake isn't natural at all.
It's a man-made reservoir built specifically to capture
those precious monsoon rains.
It was built hundreds of years ago
and covers about seven square kilometres.
But even a reservoir this size doesn't guarantee
the people of Udaipur a permanent supply of water.
As recently as 2009, when the monsoon failed here,
the entire lake dried up.
A stark reminder that the balance of life in this part of India
is totally dependent on the differing ways
that the land and the ocean respond to the sun.
The monsoon is the Earth's biggest global weather event.
But it shares the same root cause as the smallest local rain shower
and that's the Earth's tilt,
which drives seasonal variations in temperatures
of the land, sea and air.
So the question is,
why is the Earth tilted in the first place?
Our 23 degree tilt is just right.
It's enough to provide a relatively benign seasonal shift.
It makes our planet habitable.
Here in America, we can get an insight
into how the Earth might have got its tilt.
This is the Barringer Crater in Arizona.
50,000 years ago a meteorite struck this site
and just look what it left behind - this enormous hole in the ground.
'This impact would have thrown debris out
'over tens of thousands of square kilometres.'
And all the rock around here, like this,
is what's left after that explosive event.
This enormous crater is like a lesson in how size isn't everything,
because the crater itself is a kilometre across,
but the thing that caused it was only about 50 metres in diameter,
which is really quite small.
And the reason that such a small thing could cause such a big hole
is because it was travelling so fast.
'Impacts like these are extremely rare
'but in the Earth's past, they were far more common
'and a lot bigger.'
Around four and a half billion years ago,
the solar system was still in the process of formation.
The Earth was just one of many of protoplanets that orbited the sun.
Amongst these protoplanets
was a small Mars-sized planet that's been named Theia.
Its orbit put it on a collision course with the Earth.
Theia smashed into the larger Earth and was obliterated.
The impact very nearly destroyed our planet too.
The collision knocked the planet over,
tilting the Earth's axis of rotation.
This tilted Earth might still be oscillating madly,
were it not for another consequence of Theia's impact.
A huge amount of debris was blasted into space.
Gradually, this debris coalesced,
captured by the Earth's gravity...
...and it formed the moon.
Billions of years later, the gravity of the sun and the moon together,
act as a sort of counterweight, stabilising our tilt.
It's extraordinary to think that the moon is both evidence
of what caused Earth's 23 degree tilt
and the celestial object that helps maintain it.
But the stabilisation the moon provides isn't perfect.
And the smallest variations in the angle of tilt
can have profound consequences.
Remarkable evidence for this can be found in the Egyptian desert.
This is the Sahara.
Hidden in this apparently lifeless landscape
is proof that the Earth's tilt has changed, and in the recent past.
And that change has transformed climate
and history.
With me is geographer Nick Drake.
He's a veteran explorer of this region.
We travel through the desert for 600 kilometres
to reach our destination.
This is the Gilf Kebir - the Great Barrier.
For hundreds of years,
explorers have come here in search of a lost world.
A decade ago, one group succeeded.
In 2002, a couple of Italians were exploring this region
when they spotted that cave.
I don't think even they could have hoped
for something as spectacular as this.
The extraordinary paintings in the Cave of Beasts
are around 8,000 years old.
More than 3,000 years older than the pyramids.
When you start to look more closely at the figures on the wall,
this seemed to be a very athletic population of people.
They all seem to be running or jumping or throwing things.
But you've also got wonderful pictures of antelope here,
there's one, two, three, four... four of them.
Could be a springbok with their dark and light colouring.
You've also got lots of images of giraffe.
That is undoubtedly a giraffe -
you see the head, the long neck coming down,
long legs.
But there are some figures
that are in a very...
..strange position indeed. Here's one...
..and it's a bit worn...
There's another one here - there's a line of them.
There's one there.
And there is a theory
that they could be swimming.
A whole line.
So where did the waters that sustained those people and animals
come from?
A day's travel away is the valley of Wadi Bakht.
Here, there are clues that have helped to resolve this mystery.
So when you come to landscapes like this,
does everything speak to you and tell you, you know,
this is what was happening X thousand years ago?
It does to me, now, most of it.
But at the beginning, you're learning to interpret the landscape.
Nick Drake studies the ancient geology of the African deserts.
This sediment here is sand. OK.
And this above it is clay. Right.
So the sand is, when it's dry, it's blowing around,
depositing here.
And then we get wet,
and we get rivers transporting these clays
and depositing them at this spot.
And we've got quite a long period, I think, here, of wet. Yeah.
Then here we get a little layer of sand,
then a layer of clay, then a layer of sand, then a layer of clay.
And I think these are annual, or maybe biannual events.
We get a really big flood, doesn't evaporate in the winter,
it lasts for more than one year,
but they're certainly drying out relatively quickly
suggesting a seasonal environment - wet, dry, wet, dry.
That pattern of highly seasonal rainfall
can mean only one thing -
this now barren desert, once received a monsoon.
