0:00:10 > 0:00:14All of us, every day of our lives, are on the move.
0:00:14 > 0:00:18And we don't mean the morning commute or taking the kids to school,
0:00:18 > 0:00:22but a journey of epic proportions.
0:00:22 > 0:00:26Even now as you are watching this, you're hurtling through space
0:00:26 > 0:00:29at 100,000 kilometres an hour.
0:00:31 > 0:00:36KATE: Every year our planet travels around the sun and we go with it.
0:00:38 > 0:00:39I'm Kate Humble.
0:00:39 > 0:00:44This is it, the sun is directly overhead.
0:00:44 > 0:00:48My shadow is directly below me.
0:00:48 > 0:00:53In this series, we'll follow the Earth's voyage through space
0:00:53 > 0:00:55for one whole year
0:00:55 > 0:00:59to witness the astonishing consequences this journey
0:00:59 > 0:01:00has for us all.
0:01:00 > 0:01:02HELEN: I'm Dr Helen Czerski,
0:01:02 > 0:01:06and I study the physics of the natural world.
0:01:06 > 0:01:08Wow, look at that.
0:01:08 > 0:01:14I'll be investigating how our orbit powers the most spectacular weather
0:01:14 > 0:01:17and how it's also shaped and reshaped our planet.
0:01:19 > 0:01:21HELEN: As we travel through space,
0:01:21 > 0:01:25the Earth orbits the sun at an angle of just over 23 degrees.
0:01:28 > 0:01:30KATE: We're going to experience first hand
0:01:30 > 0:01:33the dramatic effects of the Earth's tilt.
0:01:33 > 0:01:37This is the moment we've been waiting for all day!
0:01:37 > 0:01:38HELEN: Through wild weather.
0:01:41 > 0:01:43It's really raining hard now!
0:01:44 > 0:01:48KATE: And back in time.
0:01:48 > 0:01:50All this here would have been covered in water.
0:01:50 > 0:01:56Join us on the most remarkable journey of your life.
0:02:09 > 0:02:12Since our journey began in July,
0:02:12 > 0:02:18we've travelled over 700 million kilometres around the sun.
0:02:18 > 0:02:22We've explored how our planet's orbit and spin
0:02:22 > 0:02:27have a fundamental effect on how we live on Earth.
0:02:27 > 0:02:32In this episode, we'll complete our year-long voyage and on the way,
0:02:32 > 0:02:34discover how another aspect
0:02:34 > 0:02:37of the Earth's relationship with the sun
0:02:37 > 0:02:40has changed the course of history.
0:02:42 > 0:02:44It's now March.
0:02:47 > 0:02:51BIRDSONG
0:02:51 > 0:02:54'And we start on a very special day...
0:02:55 > 0:02:57'...at a very special place.'
0:02:57 > 0:02:57'...at a very special place.'
0:03:08 > 0:03:11This is the great pyramid in Chichen Itza -
0:03:11 > 0:03:14an ancient Mayan city in Mexico.
0:03:19 > 0:03:21Built 1,500 years ago,
0:03:21 > 0:03:22the city is one of the world's great archaeological sites.
0:03:22 > 0:03:25the city is one of the world's great archaeological sites.
0:03:25 > 0:03:26the city is one of the world's great archaeological sites.
0:03:26 > 0:03:29And it contains a remarkable insight
0:03:29 > 0:03:32into our journey through space.
0:03:34 > 0:03:37The ancient Maya had developed a deep understanding
0:03:37 > 0:03:40of the Earth's movement around the sun,
0:03:40 > 0:03:43and they built it into the very fabric of this city.
0:03:46 > 0:03:49But it's something that can only be seen at two very precise
0:03:49 > 0:03:53and magical times of the year.
0:03:53 > 0:03:58One of those is today, March the 20th.
0:04:00 > 0:04:03As afternoon approaches, the city fills
0:04:03 > 0:04:05with followers of Mayan beliefs...
0:04:08 > 0:04:12..and those curious to see a millennia-old wonder.
0:04:19 > 0:04:23There is a unique and particular feature of our planet
0:04:23 > 0:04:25as it orbits the sun
0:04:25 > 0:04:28and it's encoded in the way that light and stone
0:04:28 > 0:04:31interact at the great pyramid.
0:04:31 > 0:04:36CHEERING
0:04:36 > 0:04:39This is the moment that all these thousands
0:04:39 > 0:04:41of people have been waiting for,
0:04:41 > 0:04:45they've all stood up and there are hands raised to welcome in the sun,
0:04:45 > 0:04:50and it's now aligned perfectly on the edge of the steps here,
0:04:50 > 0:04:53creating this very specific pattern of light and shade
0:04:53 > 0:04:56which resembles the body of a snake.
0:04:56 > 0:04:58And that's no coincidence
0:04:58 > 0:05:01because it joins up with the carved snake's head
0:05:01 > 0:05:03at the bottom of the pyramid.
0:05:09 > 0:05:13The Maya believed the snake, known as Kukulcan,
0:05:13 > 0:05:15was a messenger between gods and man.
0:05:18 > 0:05:24This is a remarkable display of Mayan architectural design.
0:05:24 > 0:05:29The appearance of this snake isn't an accident,
0:05:29 > 0:05:31they absolutely planned it
0:05:31 > 0:05:35and it happens on the same day every year.
0:05:35 > 0:05:37This is the spring equinox.
0:05:37 > 0:05:40DRUMS BEAT
0:05:42 > 0:05:45So, more than 1000 years ago,
0:05:45 > 0:05:49the Maya recognised the equinox as a pivotal moment in the year.
0:05:49 > 0:05:52CHEERING
0:05:52 > 0:05:55And they were able to align this pyramid
0:05:55 > 0:05:58with the sun's annual progress,
0:05:58 > 0:06:01causing the snake to appear each equinox.
0:06:01 > 0:06:04CHEERING
0:06:10 > 0:06:14CRASHING WAVES
0:06:17 > 0:06:20Here on Earth, there are a few moments that we all share,
0:06:20 > 0:06:24because we're all on the same journey around the sun.
0:06:24 > 0:06:27And one of those moments is the equinox,
0:06:27 > 0:06:29when day and night are equal.
0:06:33 > 0:06:37'It's a time of balance we can all experience,
0:06:37 > 0:06:39'wherever we are on the planet.'
0:06:42 > 0:06:46So whether you are here in Britain, amongst the fitful showers
0:06:46 > 0:06:48and overcast skies,
0:06:48 > 0:06:50'or in the bright spring sunshine of Mexico,'
0:06:50 > 0:06:54on the March equinox you'll get 12 hours of daylight
0:06:54 > 0:06:56and 12 hours of night time.
0:06:56 > 0:07:00That's if the sun ever comes through the clouds!
0:07:02 > 0:07:05But it's more than just a time of balance.
0:07:05 > 0:07:08It's also a turning point in our year.
0:07:10 > 0:07:11From the March equinox onwards,
0:07:11 > 0:07:14the days get longer in the northern hemisphere,
0:07:14 > 0:07:18'while in the southern hemisphere, the opposite occurs.
