0:00:10 > 0:00:13All of us, every day of our lives, are on the move.
0:00:13 > 0:00:18And we don't mean the morning commute or taking the kids to school,
0:00:18 > 0:00:21but a journey of epic proportions.
0:00:21 > 0:00:24Even now, as you're watching this,
0:00:24 > 0:00:28you're hurtling through space at 100,000 kilometres an hour.
0:00:30 > 0:00:33Every year, our planet, the Earth, travels around the sun
0:00:33 > 0:00:35and we go with it.
0:00:38 > 0:00:40I'm Kate Humble.
0:00:40 > 0:00:43This is it. The sun is directly overhead.
0:00:43 > 0:00:47My shadow is directly below me.
0:00:47 > 0:00:51In this series, we are going to follow the Earth's voyage
0:00:51 > 0:00:54through space for one whole year
0:00:54 > 0:01:00to witness the astonishing consequences this journey has for us all.
0:01:02 > 0:01:06I'm Dr Helen Czerski and I study the physics of the natural world.
0:01:06 > 0:01:07Wow, look at that!
0:01:07 > 0:01:10SHOUTING: I'll be investigating how our orbit powers
0:01:10 > 0:01:13the most spectacular weather
0:01:13 > 0:01:16and how it's also shaped and reshaped our planet.
0:01:18 > 0:01:21But our planet's journey isn't quite as smooth as you might think
0:01:21 > 0:01:28and its orbit changes over time with significant consequences.
0:01:29 > 0:01:32The bottom here is 120 metres down.
0:01:32 > 0:01:35And full of sharks.
0:01:35 > 0:01:36Wow!
0:01:36 > 0:01:39In this episode, we explore what it means
0:01:39 > 0:01:44to live on a planet locked in a never-ending voyage around the sun.
0:01:44 > 0:01:49Join us on the most remarkable journey of your life.
0:01:59 > 0:02:01Since our journey began,
0:02:01 > 0:02:06we've travelled almost 500 million kilometres around the sun to the end of December.
0:02:07 > 0:02:10In this episode, we continue our journey,
0:02:10 > 0:02:13travelling from the beginning of January
0:02:13 > 0:02:15to the spring equinox in March.
0:02:22 > 0:02:27In the northern hemisphere, that means we're in winter,
0:02:27 > 0:02:28the harshest season.
0:02:30 > 0:02:33Whilst in the southern hemisphere, it's summer,
0:02:33 > 0:02:37although it's a little different to the one in the north.
0:02:44 > 0:02:46I'm starting in the Scottish Highlands
0:02:46 > 0:02:52on a particularly significant day in our journey around the sun.
0:02:55 > 0:02:57It's the third of January,
0:02:57 > 0:02:59it's minus five...
0:03:01 > 0:03:04..and the winds are gusting to over 60 kilometres an hour.
0:03:09 > 0:03:13I'm walking...up Aonach Mor...
0:03:15 > 0:03:17..one of the highest mountains in Scotland.
0:03:20 > 0:03:24If it was a beautiful, clear, sunny day,
0:03:24 > 0:03:28you'd be able to see Ben Nevis over there.
0:03:29 > 0:03:32And if I was going to be very British
0:03:32 > 0:03:36and stiff upper lip about this, I'd say it was a little bit chilly.
0:03:38 > 0:03:39But I'm not.
0:03:40 > 0:03:42It's absolutely freezing.
0:03:45 > 0:03:48So, why, one might ask,
0:03:48 > 0:03:51am I going to the effort of climbing over 1,000 metres
0:03:51 > 0:03:53in these conditions?
0:03:55 > 0:03:58Well, by being here on Aonach Mor,
0:03:58 > 0:04:01I'm about as close to the sun as I'll ever be
0:04:01 > 0:04:03and it's actually not because of where
0:04:03 > 0:04:06but when I'm making this climb.
0:04:11 > 0:04:14Today, we're physically closer to the sun
0:04:14 > 0:04:16than on any other day of the year.
0:04:16 > 0:04:20It's a day with a special name. It's called perihelion,
0:04:20 > 0:04:23and although it's impossible to believe in conditions like this,
0:04:23 > 0:04:28the Earth is five million kilometres closer to the sun today
0:04:28 > 0:04:30than it will be in July.
0:04:33 > 0:04:37At perihelion, being on top of this mountain on this day,
0:04:37 > 0:04:41brings me one more kilometre closer to the sun.
0:04:43 > 0:04:46It may seem strange that on some days
0:04:46 > 0:04:48we can be much closer to the sun than on others,
0:04:48 > 0:04:52but it's the consequence of a particular feature
0:04:52 > 0:04:54of the Earth's orbit.
0:05:06 > 0:05:11The Earth's journey through space is controlled by the sun's gravity.
0:05:14 > 0:05:17But it isn't quite the orbit you might expect.