The geology of this site tells us that the rains fell in this area
between 5,000 and 10,000 years ago...
..transforming the landscape of Wadi Bakht
and creating a lake.
You can see these clay sediments, these grey sediments...
This, all this here, would have been covered with water?
Yep, probably going almost
to the edge of where those rocks are, over there.
The landscape we're looking at now would've been completely different?
It would've been green,
it would've been full of plants, possibly trees,
the animals in the cave paintings would've been wandering round,
drinking from this lake, and maybe even people swimming in it?
Exactly. A savanna environment.
Nick's research has revealed that the ancient African monsoon
helped feed a verdant Sahara,
a place crisscrossed by many rivers, with huge lakes -
one was 20% bigger than the UK.
The mystery, then, is what could have brought these rains here?
We know from the Indian monsoon
that when the land is hottest in the summer months,
it creates a low pressure system which draws in cold, moist air.
So the irony is, this part of the Sahara must have been receiving
more of the sun's energy - it must have been hotter back then,
5,000 years ago, than it is today.
And that's what allowed the monsoon rains
to cover this area with water.
What's remarkable is that the higher temperatures that drove
the Saharan monsoon were the consequence of a tiny change
in the angle of the Earth's tilt.
Although the gravitational pull
of the moon and sun together have stabilised our tilt,
they don't do it perfectly.
Today, the angle of tilt is 23.4 degrees,
but over regular, 41,000-year cycles,
the angle swings between 22 and 24.5 degrees.
Back when the Sahara was green,
the Earth's tilt was close to its maximum angle.
Together with small cyclical changes in the direction of the tilt
and the shape of our orbit,
the result was the sun shone more intensely
over the northern hemisphere,
powering a monsoon in the Sahara.
About 5,000 years ago, the monsoons failed here,
and very quickly, the vegetation started to disappear.
Within a few hundred years or so,
this area had gone from savanna to desert.
And the people who settled this once verdant land
were forced to move north and east to a still-fertile river valley -
the Nile.
It's rather wonderful
to think that because the changes in our tilt and orbit
are cyclical, there may come a day
when the Sahara will be green once again.
But not for another 15,000 years.
Since we began our journey at the spring equinox in March,
the days have grown longer in the northern hemisphere
and the sun has arced higher in the sky.
That process reaches its climax on June the 21st -
the summer solstice.
Wherever you are north of the equator,
on the solstice, you'll experience the longest day of the year.
And there are few more significant places to be for the solstice
than one particular place, here in Egypt.
I've left the desert and travelled to the temple of Kom Ombo
near the ancient city of Aswan, on the Nile.
I've come in search of a famous shaft of solstice light.
The Earth's tilt reveals itself every time we step out
into the sun.
'And we can see it in the shadows that it casts.
'The most revealing of all are those cast by the noonday sun.
'In the temple precinct, there's a 2,000-year-old water well.
'It's also a perfect light well
'and that becomes obvious on the day of the solstice.'
Here at the bottom of the well, my shadow is directly beneath me
and there's no shadow at all being cast by the walls of the well.
It's midday on the summer solstice
and the sun is directly overhead.
'The solstice marks the day
'on which the Earth's tilt has its strongest impact
'on the northern hemisphere
'which is leaning to its maximum extent towards the sun.
'It's revealed in this way here in Aswan
'because I'm standing on a very particular point'
on the Earth's surface.
If we were to trace a line from Aswan right around the globe,
we'd be marking a line of latitude,
but also the furthest point north
at which the midday sun is directly overhead.
This is the tropic of Cancer.
And because the Earth is tilted at 23.4 degrees from the vertical,
the tropic of Cancer is 23.4 degrees above the equator.
The June solstice also defines
another significant line of latitude.
As the northern hemisphere points towards the sun,
the Arctic experiences 24 hours of daylight.
On the solstice, the midnight sun reaches its maximum extent -
a line marked by the Arctic Circle
which is 23.5 degrees from the North Pole.
Isn't it astonishing the Earth's tilt has such a dramatic impact?
It's that tilt that drives our seasons and powers our weather.
It's had a profound influence on our human history,
and even today it dictates how and where we live
on this extraordinary, unique planet of ours.
WAVES CRASH
BIRDS CRY
The summer solstice is where we end this part
of our voyage around the sun.
When we started at the spring equinox, day and night were equal
and we all had 12 hours of each one.
At our end point, the solstice,
the contrast between day and night, is at its greatest.
We've also reached the end of our year-long journey around the sun.
In this series,
we've travelled more than 900 million kilometres through space.
And in that time, we've seen how the Earth's spin
dictates the Earth's climate patterns...
..how changes in our orbit can transform our planet...
..and how the Earth's tilt controls the seasons.
Now our voyage is over.
But the planet goes on,
each new orbit creating its own unique mix
of endlessly varied, natural phenomena.
It's quite a ride!
Subtitles by Red Bee Media Ltd