0:07:20 > 0:07:23'This is because of a special feature of our planet
0:07:23 > 0:07:26'as it journeys through space.'
0:07:26 > 0:07:29Let's say this rock is the sun.
0:07:29 > 0:07:32This is going to be our Earth,
0:07:32 > 0:07:36and as the Earth travels around its orbit
0:07:36 > 0:07:40spinning like this, it travels around on a flat plane.
0:07:40 > 0:07:43So you would think that its axis would point upwards
0:07:43 > 0:07:48but it isn't, it's tilted over at 23.4 degrees.
0:07:48 > 0:07:52'This means that the North Pole, the stem of the apple,
0:07:52 > 0:07:56'isn't vertical, it's at an angle.'
0:07:56 > 0:07:59And that tilt stays pointing in the same direction
0:07:59 > 0:08:01as the Earth travels around on its orbit.
0:08:01 > 0:08:01as the Earth travels around on its orbit.
0:08:05 > 0:08:08Because of this tilt for part of our orbit,
0:08:08 > 0:08:11the hemisphere north of the equator leans towards the sun.
0:08:14 > 0:08:17This brings with it extra solar energy,
0:08:17 > 0:08:20which fuels spring and then summer.
0:08:26 > 0:08:30Six months later, the situation is reversed.
0:08:30 > 0:08:33The southern hemisphere now leans towards the sun,
0:08:33 > 0:08:37while the northern hemisphere experiences declining energy,
0:08:37 > 0:08:39ushering in winter.
0:08:39 > 0:08:42Tilt creates the Earth's seasons.
0:08:43 > 0:08:46But there's a moment, twice a year as we orbit,
0:08:46 > 0:08:49when the sun favours neither hemisphere.
0:08:49 > 0:08:54At this point, both experience 12 hours of daylight and night time.
0:08:56 > 0:08:58This is the equinox.
0:09:03 > 0:09:05If the Earth wasn't tilted,
0:09:05 > 0:09:08every day would be like the equinox,
0:09:08 > 0:09:12with the 24 hours equally split between day and night.
0:09:12 > 0:09:14And that would mean no seasons.
0:09:18 > 0:09:23'his time we're following the Earth's journey from the spring equinox'
0:09:23 > 0:09:25to the point when the tilt of the Earth gives us
0:09:25 > 0:09:27our longest day of the year -
0:09:27 > 0:09:30June 21st, the summer solstice.
0:09:32 > 0:09:36Over this three-month period, seasonal warming sets in motion
0:09:36 > 0:09:40the greatest planetary transformations of the year.
0:09:44 > 0:09:48Winter has covered a great swathe of the northern hemisphere in snow.
0:09:50 > 0:09:54But now it's melting, receding to the edge of the Arctic Circle...
0:09:57 > 0:10:01..where spring is about to arrive in dramatic fashion.
0:10:03 > 0:10:06BIRDSONG
0:10:11 > 0:10:13This is the Hay River,
0:10:13 > 0:10:16which meanders north for 700 kilometres
0:10:16 > 0:10:19through the Canadian tundra.
0:10:19 > 0:10:22Here in the north,
0:10:22 > 0:10:25the river is still in the grip of winter ice.
0:10:29 > 0:10:32But upstream to the south, the ice
0:10:32 > 0:10:35has been cracked by the spring warmth.
0:10:35 > 0:10:40By the end of April, the broken ice is on the move.
0:10:44 > 0:10:47At this point in its journey north,
0:10:47 > 0:10:50the river tumbles over a 35-metre drop,
0:10:50 > 0:10:53giving us this spectacular sight.
0:10:53 > 0:10:55This is Alexandra Falls.
0:10:55 > 0:10:59And you can see that the central flow is flowing strongly
0:10:59 > 0:11:00and does all winter
0:11:00 > 0:11:04but the majority of the falls are still frozen solid.
0:11:06 > 0:11:11For six months, hardly anything at these falls has changed.
0:11:11 > 0:11:14Now, in the space of just a few hours,
0:11:14 > 0:11:16a transformation has begun...
0:11:19 > 0:11:23..as the ice armada approaches from the warmer south.
0:11:23 > 0:11:26ICE BOULDERS GRIND
0:11:31 > 0:11:33But there is still not enough water in the river
0:11:33 > 0:11:37to force the ice over the falls...
0:11:38 > 0:11:43..and it piles up in a great ice dam.
0:11:43 > 0:11:46ICE GRINDS
0:11:48 > 0:11:50But eventually,
0:11:50 > 0:11:52it gives way.
0:11:59 > 0:12:01ROARING AND CRACKING
0:12:01 > 0:12:05This is the moment we've been waiting for all day.
0:12:05 > 0:12:07All this broken ice
0:12:07 > 0:12:10has been backing up behind the waterfall,
0:12:10 > 0:12:13and what needed to happen to shift it
0:12:13 > 0:12:16was for the water level just to come up.
0:12:16 > 0:12:19And it's literally just happened and as you can see,
0:12:19 > 0:12:19And it's literally just happened and as you can see,
0:12:19 > 0:12:23it is pouring and pouring
0:12:23 > 0:12:27over the waterfall, in this great dramatic jumble
0:12:27 > 0:12:29of mud and broken ice.
0:12:29 > 0:12:32ROARING
0:12:46 > 0:12:50It's just mesmerising to watch.
0:12:50 > 0:12:53RUSHING WATER
0:12:55 > 0:12:58Look at this huge piece now, falling off.
0:13:00 > 0:13:04And you know at home when we talk about the arrival of spring,
0:13:04 > 0:13:09and we talk about the snowdrops coming and the first birds tweeting,
0:13:09 > 0:13:11well, this is spring, Hay River-style!
0:13:11 > 0:13:15There's nothing gentle or quiet about it.
0:13:15 > 0:13:19It's violent, it's noisy
0:13:19 > 0:13:25and it's entirely speeded up by these warm meltwaters
0:13:25 > 0:13:27that have come from the south.
0:13:33 > 0:13:39By April, increasing warmth from the spring sun is transforming
0:13:39 > 0:13:41the landscape of the northern hemisphere.
0:13:41 > 0:13:44And it's turning green.
0:13:45 > 0:13:49Using photosynthesis, plants convert the sun's energy
0:13:49 > 0:13:53into the fuel needed for them to grow and flower.
0:14:00 > 0:14:02CUBS BARK
0:14:02 > 0:14:05Everywhere, nature is responding to these changing conditions.
0:14:08 > 0:14:11CHICKS SQUAWK
0:14:11 > 0:14:15North of the Hay River, the caribou are on the move,
0:14:15 > 0:14:19heading towards the newly revealed pastures.
0:14:21 > 0:14:25The first to arrive are the pregnant females.
0:14:25 > 0:14:28Within a couple of weeks, the rest of the herd gathers
0:14:28 > 0:14:33until as many as 150,000 animals are together.
0:14:33 > 0:14:36The Arctic has come to life.