0:05:19 > 0:05:23Now, say that this stone is our sun.
0:05:23 > 0:05:27Now, the Earth doesn't orbit the sun in a perfect circle.
0:05:27 > 0:05:30Instead, we go around, on an ellipse.
0:05:32 > 0:05:36Not only is the Earth's orbit elliptical,
0:05:36 > 0:05:38the sun isn't in the centre of it.
0:05:38 > 0:05:44That means that our distance from the sun varies continuously throughout the year.
0:05:44 > 0:05:49And, today, on our orbit, January the 3rd, we're there -
0:05:49 > 0:05:53closer to the sun than we will be for the whole of the rest of the year.
0:05:55 > 0:06:00The Earth's elliptical orbit means that in January, at perihelion,
0:06:00 > 0:06:06the Earth receives about 7% more solar energy than it does in July,
0:06:06 > 0:06:09when the Earth is at its furthest point from the sun.
0:06:11 > 0:06:15You might think that this extra energy would mean that January would be warm
0:06:15 > 0:06:18and July would be cold.
0:06:19 > 0:06:24Well, it turns out that proximity to the sun doesn't guarantee warmth.
0:06:27 > 0:06:30The reason for this apparent anomaly
0:06:30 > 0:06:34is that there's a second, more powerful factor at work.
0:06:36 > 0:06:38As it orbits the sun,
0:06:38 > 0:06:43the Earth is tilted on its axis at an angle of just over 23 degrees.
0:06:46 > 0:06:49Because of this 23.4 degree tilt,
0:06:49 > 0:06:55in January, the northern hemisphere is pointing away from the sun.
0:06:57 > 0:07:02The Earth's tilt reduces the amount of solar radiation in the northern hemisphere
0:07:02 > 0:07:08by up to 50%, far more than perihelion increases it,
0:07:08 > 0:07:10which is why it's winter in Britain,
0:07:10 > 0:07:15even though this is when we're at our closest to the sun.
0:07:19 > 0:07:23But perihelion in the southern hemisphere coincides with summer,
0:07:23 > 0:07:29so, in theory, the relative proximity of the sun and the extra energy this brings
0:07:29 > 0:07:33should mean this part of the world has particularly hot summers.
0:07:35 > 0:07:41I've come to Chile to discover whether this holds true.
0:07:41 > 0:07:45This is Puerto Williams. It's not just the southernmost town in Chile,
0:07:45 > 0:07:47it's the southernmost town in the world.
0:07:47 > 0:07:52The next significant land mass from here is Antarctica.
0:07:54 > 0:07:57Puerto Williams is a good place for us to be
0:07:57 > 0:08:01because it's at an equivalent latitude to the UK.
0:08:04 > 0:08:08So we can find out how summers here, in the southern hemisphere,
0:08:08 > 0:08:10compare to ours.
0:08:13 > 0:08:16I'm heading into a stretch of water called the Beagle Channel
0:08:16 > 0:08:19that crosses the bottom of the continent.
0:08:21 > 0:08:23It's named after HMS Beagle,
0:08:23 > 0:08:28the boat that carried Charles Darwin here almost 200 years ago.
0:08:32 > 0:08:37Truth be told, so far, conditions are not hugely different to summers back home.
0:08:40 > 0:08:46But there is a difference and it's something you'd never see in the UK.
0:08:49 > 0:08:51A glacier.
0:09:06 > 0:09:11I am absolutely...blown away by where we are.
0:09:11 > 0:09:15It's just the scale of it that takes your breath away.
0:09:15 > 0:09:17It comes right down into the water,
0:09:17 > 0:09:20and, as you can see, there are just great chunks of ice everywhere you look
0:09:20 > 0:09:22that have broken off the glacier.
0:09:22 > 0:09:26It's like floating in a giant gin and tonic.
0:09:26 > 0:09:28But look at that!
0:09:28 > 0:09:30Ooh, there's ice falling off it now!
0:09:38 > 0:09:41And what's so astonishing about this is its location.
0:09:41 > 0:09:45This isn't the only glacier in this region, not by a long way.
0:09:45 > 0:09:49And yet, we're at 55 degrees latitude south.
0:09:49 > 0:09:54If you go to the equivalent latitude in the north, 55 degrees north,
0:09:54 > 0:09:56you get to the Lake District in England.
0:09:56 > 0:09:58Now, we all know that the Lake District is very pretty.
0:09:58 > 0:10:01But it hasn't got one of those.
0:10:08 > 0:10:13The presence of this glacier is evidence that, rather than being hotter,
0:10:13 > 0:10:16summers in the southern hemisphere are actually cooler
0:10:16 > 0:10:18than in the northern hemisphere.
0:10:18 > 0:10:23In fact, on average, they're a full four degrees Celsius cooler,
0:10:23 > 0:10:29despite the added boost that perihelion gives to the southern hemisphere summer.
0:10:30 > 0:10:36So something else is at work here, counteracting the effects of perihelion.
0:10:36 > 0:10:41To discover what it is, I'm heading back out to sea.
0:10:46 > 0:10:50Well, we're now out in the open ocean
0:10:50 > 0:10:53and, I have to say, if you're not a sailor,
0:10:53 > 0:10:55and I'm not...
0:10:56 > 0:11:00..it makes you feel very small...
0:11:01 > 0:11:07Whoo! ..a little bit scared and quite sick.
0:11:08 > 0:11:11We're sailing in the Southern Ocean...
0:11:13 > 0:11:16..where strong winds and icebergs
0:11:16 > 0:11:21have made these waters notorious as a sailor's graveyard.
0:11:23 > 0:11:26This is a very exposed stretch of water.
0:11:28 > 0:11:31To the west is the Pacific Ocean,
0:11:31 > 0:11:33whilst to the east is the Atlantic.
0:11:33 > 0:11:37To the south, the nearest land mass is Antarctica.
0:11:39 > 0:11:43It's the very vastness of this expanse of water
0:11:43 > 0:11:47that's the reason why summers in the southern hemisphere are so cool.
0:11:51 > 0:11:55If you look at the whole of the southern hemisphere,
0:11:55 > 0:11:57over 80% of it is covered by oceans
0:11:57 > 0:12:02and these huge expanses of water have a powerful effect on the climate.
0:12:02 > 0:12:05That's because water has an important characteristic.
0:12:05 > 0:12:09It takes a lot more of the sun's energy to warm up the sea than it does the land.
0:12:09 > 0:12:14In other words, water has a high heat capacity.
0:12:16 > 0:12:19This means that, even in midsummer,
0:12:19 > 0:12:22and even with the added warmth provided by perihelion,
0:12:22 > 0:12:25the oceans in the southern hemisphere are still cool.
0:12:25 > 0:12:28And this keeps the air cool too.
0:12:30 > 0:12:34Even at this time of year, when the Earth is physically closest to the sun,
0:12:34 > 0:12:38and the southern hemisphere is tilted towards it,
0:12:38 > 0:12:41the influence of the oceans keep it much cooler.
0:12:45 > 0:12:48It's a sobering thought that without perihelion,
0:12:48 > 0:12:53southern hemisphere summers would be even cooler than they are now.
0:13:00 > 0:13:03The Earth's slightly off-centre orbit
0:13:03 > 0:13:05is a reminder that we live on a planet
0:13:05 > 0:13:09that's hurtling through space around the sun.
0:13:09 > 0:13:14This journey is controlled by the immense power of the sun's gravity.
0:13:14 > 0:13:19But the sun's gravity is also responsible for significant dangers.
0:13:33 > 0:13:37I've travelled to a place where you can see these dangers
0:13:37 > 0:13:39written into the Earth's surface.
0:13:46 > 0:13:50This is the Barringer Crater in Arizona.
0:13:50 > 0:13:5250,000 years ago,
0:13:52 > 0:13:57a meteorite struck this site and excavated this dramatic hole.
0:13:57 > 0:14:03That impact spread debris over tens of thousands of square kilometres.
0:14:03 > 0:14:06This crater itself is more than a kilometre,
0:14:06 > 0:14:09or three-quarters of a mile, across,
0:14:09 > 0:14:13so as you can imagine, it was an incredibly violent event.
0:14:14 > 0:14:17To get an idea of the force involved in that impact,
0:14:17 > 0:14:20we can look at two types of rock that you find round here.
0:14:20 > 0:14:25Now, this, this is Coconino sandstone.
0:14:25 > 0:14:29This is what was present before the meteorite hit.
0:14:29 > 0:14:35Now, down here, we can see what happened to this kind of stone after the impact.
0:14:35 > 0:14:38So, this rock here, it's chemically exactly the same,
0:14:38 > 0:14:42but what the impact did to it was just pulverise it. Look at this.
0:14:42 > 0:14:44It's just breaking apart in my fingers.
0:14:44 > 0:14:48And the reason for that is that the shock that went through from this impact
0:14:48 > 0:14:51just fractured all the tiny grains of quartz.
0:15:01 > 0:15:07Incredibly, all this was done by a meteorite just 50 metres across.
0:15:08 > 0:15:12There are thousands of objects circling the sun,
0:15:12 > 0:15:15trapped by its immense gravitational field.
0:15:18 > 0:15:21Every now and then, we collide with one.
0:15:21 > 0:15:26But not all of them are as small as the one that created the Barringer Crater.
0:15:31 > 0:15:35Hidden underneath what is today a place called Chicxulub in Mexico
0:15:35 > 0:15:38is a huge crater.
0:15:41 > 0:15:45The impact of the Chicxulub meteorite was cataclysmic.