0:14:39 > 0:14:42On the other side of the planet, in the southern hemisphere,
0:14:42 > 0:14:46the opposite seasonal change is underway.
0:14:46 > 0:14:46Temperatures are plummeting with the shorter days of autumn.
0:14:46 > 0:14:50Temperatures are plummeting with the shorter days of autumn.
0:14:50 > 0:14:53The average temperature at this time of year
0:14:53 > 0:14:56is minus 40 degrees Celsius.
0:14:57 > 0:15:01Animals, rather sensibly, abandon the continent,
0:15:01 > 0:15:04with one notable exception.
0:15:04 > 0:15:07The Emperor Penguin.
0:15:07 > 0:15:11They choose these short, freezing days to mate,
0:15:11 > 0:15:14because the sea ice has re-formed,
0:15:14 > 0:15:18and is now strong enough to support their vast breeding colonies.
0:15:21 > 0:15:26All over our planet, the natural world reacts to the shifting energy
0:15:26 > 0:15:28we receive from the sun.
0:15:38 > 0:15:41HELEN: As our planet's orbit takes us towards June,
0:15:41 > 0:15:45the Earth's tilt powers great seasonal change.
0:15:45 > 0:15:48And this gives rise to some dramatic weather phenomena
0:15:48 > 0:15:52that are concentrated at this time of year.
0:15:54 > 0:16:00The most extreme occurs over the Midwest of the United States.
0:16:04 > 0:16:07Every spring day we experience the interaction
0:16:07 > 0:16:10between Earth's orbit and its tilt.
0:16:12 > 0:16:16At its most simple, the days get longer and the land gets warmer.
0:16:16 > 0:16:19But it also affects the atmosphere,
0:16:19 > 0:16:21with important consequences.
0:16:23 > 0:16:26It's the driving force behind the most significant weather events
0:16:26 > 0:16:28on our planet.
0:16:30 > 0:16:35WIND WHIRLS FEROCIOUSLY
0:16:35 > 0:16:40A tornado is the most volatile of these seasonal weather events.
0:16:40 > 0:16:43They occur most frequently in the spring
0:16:43 > 0:16:46and especially in the Midwest of America -
0:16:46 > 0:16:49a region known as Tornado Alley.
0:16:50 > 0:16:55MAN: 'Did you see that? The whole house came apart!
0:16:55 > 0:16:57'Oh, my God!'
0:17:04 > 0:17:06HELEN: But despite its violence,
0:17:06 > 0:17:11at the core of a tornado is a very simple process.
0:17:16 > 0:17:18This goes on like a backpack.
0:17:18 > 0:17:21To experience it, I'm taking to the air,
0:17:21 > 0:17:25over the Midwestern state of Colorado.
0:17:25 > 0:17:28One, two, three, go. Run!
0:17:30 > 0:17:33Paragliding pilots like Honza Rejmanek,
0:17:33 > 0:17:35love this time of year.
0:17:44 > 0:17:47Spring provides the perfect conditions for soaring...
0:17:55 > 0:17:59..because the increasing temperatures generate thermals.
0:18:03 > 0:18:05So right now we are in a thermal.
0:18:05 > 0:18:09These are basically almost like invisible smokestacks of rising air.
0:18:09 > 0:18:12Right now we've found one, I'm going to take a turn in it
0:18:12 > 0:18:15and circle around and try to gain height.
0:18:15 > 0:18:16and circle around and try to gain height.
0:18:16 > 0:18:20'Thermals form when the sun warms the ground,
0:18:20 > 0:18:23'and the ground, in turn, warms the air above it.'
0:18:23 > 0:18:26What I'm experiencing
0:18:26 > 0:18:30is one of the most fundamental principles of atmospheric physics -
0:18:30 > 0:18:30warmer air rises.
0:18:30 > 0:18:32warmer air rises.
0:18:34 > 0:18:38'When air warms, it expands and becomes less dense.
0:18:38 > 0:18:42'So this air has a lower atmospheric pressure
0:18:42 > 0:18:45'than the cooler air that surrounds it.'
0:18:45 > 0:18:49So it floats upwards, forming this rising thermal column.
0:18:55 > 0:18:57The atmosphere tries to even out differences
0:18:57 > 0:18:59in air temperature and pressure,
0:18:59 > 0:19:03attempting to return to equilibrium.
0:19:03 > 0:19:08So the rising thermal will mix with the cooler air above.
0:19:09 > 0:19:13This basic process of moving towards equilibrium
0:19:13 > 0:19:17lies at the heart of every significant weather event
0:19:17 > 0:19:18on the planet.
0:19:29 > 0:19:32'But in the springtime air over Tornado Alley,
0:19:32 > 0:19:34'there's a regional anomaly
0:19:34 > 0:19:39'that intensifies this basic atmospheric process.
0:19:39 > 0:19:42'The result is that here,
0:19:42 > 0:19:45'particularly powerful storms can develop.'
0:19:47 > 0:19:51There's a stable layer of dry air that acts as a barrier
0:19:51 > 0:19:54between the warm air down below and the cooler air higher up.
0:19:54 > 0:19:56So the warm air is trapped,
0:19:56 > 0:19:59and what's more, the ground keeps heating it as the day goes on.
0:19:59 > 0:20:02WIND WHISTLES
0:20:02 > 0:20:04THUNDERCLAP
0:20:04 > 0:20:09The thermals get more and more powerful until, by late afternoon,
0:20:09 > 0:20:14they finally punch through the barrier layer at colossal speed.
0:20:14 > 0:20:19These rapid updraughts of less dense, lower pressure air
0:20:19 > 0:20:23are so strong that they generate huge thunderstorms.
0:20:23 > 0:20:26THUNDER RUMBLES
0:20:28 > 0:20:32It's from these thunderstorms that, in certain conditions,
0:20:32 > 0:20:35tornadoes can emerge.
0:20:43 > 0:20:46'I'm going to investigate how this happens...'
0:20:46 > 0:20:49Not as bad as north of us.
0:20:50 > 0:20:54...with the help of atmospheric scientist, Josh Wurman.
0:20:54 > 0:20:57I don't know what to make of these stringy little features.
0:20:59 > 0:21:02The first step in our quest for a tornado
0:21:02 > 0:21:04is locating a promising storm.
0:21:04 > 0:21:07After a couple of days on the road, we manage to intercept
0:21:07 > 0:21:11one moving north through Colorado.
0:21:13 > 0:21:16So what's happening behind me is the storm is building
0:21:16 > 0:21:18and in the middle of that storm over there,
0:21:18 > 0:21:21there's an updraught with low pressure at the centre of it.
0:21:21 > 0:21:25And all the air around the outside has higher pressure,
0:21:25 > 0:21:27and that high pressure is pushing air into the centre
0:21:27 > 0:21:31and up into the storm, and that's what building the storm.
0:21:35 > 0:21:37The atmosphere tries to even out
0:21:37 > 0:21:40the extreme differences in temperature
0:21:40 > 0:21:41that have been generated.
0:21:41 > 0:21:44So the air movements at the core of the storm
0:21:44 > 0:21:46become exceptionally powerful.
0:21:48 > 0:21:52'Hail is one characteristic product of this atmospheric violence.'
0:21:56 > 0:22:00'The hail formed when an updraught cooled rapidly,
0:22:00 > 0:22:05'so that water condensed out of the air, and turned immediately to ice.'
0:22:05 > 0:22:09SHOUTING: This is what was carried from the south,
0:22:09 > 0:22:12and it was pushed up into the storm
0:22:12 > 0:22:15and it gave the storm its energy.
0:22:15 > 0:22:18And now it's falling back down on me!
0:22:18 > 0:22:20GIGGLES
0:22:20 > 0:22:22Wow!
0:22:22 > 0:22:26CAMERAMAN: That's it. Let's get inside. This is too hard now.
0:22:26 > 0:22:30And even though this is chaotic and messy,
0:22:30 > 0:22:34what this is, is a demonstration that the atmosphere is an unstable place,
0:22:34 > 0:22:38and there are all these differences in temperatures and pressures.
0:22:38 > 0:22:42And this is what happens when the atmosphere moves around
0:22:42 > 0:22:45to even everything out, and make it all the same.
0:22:45 > 0:22:47It's not looking very peaceful at the moment
0:22:47 > 0:22:49but that's what it's trying to get back to.
0:22:51 > 0:22:54THUNDER RUMBLES
0:22:56 > 0:23:01When tornadoes do form, they are often preceded by hail.
0:23:01 > 0:23:03'But this time, there's no twister.
0:23:06 > 0:23:07'So we're back on the road,
0:23:07 > 0:23:10'still trying to see a storm spawn a tornado.
0:23:13 > 0:23:17'After a week of tracking promising storms without success,
0:23:17 > 0:23:19'Josh's specialist radar detects one
0:23:19 > 0:23:22'which shows a revealing swirl of clouds.
0:23:25 > 0:23:31'We have to move fast - tornadoes form and vanish very quickly.'
0:23:31 > 0:23:33JOSH: Going out ahead, this big dark area's the core.
0:23:33 > 0:23:36So we're basically going to penetrate through the core
0:23:36 > 0:23:38and see what's interesting.
0:23:40 > 0:23:44'Tornadoes form when powerful rotating cylinders of air
0:23:44 > 0:23:45'within the storm
0:23:45 > 0:23:49'get caught by an updraught and are knocked on their side.'
0:23:49 > 0:23:53Right now, we're kind of in the centre of the coiled part of this...
0:23:53 > 0:23:56'When that column of rotating air touches the ground,
0:23:56 > 0:23:58'a tornado is born.'
0:23:58 > 0:24:04INDISTINCT RADIO CONVERSATION
0:24:12 > 0:24:16At the tornado's core is an area of intense low pressure,
0:24:16 > 0:24:19which draws high pressure air towards it.
0:24:21 > 0:24:24The dust and debris picked up by the tornado
0:24:24 > 0:24:26reveal the swirling pattern of winds.
0:24:48 > 0:24:50So, this is it.
0:24:50 > 0:24:51The high pressure is swirling inwards and up that funnel.
0:24:51 > 0:24:53The high pressure is swirling inwards and up that funnel.
0:24:53 > 0:24:55And it's enormous!
0:24:57 > 0:25:00I had no idea it would look that big!
0:25:00 > 0:25:02That's just amazing!
0:25:02 > 0:25:04And here it's almost calm.
0:25:04 > 0:25:08But over there, those winds are going at hundreds of miles an hour,
0:25:08 > 0:25:13pushing stuff right up into the heart of the storm.
0:25:13 > 0:25:16I just... I can't stop looking at it, it's incredible.
0:25:22 > 0:25:28Just 15 minutes after it first touched down, the tornado dissipates.
0:25:31 > 0:25:34There's still so much that we don't understand about storms.
0:25:34 > 0:25:36We don't understand when they're going to produce hail,
0:25:36 > 0:25:38when they're going to produce rain,
0:25:38 > 0:25:41when they are going to produce tornadoes.
0:25:41 > 0:25:45But what we do understand is that a storm like this
0:25:45 > 0:25:49is a manifestation of something happening round us all the time.
0:25:53 > 0:25:57Our planet's atmosphere is a mosaic of warmer and cooler air masses,
0:25:57 > 0:25:59constantly in motion.
0:26:01 > 0:26:05The air is rising, falling and swirling around
0:26:05 > 0:26:09as it seeks to balance differences in temperature and pressure
0:26:09 > 0:26:12and return to equilibrium.
0:26:12 > 0:26:15During April and May,
0:26:15 > 0:26:19the effect of the Earth's tilt is to enhance those differences
0:26:19 > 0:26:24by increasing surface temperatures, which in turn heat the air.
0:26:27 > 0:26:30So all over the northern hemisphere,
0:26:30 > 0:26:34spring is the season for volatile storms.
0:26:34 > 0:26:40Tornadoes are only one consequence.
0:26:40 > 0:26:46The heavy and sudden downpours from storms can result in flash floods...
0:26:46 > 0:26:49..like the one that hit the town of Barranquilla in Colombia
0:26:49 > 0:26:52in May 2011.
0:26:53 > 0:26:58These occur when the rain inundates densely saturated ground.
0:26:58 > 0:27:04The water isn't fully absorbed, but instead flows rapidly downhill
0:27:04 > 0:27:07in a near-instantaneous torrent.
0:27:09 > 0:27:14Thunderstorms can also give birth to an unexpected phenomenon...
0:27:17 > 0:27:21..massive dust storms called haboobs.
0:27:21 > 0:27:24This one blew into Phoenix, Arizona in 2011.
0:27:29 > 0:27:32Haboobs are produced in normally arid regions,
0:27:32 > 0:27:36when the leading edge of a storm collapses,
0:27:36 > 0:27:38generating a super-fast downdraught
0:27:38 > 0:27:42that kicks up a wall of dust and sand in front of it.
0:27:54 > 0:27:58As May turns to June, the volatility in our atmosphere
0:27:58 > 0:28:02drives the biggest single weather event on the planet.
0:28:07 > 0:28:11An event centred on the Indian subcontinent.
0:28:14 > 0:28:16TRAFFIC HUMS
0:28:16 > 0:28:19CAR HORNS TOOT
0:28:19 > 0:28:22This is the city of Udaipur in Rajasthan.
0:28:26 > 0:28:31It's in the northwestern corner of India.
0:28:34 > 0:28:37Since March, temperatures here
0:28:37 > 0:28:40have been steadily rising as the Earth's tilt
0:28:40 > 0:28:42has warmed the northern hemisphere.
0:28:42 > 0:28:48But by June, everything is on the brink of an exhilarating change.
0:28:55 > 0:29:00I'm here at the time of an epic weather event of huge importance
0:29:00 > 0:29:03not just to Rajasthan but to the whole subcontinent
0:29:03 > 0:29:07and the over billion people who live here.
0:29:11 > 0:29:15'There's a wonderful place to appreciate the event's significance,
0:29:15 > 0:29:19'on one of the hills that overlook the city,
0:29:19 > 0:29:23'here, at this cliff-top palace.'