0:15:53 > 0:15:56It blasted so much hot debris into the atmosphere
0:15:56 > 0:16:00that almost the whole planet caught fire.
0:16:00 > 0:16:02The overall impact was so great
0:16:02 > 0:16:06it eventually contributed to the extinction of the dinosaurs.
0:16:07 > 0:16:09ROARING
0:16:16 > 0:16:21Our orbit regularly takes us into the path of asteroids and comets.
0:16:21 > 0:16:25And it's a sobering thought that our voyage through space
0:16:25 > 0:16:28could deliver a random disaster to the whole planet.
0:16:29 > 0:16:35The good news is that the bigger the potential disaster, the rarer it is.
0:16:35 > 0:16:40But there's another potential danger that comes from our orbit around the sun.
0:16:40 > 0:16:44And the best time to see it is at this time of year,
0:16:44 > 0:16:46in the middle of winter.
0:16:51 > 0:16:56The long nights mean that this is the peak season for an extraordinary spectacle.
0:17:07 > 0:17:12For thousands of years, people have marvelled at the spectacular light displays
0:17:12 > 0:17:14that sometimes appear in the night sky
0:17:14 > 0:17:18and they've wondered what on earth they could possibly mean.
0:17:18 > 0:17:22The Vikings believed them to be the reflections of dead maidens.
0:17:22 > 0:17:26The Cree Native Americans called them the Dance of the Spirits,
0:17:26 > 0:17:28and, in Europe in the Middle Ages,
0:17:28 > 0:17:31they believed the lights meant that God was angry.
0:17:31 > 0:17:35But the truth is actually even more extraordinary.
0:17:40 > 0:17:44This celestial light show, or aurora, as it's known,
0:17:44 > 0:17:49is the front line in the battle between the sun and the Earth's atmosphere.
0:17:49 > 0:17:54Every second, the sun blasts out a million tonnes of radioactive particles
0:17:54 > 0:17:56and the Earth is in the firing line.
0:18:01 > 0:18:05The sun emits a continuous flow of charged particles,
0:18:05 > 0:18:08known as the solar wind.
0:18:08 > 0:18:12This streams outward, in a wash of radiation.
0:18:12 > 0:18:16But when it reaches the Earth, it encounters a barrier.
0:18:16 > 0:18:19The Earth's magnetic field deflects the particles
0:18:19 > 0:18:23and funnels them towards the poles.
0:18:23 > 0:18:28Here, they collide with atoms of nitrogen and oxygen in the atmosphere.
0:18:28 > 0:18:34These collisions emit energy in the form of light, giving us the aurora.
0:18:45 > 0:18:48From the International Space Station,
0:18:48 > 0:18:52you get a better sense of the awesome scale of the aurora.
0:18:55 > 0:18:58We don't often think of it this way,
0:18:58 > 0:19:04but the aurora is graphic evidence that we live inside the atmosphere of the sun.
0:19:07 > 0:19:14This is the sun's atmosphere colliding with the Earth's atmosphere.
0:19:22 > 0:19:27So our orbit, close to the sun, is full of risk.
0:19:27 > 0:19:31But it's also vital for our survival.
0:19:39 > 0:19:42Almost all life on our planet
0:19:42 > 0:19:45depends on the energy we receive from the sun.
0:19:47 > 0:19:51Our location close to the sun provides one critical benefit -
0:19:51 > 0:19:56it allows the presence of liquid water.
0:19:59 > 0:20:01If our planet was much closer to the sun,
0:20:01 > 0:20:05it would be too hot and the water would boil away.
0:20:05 > 0:20:08Too far away, and it would freeze.
0:20:09 > 0:20:13Our planet is in what's known as the habitable zone.
0:20:15 > 0:20:20The zone where water can exist and life can flourish.
0:20:27 > 0:20:30Earth may be dangerously close to the sun,
0:20:30 > 0:20:33but this is the price that has to be paid to sustain life.
0:20:36 > 0:20:42And our location close to the sun is even more favourable than it first appears.
0:20:43 > 0:20:48Earth orbits the sun at just the right distance to allow water to exist
0:20:48 > 0:20:52in all three states - as a solid, a liquid and a gas.
0:20:52 > 0:20:56And it's switching between those states all the time.
0:20:56 > 0:21:00But water in each of those states behaves very differently,
0:21:00 > 0:21:04and it's those differences that generate the climate system as we know it on Earth.
0:21:09 > 0:21:12It's now the middle of January.
0:21:12 > 0:21:15This time of year gives us a great opportunity
0:21:15 > 0:21:18to see two ways in which water changes state,
0:21:18 > 0:21:21with very different consequences.
0:21:28 > 0:21:31I'm back in the southern hemisphere,
0:21:31 > 0:21:34in the foothills of the Andes in Argentina.
0:21:34 > 0:21:38Here, you can see water moving between states
0:21:38 > 0:21:42and how this process transforms our planet.
0:21:45 > 0:21:50This is the cloud forest of Calilegua, 2,000 metres above sea level.
0:21:50 > 0:21:54And, as you can see, clouds are definitely a feature here.
0:21:54 > 0:21:59There's a wonderful thick wisp of cloud down in the valley there
0:21:59 > 0:22:02and then this great bank over the trees on the horizon.
0:22:03 > 0:22:08And then you've got these ghostly wisps climbing up above the trees.
0:22:12 > 0:22:16It really is a magical place.
0:22:21 > 0:22:25This is a classic summer's day in the cloud forest.
0:22:25 > 0:22:28It's hot, it's humid...
0:22:30 > 0:22:32It's like being in a giant steam room.
0:22:32 > 0:22:34BIRDS CAW
0:22:34 > 0:22:39The humidity I'm feeling is because the heat has evaporated water,
0:22:39 > 0:22:46so the air is laden with vapour, the gaseous form of water.
0:22:46 > 0:22:49As the day progresses, some of this warm, moist air
0:22:49 > 0:22:51will change state again.
0:22:56 > 0:23:00It's mid-afternoon and it's getting increasingly hot and steamy.
0:23:00 > 0:23:02In fact, if feels like this heat
0:23:02 > 0:23:06is about to trigger something absolutely spectacular.
0:23:09 > 0:23:15Throughout the day, the land has been absorbing more and more heat.
0:23:15 > 0:23:18That heat warms up the moist air
0:23:18 > 0:23:21and forces it to rise high into the atmosphere,
0:23:21 > 0:23:24forming towering cumulus clouds.
0:23:24 > 0:23:29These clouds are the transformation of water made visible.
0:23:29 > 0:23:32The rising water vapour has cooled and changed state
0:23:32 > 0:23:35and become liquid again.
0:23:35 > 0:23:41What's incredible is watching this cumulus cloud growing in front of my very eyes.
0:23:41 > 0:23:45It has to be eight, ten kilometres tall already
0:23:45 > 0:23:49and you can almost feel the energy crackling away inside it.
0:23:49 > 0:23:55There's a tremendous sense of build-up and anticipation in the air.
0:23:59 > 0:24:05Powerful updraughts push the cloud so high, the top spreads out
0:24:05 > 0:24:07to form a characteristic anvil shape.
0:24:17 > 0:24:20An approaching storm like this could last half an hour.
0:24:20 > 0:24:22It could last 10 or 12 hours.
0:24:22 > 0:24:27Sometimes they even join up with other storms to create destructive megastorms
0:24:27 > 0:24:30that can devastate the whole region.
0:24:34 > 0:24:40These tropical storms are an extreme version of a familiar phenomenon.
0:24:47 > 0:24:48Rain.
0:24:48 > 0:24:50RAIN SPATTERING
0:24:55 > 0:24:59Rain is so familiar that it's easy to forget
0:24:59 > 0:25:01what a critical role it plays on Earth.
0:25:01 > 0:25:08It's the way in which water is transported from the oceans and deposited over land.
0:25:08 > 0:25:13Without the ability of water to change from liquid to gas, and back again,
0:25:13 > 0:25:17the land would be a dry and barren desert.
0:25:22 > 0:25:26Meanwhile, in the northern hemisphere at this time of year,
0:25:26 > 0:25:31a different transformation of water occurs, from liquid to solid.
0:25:41 > 0:25:46I've come to the edge of Lake Ontario in North America
0:25:46 > 0:25:51to see one of the most extreme examples of this transformation in action.
0:25:55 > 0:25:59This area is home to some of the heaviest snowfalls in the world.
0:25:59 > 0:26:04But it's not immediately obvious why this should be so.
0:26:05 > 0:26:07It's so peaceful here.
0:26:07 > 0:26:10There's a beautiful blue sky. It's been a stunning day.
0:26:10 > 0:26:13But tomorrow, from across the lake over there,
0:26:13 > 0:26:16there's a huge storm coming our way,
0:26:16 > 0:26:19although you'd never know that to look at it now.
0:26:20 > 0:26:23The snowstorm is likely to be particularly heavy
0:26:23 > 0:26:27because of a unique set of conditions.
0:26:41 > 0:26:45The air outside is cold and dry.
0:26:45 > 0:26:47It's come straight from the Arctic.
0:26:47 > 0:26:50But this frigid air is about to be transformed.
0:26:52 > 0:26:54You can see what does it right below me.
0:26:54 > 0:26:56Water.
0:26:56 > 0:26:57Warm water.
0:26:57 > 0:27:00Even though it looks pretty chilly down there,
0:27:00 > 0:27:03the water's significantly warmer than the land around it.
0:27:03 > 0:27:04And there's lots of it.