0:29:23 > 0:29:26It was built at the end of the 19th century
0:29:26 > 0:29:29by the 72nd Maharana of Udaipur
0:29:29 > 0:29:33and it's known as Sajjan Garh.
0:29:35 > 0:29:38'Although now abandoned, Sajjan Garh's halls
0:29:38 > 0:29:42'and courtyards still have an evocative, fading grandeur.'
0:29:47 > 0:29:51The palace was designed with a whole series of balconies and verandas
0:29:51 > 0:29:54and you do get the most staggering view of the city from up here.
0:29:56 > 0:29:59But that's not what the Maharana was interested in.
0:30:05 > 0:30:08He built this palace to get a pure, unadulterated view of the sky
0:30:08 > 0:30:11He built this palace to get a pure, unadulterated view of the sky
0:30:11 > 0:30:15and the clouds that start to build at this time of year.
0:30:15 > 0:30:18Sajjan Garh is the monsoon palace.
0:30:23 > 0:30:26When the rains do eventually arrive,
0:30:26 > 0:30:31they'll be an essential relief from the heat of the Indian summer.
0:30:31 > 0:30:32But what's intriguing is that the monsoon is actually a consequence
0:30:32 > 0:30:35But what's intriguing is that the monsoon is actually a consequence
0:30:35 > 0:30:40of the rising seasonal temperatures that precede it.
0:30:40 > 0:30:43To reveal why this is, we need to travel 2,000 kilometres...
0:30:43 > 0:30:45To reveal why this is, we need to travel 2,000 kilometres...
0:30:45 > 0:30:46..south.
0:30:56 > 0:31:00I'm in the coastal state where the monsoon first arrives in India -
0:31:00 > 0:31:02Kerala.
0:31:05 > 0:31:11The key to understanding the monsoon is here, on the beach.
0:31:11 > 0:31:13The monsoon is powered by a simple,
0:31:13 > 0:31:16but incredibly significant difference -
0:31:16 > 0:31:18the difference between land and sea.
0:31:18 > 0:31:20the difference between land and sea.
0:31:20 > 0:31:25And in particular, the differing ways in which they respond to the sun.
0:31:30 > 0:31:32Take this sand as an example.
0:31:32 > 0:31:35The sun's energy is heating all of this surface,
0:31:35 > 0:31:39but if I dig down just a little way...
0:31:39 > 0:31:42..the sand underneath is quite cool, and that's quite familiar,
0:31:42 > 0:31:44we see that on sunny beaches all the time.
0:31:44 > 0:31:46And here, where it gets really hot,
0:31:46 > 0:31:49the surface can reach 40 degrees Celsius.
0:31:49 > 0:31:52Just 15 centimetres down into the sand,
0:31:52 > 0:31:54it can be only 7 degrees Celsius.
0:31:54 > 0:31:57So, all the sun's energy is going into a really thin surface layer,
0:31:57 > 0:32:02and that layer heats up really, really, quickly.
0:32:02 > 0:32:04The sun is also beating down on the ocean,
0:32:04 > 0:32:07and that responds very, very differently.
0:32:13 > 0:32:16This water is much warmer than the sea at home
0:32:16 > 0:32:19but it's much cooler than the beach,
0:32:19 > 0:32:20and the reason for that
0:32:20 > 0:32:24is that the ocean takes much more of the sun's energy to heat it up.
0:32:24 > 0:32:27So a kilogram of water will take three times as much energy
0:32:27 > 0:32:30as a kilogram of sand to heat by one degree.
0:32:32 > 0:32:35The ocean is also relatively cool because to heat the surface
0:32:35 > 0:32:38you have to heat much more than just a thin layer.
0:32:38 > 0:32:42What happens is that winds that blow across the surface of the ocean
0:32:42 > 0:32:44generate turbulence which mixes that top layer.
0:32:44 > 0:32:47So as soon as some water's been heated at the top,
0:32:47 > 0:32:49it gets mixed down below.
0:32:51 > 0:32:56'This means that, unlike the land, the ocean warms up only very slowly,
0:32:56 > 0:32:59'as the sun's energy is absorbed.
0:33:01 > 0:33:03'So as we enter summer, the land heats up quickly,
0:33:03 > 0:33:07'while the ocean lags further and further behind.'
0:33:09 > 0:33:12This increasing temperature difference is critical,
0:33:12 > 0:33:16because both land and sea heat the air above them.
0:33:19 > 0:33:22As the sun has baked the Indian subcontinent,
0:33:22 > 0:33:25the land has warmed the air above it.
0:33:25 > 0:33:29The warmer air is less dense, so it rises.
0:33:29 > 0:33:32This draws in the cooler air from the ocean.
0:33:32 > 0:33:36Because of India's particular geography,
0:33:36 > 0:33:39this process is magnified.
0:33:39 > 0:33:42It's a triangular peninsula, with wide, hot plains
0:33:42 > 0:33:45and, crucially, a very long coastline.
0:33:45 > 0:33:48This combination sets up a powerful
0:33:48 > 0:33:52and sustained movement of cooler ocean air -
0:33:52 > 0:33:54the monsoon wind.
0:33:57 > 0:33:59Of course when most of us think of a monsoon
0:33:59 > 0:34:01we think not of seasonal winds, but of rain.
0:34:04 > 0:34:05'By setting up a time-lapse camera,
0:34:05 > 0:34:08'I'm hoping to watch the rain clouds forming.'
0:34:14 > 0:34:17THUNDERCLAP
0:34:24 > 0:34:26Wow!
0:34:36 > 0:34:38There is an enormous process on the go here.
0:34:38 > 0:34:40When the sun shines down on the ocean surface,
0:34:40 > 0:34:43some of the water at the surface will evaporate,
0:34:43 > 0:34:46so water and energy are carried up into the atmosphere.
0:34:46 > 0:34:48And as the monsoon winds come inland
0:34:48 > 0:34:51and they carry that water vapour with them,
0:34:51 > 0:34:54the heated land makes that moist air rise,
0:34:54 > 0:34:57goes up into the clouds and there droplets condense -
0:34:57 > 0:35:00the water condenses out, becomes visible, we see clouds.
0:35:00 > 0:35:04When those droplets join together to form droplets which are large enough,
0:35:04 > 0:35:06we get rain like this.
0:35:06 > 0:35:09And it's really raining hard now!
0:35:11 > 0:35:15So, what we are seeing now is a thin layer of the ocean
0:35:15 > 0:35:16that's been lifted up,
0:35:16 > 0:35:20shifted over here and is now being dumped on top of me.
0:35:24 > 0:35:27'None of this would be happening if it wasn't for the Earth's tilt.
0:35:29 > 0:35:32'It's the seasonal heating is what widens the gap in temperature
0:35:32 > 0:35:35'between the land and the sea', and this drives everything.
0:35:35 > 0:35:40And this massive system of rain and wind rushes inland
0:35:40 > 0:35:41and that's the monsoon.
0:35:45 > 0:35:48I'm wet! So drenched!