0:27:04 > 0:27:06Even though it's frozen round the edges,
0:27:06 > 0:27:10there's plenty of open water in the middle.
0:27:12 > 0:27:17Lake Ontario is one of the Great Lakes, so it's a huge body of water.
0:27:17 > 0:27:19Water's high heat capacity
0:27:19 > 0:27:24means it's held onto much of the heat it absorbed during the summer.
0:27:24 > 0:27:28As the cold, dry air passes over this relatively warm lake,
0:27:28 > 0:27:31water evaporates.
0:27:31 > 0:27:36As it rises over Upstate New York, it forms clouds.
0:27:36 > 0:27:39Those clouds are the start of a special type of snowstorm,
0:27:39 > 0:27:43which leads to some of the biggest and fastest accumulations of snow
0:27:43 > 0:27:44anywhere in the world.
0:27:44 > 0:27:47And it's called a lake-effect snowstorm.
0:27:53 > 0:27:56These snowstorms are particularly intense
0:27:56 > 0:28:00because the cold air can keep on blowing across the lake for days.
0:28:00 > 0:28:04It's like a conveyor belt of cloud formation.
0:28:07 > 0:28:11Within these clouds, the cold air means that water turns from liquid
0:28:11 > 0:28:14to its solid, crystalline state...
0:28:15 > 0:28:16..a snowflake.
0:28:22 > 0:28:27And they start because there are tiny grains of dust, way up in the clouds
0:28:27 > 0:28:32and the warm lake air provides moisture, which condenses onto those droplets.
0:28:32 > 0:28:35And as they're carried up and up into the cloud,
0:28:35 > 0:28:38the temperature goes down and so they freeze into a crystal.
0:28:38 > 0:28:40And that crystal is a snowflake.
0:28:43 > 0:28:47Here, conditions produce a very particular type of snowflake.
0:28:47 > 0:28:51Because the air is so cold, it produces crystals with sharper tips.
0:28:53 > 0:28:57These grow more branches, called dendrites,
0:28:57 > 0:29:00which make the snowflakes fluffier.
0:29:00 > 0:29:05It's the kind of snow we all love - as long as there isn't too much of it!
0:29:08 > 0:29:13It's now approaching nightfall and the snowstorm is almost upon us.
0:29:13 > 0:29:18How much snow falls will depend on one final factor.
0:29:20 > 0:29:22The wind direction.
0:29:22 > 0:29:27If the wind comes from the north, it passes over the narrow part of the lake
0:29:27 > 0:29:31and so picks up only a small amount of moisture,
0:29:31 > 0:29:33making just a light shower of snow.
0:29:33 > 0:29:35But if the wind comes from the west,
0:29:35 > 0:29:39it passes over almost the full length of the lake
0:29:39 > 0:29:43and picks up a lot of moisture, producing much more snow.
0:29:49 > 0:29:52At night, the storm finally arrives.
0:29:57 > 0:29:59I'm here in the middle of the snowstorm
0:29:59 > 0:30:01and the winds are really strong.
0:30:01 > 0:30:05The thing is that powerful winds like this are exactly what you get up in the clouds
0:30:05 > 0:30:08where snowflakes form.
0:30:08 > 0:30:11So next time you see a peaceful snow scene,
0:30:11 > 0:30:14remember that all of those delicate snowflakes
0:30:14 > 0:30:18are formed in a violent, windy environment, just like this.
0:30:21 > 0:30:23WIND HOWLS
0:30:36 > 0:30:41Next morning, the town beside the lake wakes up to a heavy coating of snow.
0:30:44 > 0:30:49But because it's a regular event, people here are prepared.
0:30:51 > 0:30:53Across the northern hemisphere,
0:30:53 > 0:30:57the same interaction of cold land and relatively warm moisture
0:30:57 > 0:31:02produces many other spectacular weather phenomena.
0:31:06 > 0:31:11In January 2005, these remarkable ice sculptures formed
0:31:11 > 0:31:16when spray from Lake Geneva in Switzerland was thrown up by strong winds
0:31:16 > 0:31:18and froze as soon as it landed.
0:31:23 > 0:31:28In Canada in 1998, rain falling on frozen ground
0:31:28 > 0:31:33turned to ice as it landed, a phenomenon known as an ice storm.
0:31:33 > 0:31:36It continued for 80 hours.
0:31:36 > 0:31:41The sheer weight of ice crushed over 1,000 steel pylons,
0:31:41 > 0:31:44leaving four million people without electricity.
0:31:46 > 0:31:50Closer to home, frost forms when air saturated with moisture
0:31:50 > 0:31:53touches surfaces that are already frozen.
0:31:59 > 0:32:06Our orbit around the sun exposes our planet to potentially deadly radiation.
0:32:07 > 0:32:10But the payoff is a big one...
0:32:11 > 0:32:16..a planet where water can be distributed across the whole Earth,
0:32:16 > 0:32:20providing spectacular weather and making it habitable.
0:32:23 > 0:32:29It's now late January and the northern hemisphere is locked in winter.
0:32:29 > 0:32:33And yet there is a paradox about our winter,
0:32:33 > 0:32:38because in January, winter is still getting colder,
0:32:38 > 0:32:42even though the northern hemisphere is receiving more energy from the sun.
0:32:51 > 0:32:53I've come to Northern Canada,
0:32:53 > 0:32:59to the best - or perhaps the worst - place to explore this paradox.
0:33:02 > 0:33:04Whoo!
0:33:05 > 0:33:07Cor! This...
0:33:09 > 0:33:10..is Yellowknife.
0:33:12 > 0:33:17It has the dubious distinction of being the coldest city
0:33:17 > 0:33:21in the whole of North America.
0:33:23 > 0:33:26Today is January the 19th.
0:33:26 > 0:33:30On average, this is the coldest day of the year across the northern hemisphere.
0:33:30 > 0:33:36It's minus 35 degrees Celsius, which certainly qualifies as cold to me.
0:33:38 > 0:33:40It's pretty hard to describe to you
0:33:40 > 0:33:45just how it feels to be at minus 35, but I'm going to give it a go.
0:33:45 > 0:33:48When you breathe, it hurts.
0:33:48 > 0:33:51It kind of gets you at the back of the throat.
0:33:51 > 0:33:54Your nose feels like it's permanently frozen solid.
0:33:54 > 0:33:58And despite the fact that I've got the feathers of about 25 geese
0:33:58 > 0:34:01stuffed into this jacket, and more thermal underwear
0:34:01 > 0:34:05than I thought possible to wear at exactly the same time,
0:34:05 > 0:34:08I still feel cold.
0:34:08 > 0:34:13In these conditions, even familiar things behave in unfamiliar ways.
0:34:13 > 0:34:16You can take a lovely, hot, steaming cup of coffee,
0:34:16 > 0:34:22throw it in the air, and the steam from that coffee will freeze instantly.
0:34:22 > 0:34:25Well, you've got to give it a go, haven't you?
0:34:27 > 0:34:28Right...
0:34:28 > 0:34:30Here goes.
0:34:33 > 0:34:35Wow!
0:34:36 > 0:34:38That is amazing!
0:34:38 > 0:34:42Oh, my word!
0:34:49 > 0:34:55There's something curious about the way winter peaks towards the end of January.
0:34:55 > 0:34:58The winter solstice falls on December the 21st
0:34:58 > 0:35:01and this marks the day when the northern hemisphere
0:35:01 > 0:35:05receives the least amount of solar energy from the sun.
0:35:05 > 0:35:10So you might expect the December solstice to be the coldest day of the year.
0:35:10 > 0:35:12But it's not.
0:35:13 > 0:35:17On average, temperatures on the 19th of January are colder
0:35:17 > 0:35:20than they are in mid-December.
0:35:20 > 0:35:22But, you say, the days are getting longer.
0:35:22 > 0:35:26The northern hemisphere is getting more sun.
0:35:26 > 0:35:28It should be warming up.
0:35:32 > 0:35:34In Yellowknife, there are people
0:35:34 > 0:35:39whose livelihoods depend on the way winter's peak is delayed.
0:35:40 > 0:35:42In the driving seat is Blair Weatherby.
0:35:42 > 0:35:46His family have been driving through the bitter cold of this region
0:35:46 > 0:35:48for three generations.
0:35:48 > 0:35:53He's not an ordinary trucker. He's an ice road trucker.
0:35:53 > 0:35:56And this is his highway.
0:36:14 > 0:36:17In the summer, what happens here?
0:36:17 > 0:36:19We'd be in a boat!
0:36:20 > 0:36:26That's because we're not driving on land, but on a frozen lake.
0:36:32 > 0:36:36So really to appreciate Yellowknife's splendid isolation,
0:36:36 > 0:36:37you have to look at a map.
0:36:37 > 0:36:42And here it is, right on Great Slave Lake.
0:36:42 > 0:36:45But it's surrounded by water and tundra.
0:36:45 > 0:36:49So at this time of year, of course, it freezes, and Yellowknife,
0:36:49 > 0:36:52and all these tiny, little, incredibly remote communities
0:36:52 > 0:36:55can get linked up by the ice roads.
0:36:55 > 0:37:00So what time of year can you start driving on the lake,
0:37:00 > 0:37:03as opposed to boating on the lake?
0:37:03 > 0:37:07The season starts towards the end of January.
0:37:07 > 0:37:13It's about 30 inches thick at this point. It just keeps getting thicker and thicker.
0:37:15 > 0:37:20So whilst the northern hemisphere's coldest day is the 19th of January,
0:37:20 > 0:37:24here in Yellowknife, it's still bitterly cold for many weeks to come.