0:35:48 > 0:35:52I feel like I've been in a shower for about ten minutes!
0:35:52 > 0:35:53I suppose I have(!)
0:35:55 > 0:35:5880% of all India's rains
0:35:58 > 0:36:01arrive in this seasonal deluge.
0:36:02 > 0:36:07It's not just the volume of the monsoon rains which is impressive.
0:36:07 > 0:36:08It's the distance they travel.
0:36:12 > 0:36:14As summer progresses in India,
0:36:14 > 0:36:19the difference in temperature between land and ocean actually increases.
0:36:20 > 0:36:25This makes the whole monsoon system more powerful,
0:36:25 > 0:36:29drawing this moisture-laden air further and further inland.
0:36:33 > 0:36:37From when the monsoon first arrives on the Kerala coast
0:36:37 > 0:36:38around June the 1st,
0:36:38 > 0:36:41it spreads more than 2,000 kilometres
0:36:41 > 0:36:46until it eventually reaches the far north of the country.
0:36:46 > 0:36:48Including Rajasthan.
0:36:52 > 0:36:54VOICES CLAMOUR
0:36:54 > 0:36:58CAR HORNS TOOT
0:36:58 > 0:37:01It's remarkable that the moisture-laden winds
0:37:01 > 0:37:05that originate many hundreds of kilometres to the south from here
0:37:05 > 0:37:09are still capable of delivering rain in Rajasthan.
0:37:11 > 0:37:15The rains aren't nearly as heavy here as they are in Kerala.
0:37:15 > 0:37:17They tend to fall in short bursts
0:37:17 > 0:37:21and sometimes there are several days between downpours.
0:37:24 > 0:37:29And sometimes the monsoon fails altogether.
0:37:29 > 0:37:33So effective systems of storing rainwater are critical.
0:37:43 > 0:37:47This is Lake Pichola, and for the tourists that flock to Udaipur
0:37:47 > 0:37:52in their thousands, it's a must see on their itinerary.
0:37:54 > 0:37:57But the jewel is this, the Lake Palace,
0:37:57 > 0:38:00which looks like it's almost floating on the surface
0:38:00 > 0:38:03and is entirely surrounded by water.
0:38:13 > 0:38:18But what's truly surprising is that this lake isn't natural at all.
0:38:18 > 0:38:22It's a man-made reservoir built specifically to capture
0:38:22 > 0:38:24those precious monsoon rains.
0:38:29 > 0:38:32It was built hundreds of years ago
0:38:32 > 0:38:36and covers about seven square kilometres.
0:38:38 > 0:38:42But even a reservoir this size doesn't guarantee
0:38:42 > 0:38:46the people of Udaipur a permanent supply of water.
0:38:46 > 0:38:50As recently as 2009, when the monsoon failed here,
0:38:50 > 0:38:53the entire lake dried up.
0:38:56 > 0:39:00A stark reminder that the balance of life in this part of India
0:39:00 > 0:39:03is totally dependent on the differing ways
0:39:03 > 0:39:07that the land and the ocean respond to the sun.
0:39:18 > 0:39:22The monsoon is the Earth's biggest global weather event.
0:39:22 > 0:39:27But it shares the same root cause as the smallest local rain shower
0:39:27 > 0:39:29and that's the Earth's tilt,
0:39:29 > 0:39:33which drives seasonal variations in temperatures
0:39:33 > 0:39:36of the land, sea and air.
0:39:36 > 0:39:38So the question is,
0:39:38 > 0:39:42why is the Earth tilted in the first place?
0:39:48 > 0:39:52Our 23 degree tilt is just right.
0:39:52 > 0:39:56It's enough to provide a relatively benign seasonal shift.
0:39:56 > 0:39:59It makes our planet habitable.
0:40:02 > 0:40:04Here in America, we can get an insight
0:40:04 > 0:40:07into how the Earth might have got its tilt.
0:40:16 > 0:40:20This is the Barringer Crater in Arizona.
0:40:20 > 0:40:2450,000 years ago a meteorite struck this site
0:40:24 > 0:40:29and just look what it left behind - this enormous hole in the ground.
0:40:34 > 0:40:37'This impact would have thrown debris out
0:40:37 > 0:40:40'over tens of thousands of square kilometres.'
0:40:40 > 0:40:44And all the rock around here, like this,
0:40:44 > 0:40:47is what's left after that explosive event.
0:40:49 > 0:40:54This enormous crater is like a lesson in how size isn't everything,
0:40:54 > 0:40:56because the crater itself is a kilometre across,
0:40:56 > 0:41:00but the thing that caused it was only about 50 metres in diameter,
0:41:00 > 0:41:02which is really quite small.
0:41:02 > 0:41:06And the reason that such a small thing could cause such a big hole
0:41:06 > 0:41:09is because it was travelling so fast.
0:41:11 > 0:41:13'Impacts like these are extremely rare
0:41:13 > 0:41:17'but in the Earth's past, they were far more common
0:41:17 > 0:41:19'and a lot bigger.'
0:41:23 > 0:41:26Around four and a half billion years ago,
0:41:26 > 0:41:29the solar system was still in the process of formation.
0:41:32 > 0:41:38The Earth was just one of many of protoplanets that orbited the sun.
0:41:38 > 0:41:40Amongst these protoplanets
0:41:40 > 0:41:45was a small Mars-sized planet that's been named Theia.
0:41:46 > 0:41:50Its orbit put it on a collision course with the Earth.
0:41:54 > 0:41:58Theia smashed into the larger Earth and was obliterated.
0:42:02 > 0:42:06The impact very nearly destroyed our planet too.
0:42:06 > 0:42:09The collision knocked the planet over,
0:42:09 > 0:42:12tilting the Earth's axis of rotation.
0:42:16 > 0:42:17This tilted Earth might still be oscillating madly,
0:42:18 > 0:42:20This tilted Earth might still be oscillating madly,
0:42:20 > 0:42:24were it not for another consequence of Theia's impact.
0:42:24 > 0:42:29A huge amount of debris was blasted into space.
0:42:29 > 0:42:32Gradually, this debris coalesced,
0:42:32 > 0:42:34captured by the Earth's gravity...
0:42:34 > 0:42:37...and it formed the moon.
0:42:43 > 0:42:47Billions of years later, the gravity of the sun and the moon together,
0:42:47 > 0:42:52act as a sort of counterweight, stabilising our tilt.
0:42:52 > 0:42:57It's extraordinary to think that the moon is both evidence
0:42:57 > 0:43:00of what caused Earth's 23 degree tilt
0:43:00 > 0:43:03and the celestial object that helps maintain it.
0:43:06 > 0:43:10But the stabilisation the moon provides isn't perfect.
0:43:12 > 0:43:15And the smallest variations in the angle of tilt
0:43:15 > 0:43:18can have profound consequences.
0:43:21 > 0:43:26Remarkable evidence for this can be found in the Egyptian desert.
0:43:38 > 0:43:41This is the Sahara.
0:43:41 > 0:43:44Hidden in this apparently lifeless landscape
0:43:44 > 0:43:49is proof that the Earth's tilt has changed, and in the recent past.
0:43:51 > 0:43:54And that change has transformed climate
0:43:54 > 0:43:56and history.
0:43:58 > 0:44:01With me is geographer Nick Drake.
0:44:01 > 0:44:04He's a veteran explorer of this region.
0:44:08 > 0:44:11We travel through the desert for 600 kilometres
0:44:11 > 0:44:13to reach our destination.
0:44:25 > 0:44:29This is the Gilf Kebir - the Great Barrier.
0:44:32 > 0:44:33For hundreds of years,
0:44:33 > 0:44:37explorers have come here in search of a lost world.
0:44:39 > 0:44:42A decade ago, one group succeeded.
0:44:47 > 0:44:52In 2002, a couple of Italians were exploring this region
0:44:52 > 0:44:55when they spotted that cave.
0:45:10 > 0:45:14I don't think even they could have hoped
0:45:14 > 0:45:17for something as spectacular as this.
0:45:21 > 0:45:24The extraordinary paintings in the Cave of Beasts
0:45:24 > 0:45:27are around 8,000 years old.
0:45:27 > 0:45:31More than 3,000 years older than the pyramids.
0:45:34 > 0:45:40When you start to look more closely at the figures on the wall,
0:45:40 > 0:45:44this seemed to be a very athletic population of people.
0:45:44 > 0:45:49They all seem to be running or jumping or throwing things.
0:45:49 > 0:45:53But you've also got wonderful pictures of antelope here,
0:45:53 > 0:45:55there's one, two, three, four... four of them.
0:45:55 > 0:45:59Could be a springbok with their dark and light colouring.
0:45:59 > 0:46:04You've also got lots of images of giraffe.
0:46:04 > 0:46:07That is undoubtedly a giraffe -
0:46:07 > 0:46:11you see the head, the long neck coming down,
0:46:11 > 0:46:13long legs.
0:46:13 > 0:46:15But there are some figures
0:46:15 > 0:46:17that are in a very...
0:46:17 > 0:46:21..strange position indeed. Here's one...
0:46:23 > 0:46:26..and it's a bit worn...
0:46:26 > 0:46:30There's another one here - there's a line of them.
0:46:32 > 0:46:33There's one there.
0:46:33 > 0:46:36And there is a theory
0:46:36 > 0:46:39that they could be swimming.
0:46:41 > 0:46:43A whole line.
0:46:50 > 0:46:54So where did the waters that sustained those people and animals
0:46:54 > 0:46:55come from?
0:46:58 > 0:47:03A day's travel away is the valley of Wadi Bakht.
0:47:03 > 0:47:07Here, there are clues that have helped to resolve this mystery.
0:47:09 > 0:47:12So when you come to landscapes like this,
0:47:12 > 0:47:15does everything speak to you and tell you, you know,
0:47:15 > 0:47:18this is what was happening X thousand years ago?
0:47:18 > 0:47:19It does to me, now, most of it.
0:47:19 > 0:47:21It does to me, now, most of it.
0:47:21 > 0:47:24But at the beginning, you're learning to interpret the landscape.
0:47:26 > 0:47:31Nick Drake studies the ancient geology of the African deserts.
0:47:32 > 0:47:35This sediment here is sand. OK.
0:47:35 > 0:47:38And this above it is clay. Right.
0:47:38 > 0:47:41So the sand is, when it's dry, it's blowing around,
0:47:41 > 0:47:43depositing here.
0:47:43 > 0:47:45And then we get wet,
0:47:45 > 0:47:48and we get rivers transporting these clays
0:47:48 > 0:47:50and depositing them at this spot.
0:47:50 > 0:47:54And we've got quite a long period, I think, here, of wet. Yeah.
0:47:54 > 0:47:56Then here we get a little layer of sand,
0:47:56 > 0:47:59then a layer of clay, then a layer of sand, then a layer of clay.
0:47:59 > 0:48:02And I think these are annual, or maybe biannual events.
0:48:02 > 0:48:05We get a really big flood, doesn't evaporate in the winter,
0:48:05 > 0:48:06it lasts for more than one year,
0:48:06 > 0:48:09but they're certainly drying out relatively quickly
0:48:09 > 0:48:12suggesting a seasonal environment - wet, dry, wet, dry.
0:48:14 > 0:48:17That pattern of highly seasonal rainfall
0:48:17 > 0:48:20can mean only one thing -
0:48:20 > 0:48:25this now barren desert, once received a monsoon.
0:48:27 > 0:48:31The geology of this site tells us that the rains fell in this area
0:48:31 > 0:48:35between 5,000 and 10,000 years ago...
0:48:37 > 0:48:40..transforming the landscape of Wadi Bakht
0:48:40 > 0:48:41and creating a lake.
0:48:43 > 0:48:46You can see these clay sediments, these grey sediments...
0:48:46 > 0:48:49This, all this here, would have been covered with water?
0:48:49 > 0:48:51Yep, probably going almost
0:48:51 > 0:48:54to the edge of where those rocks are, over there.
0:48:54 > 0:48:58The landscape we're looking at now would've been completely different?
0:48:58 > 0:48:59It would've been green,
0:48:59 > 0:49:03it would've been full of plants, possibly trees,
0:49:03 > 0:49:06the animals in the cave paintings would've been wandering round,
0:49:06 > 0:49:10drinking from this lake, and maybe even people swimming in it?
0:49:10 > 0:49:12Exactly. A savanna environment.
0:49:13 > 0:49:18Nick's research has revealed that the ancient African monsoon
0:49:18 > 0:49:21helped feed a verdant Sahara,
0:49:21 > 0:49:25a place crisscrossed by many rivers, with huge lakes -
0:49:25 > 0:49:29one was 20% bigger than the UK.
0:49:37 > 0:49:43The mystery, then, is what could have brought these rains here?
0:49:45 > 0:49:47We know from the Indian monsoon
0:49:47 > 0:49:51that when the land is hottest in the summer months,
0:49:51 > 0:49:57it creates a low pressure system which draws in cold, moist air.
0:49:57 > 0:50:01So the irony is, this part of the Sahara must have been receiving
0:50:01 > 0:50:06more of the sun's energy - it must have been hotter back then,
0:50:06 > 0:50:095,000 years ago, than it is today.
0:50:09 > 0:50:12And that's what allowed the monsoon rains
0:50:12 > 0:50:15to cover this area with water.
0:50:19 > 0:50:23What's remarkable is that the higher temperatures that drove
0:50:23 > 0:50:28the Saharan monsoon were the consequence of a tiny change
0:50:28 > 0:50:31in the angle of the Earth's tilt.
0:50:34 > 0:50:36Although the gravitational pull
0:50:36 > 0:50:41of the moon and sun together have stabilised our tilt,
0:50:41 > 0:50:44they don't do it perfectly.
0:50:47 > 0:50:52Today, the angle of tilt is 23.4 degrees,
0:50:52 > 0:50:56but over regular, 41,000-year cycles,
0:50:56 > 0:51:02the angle swings between 22 and 24.5 degrees.
0:51:02 > 0:51:05Back when the Sahara was green,
0:51:05 > 0:51:09the Earth's tilt was close to its maximum angle.
0:51:15 > 0:51:19Together with small cyclical changes in the direction of the tilt
0:51:19 > 0:51:21and the shape of our orbit,
0:51:21 > 0:51:25the result was the sun shone more intensely
0:51:25 > 0:51:27over the northern hemisphere,
0:51:27 > 0:51:30powering a monsoon in the Sahara.
0:51:32 > 0:51:35About 5,000 years ago, the monsoons failed here,
0:51:35 > 0:51:39and very quickly, the vegetation started to disappear.
0:51:39 > 0:51:42Within a few hundred years or so,
0:51:42 > 0:51:44this area had gone from savanna to desert.
0:51:47 > 0:51:51And the people who settled this once verdant land
0:51:51 > 0:51:56were forced to move north and east to a still-fertile river valley -
0:51:56 > 0:51:57the Nile.
0:51:59 > 0:52:01It's rather wonderful
0:52:01 > 0:52:04to think that because the changes in our tilt and orbit
0:52:04 > 0:52:07are cyclical, there may come a day
0:52:07 > 0:52:10when the Sahara will be green once again.
0:52:10 > 0:52:14But not for another 15,000 years.
0:52:21 > 0:52:25Since we began our journey at the spring equinox in March,
0:52:25 > 0:52:30the days have grown longer in the northern hemisphere
0:52:30 > 0:52:34and the sun has arced higher in the sky.
0:52:34 > 0:52:39That process reaches its climax on June the 21st -
0:52:39 > 0:52:41the summer solstice.
0:52:41 > 0:52:44Wherever you are north of the equator,
0:52:44 > 0:52:48on the solstice, you'll experience the longest day of the year.
0:52:56 > 0:52:56And there are few more significant places to be for the solstice
0:52:56 > 0:53:01And there are few more significant places to be for the solstice
0:53:01 > 0:53:05than one particular place, here in Egypt.
0:53:07 > 0:53:11I've left the desert and travelled to the temple of Kom Ombo
0:53:11 > 0:53:15near the ancient city of Aswan, on the Nile.
0:53:15 > 0:53:20I've come in search of a famous shaft of solstice light.
0:53:20 > 0:53:24The Earth's tilt reveals itself every time we step out
0:53:24 > 0:53:26into the sun.
0:53:26 > 0:53:30'And we can see it in the shadows that it casts.
0:53:30 > 0:53:36'The most revealing of all are those cast by the noonday sun.
0:53:38 > 0:53:44'In the temple precinct, there's a 2,000-year-old water well.
0:53:44 > 0:53:46'It's also a perfect light well
0:53:46 > 0:53:50'and that becomes obvious on the day of the solstice.'
0:53:50 > 0:53:55Here at the bottom of the well, my shadow is directly beneath me
0:53:55 > 0:53:59and there's no shadow at all being cast by the walls of the well.
0:53:59 > 0:54:02It's midday on the summer solstice
0:54:02 > 0:54:05and the sun is directly overhead.
0:54:08 > 0:54:10'The solstice marks the day
0:54:10 > 0:54:14'on which the Earth's tilt has its strongest impact
0:54:14 > 0:54:16'on the northern hemisphere
0:54:16 > 0:54:20'which is leaning to its maximum extent towards the sun.
0:54:23 > 0:54:27'It's revealed in this way here in Aswan
0:54:27 > 0:54:30'because I'm standing on a very particular point'
0:54:30 > 0:54:32on the Earth's surface.
0:54:38 > 0:54:43If we were to trace a line from Aswan right around the globe,
0:54:43 > 0:54:46we'd be marking a line of latitude,
0:54:46 > 0:54:48but also the furthest point north
0:54:48 > 0:54:53at which the midday sun is directly overhead.
0:54:55 > 0:54:58This is the tropic of Cancer.
0:54:58 > 0:55:04And because the Earth is tilted at 23.4 degrees from the vertical,
0:55:04 > 0:55:10the tropic of Cancer is 23.4 degrees above the equator.
0:55:10 > 0:55:13The June solstice also defines
0:55:13 > 0:55:17another significant line of latitude.
0:55:17 > 0:55:20As the northern hemisphere points towards the sun,
0:55:20 > 0:55:24the Arctic experiences 24 hours of daylight.
0:55:24 > 0:55:29On the solstice, the midnight sun reaches its maximum extent -
0:55:29 > 0:55:33a line marked by the Arctic Circle
0:55:33 > 0:55:35which is 23.5 degrees from the North Pole.
0:55:38 > 0:55:43Isn't it astonishing the Earth's tilt has such a dramatic impact?
0:55:43 > 0:55:48It's that tilt that drives our seasons and powers our weather.
0:55:48 > 0:55:51It's had a profound influence on our human history,
0:55:51 > 0:55:55and even today it dictates how and where we live
0:55:55 > 0:55:58on this extraordinary, unique planet of ours.
0:55:58 > 0:55:58on this extraordinary, unique planet of ours.
0:55:58 > 0:56:00WAVES CRASH
0:56:00 > 0:56:03BIRDS CRY
0:56:06 > 0:56:09The summer solstice is where we end this part
0:56:09 > 0:56:12of our voyage around the sun.
0:56:12 > 0:56:16When we started at the spring equinox, day and night were equal
0:56:16 > 0:56:19and we all had 12 hours of each one.
0:56:19 > 0:56:22At our end point, the solstice,
0:56:22 > 0:56:27the contrast between day and night, is at its greatest.
0:56:31 > 0:56:37We've also reached the end of our year-long journey around the sun.
0:56:37 > 0:56:39In this series,
0:56:39 > 0:56:44we've travelled more than 900 million kilometres through space.
0:56:44 > 0:56:47And in that time, we've seen how the Earth's spin
0:56:47 > 0:56:51dictates the Earth's climate patterns...
0:56:54 > 0:56:58..how changes in our orbit can transform our planet...
0:57:00 > 0:57:03..and how the Earth's tilt controls the seasons.
0:57:07 > 0:57:11Now our voyage is over.
0:57:11 > 0:57:14But the planet goes on,
0:57:14 > 0:57:17each new orbit creating its own unique mix
0:57:17 > 0:57:21of endlessly varied, natural phenomena.
0:57:21 > 0:57:23It's quite a ride!
0:57:54 > 0:57:57Subtitles by Red Bee Media Ltd
0:58:02 > 0:58:04When the first travellers crossed America, they were faced with this -
0:58:04 > 0:58:07an immense landscape of extremes,
0:58:07 > 0:58:11from snow-capped mountains to arid plains and thick forests.
0:58:11 > 0:58:14The landscape shaped the nation.
0:58:14 > 0:58:16The adversity, tenacity.
0:58:16 > 0:58:20The very nature of the American personality was defined.
0:58:20 > 0:58:23Ray Mears explores the land behind the Hollywood legend
0:58:23 > 0:58:26and discovers the wild that made the west.