0:37:24 > 0:37:29For the truckers, this delayed winter means their work season
0:37:29 > 0:37:32runs from late January well into March.
0:37:32 > 0:37:37Here, you can go for hours with your hands off the steering wheel sometimes.
0:37:37 > 0:37:41There's lakes that take two and a half hours to drive across. People watch movies.
0:37:41 > 0:37:46You put a DVD player on your dash and watch a movie when you're going across the ice.
0:37:47 > 0:37:51So why is the worst of winter delayed so long
0:37:51 > 0:37:54after the solstice on December the 21st?
0:37:58 > 0:38:01It's all about the balance between the heat coming in
0:38:01 > 0:38:02and the heat going out.
0:38:04 > 0:38:07Throughout early winter, the northern hemisphere
0:38:07 > 0:38:10receives declining amounts of the sun's energy,
0:38:10 > 0:38:14so it starts to cool down.
0:38:14 > 0:38:16But there's a lag in this cooling,
0:38:16 > 0:38:20because the Earth's surface loses heat relatively slowly.
0:38:21 > 0:38:26So well into January, the Earth's surface is still losing heat,
0:38:26 > 0:38:31even though solar energy is slowly increasing.
0:38:31 > 0:38:37It isn't until around the 19th of January that a tipping point is reached.
0:38:37 > 0:38:40From this day onwards, the increase in solar radiation
0:38:40 > 0:38:43will overwhelm the effects of the heat loss
0:38:43 > 0:38:46and the northern hemisphere will begin to warm up.
0:38:46 > 0:38:50But it'll still be a few more weeks yet
0:38:50 > 0:38:55before the ice here is too thin to support the weight of the trucks.
0:39:01 > 0:39:05We've seen how the Earth's journey through space is critical for life
0:39:05 > 0:39:10and how the Earth's angle of tilt defines our seasons.
0:39:10 > 0:39:16But you only really understand just how important our orbit is for our planet
0:39:16 > 0:39:19when you look into the Earth's past.
0:39:21 > 0:39:26There's evidence in the most unexpected places.
0:39:27 > 0:39:32A few miles out there is one of the most spectacular wonders of the world,
0:39:32 > 0:39:35but I can't see it from here because it's underwater.
0:39:35 > 0:39:37I'm in Belize in Central America
0:39:37 > 0:39:41and what I'm going to see is known as the Blue Hole.
0:39:57 > 0:40:00It's not often that nature produces something
0:40:00 > 0:40:02as beautifully symmetrical as this.
0:40:02 > 0:40:04It's almost a perfect circle.
0:40:10 > 0:40:14But it's more than just a stunning piece of natural architecture,
0:40:14 > 0:40:17because deep down there are clues
0:40:17 > 0:40:20to some of the most dramatic events in Earth's history.
0:40:41 > 0:40:44This wall seems to go down for ever
0:40:44 > 0:40:49and I'm told that the bottom here is 120 metres down,
0:40:49 > 0:40:52which sounds like a very, very long way just now.
0:40:52 > 0:40:56And I'm just dropping into the abyss.
0:41:05 > 0:41:09That shark just swam past in front of me.
0:41:09 > 0:41:14I've never, ever been so close to a reef shark.
0:41:15 > 0:41:18And there's another two just behind me.
0:41:28 > 0:41:31Finally, I've reached my goal.
0:41:43 > 0:41:45So down here at 40 metres...
0:41:47 > 0:41:49..it's really eerie.
0:41:49 > 0:41:50Gloomy.
0:41:52 > 0:41:54And this is what I've come to see.
0:41:55 > 0:41:57And they're stalactites.
0:42:09 > 0:42:14But there's only one way I know of for stalactites to form.
0:42:14 > 0:42:18And it isn't down here, in 40 metres of water,
0:42:18 > 0:42:20with sharks swimming about nearby.
0:42:25 > 0:42:31Stalactites are created when mineral-rich water drips from the roof of a cave,
0:42:31 > 0:42:34over hundreds or even thousands of years,
0:42:34 > 0:42:36leaving behind mineral deposits.
0:42:36 > 0:42:41In other words, they didn't form in the ocean.
0:42:46 > 0:42:50Stalactites like this can only ever form above ground.
0:42:52 > 0:42:54And that means that when these grew,
0:42:54 > 0:42:58the sea level was much, much lower than it is today.
0:43:02 > 0:43:06Scientists have precisely dated stalactites from the Blue Hole
0:43:06 > 0:43:11and, by comparing these and other sea level indicators from around the world,
0:43:11 > 0:43:15they've built up a picture of changing sea levels
0:43:15 > 0:43:18dating back hundreds of thousands of years.
0:43:19 > 0:43:22It reveals a striking pattern.
0:43:22 > 0:43:27Sea levels across the world have risen and fallen over time.
0:43:37 > 0:43:41Genuinely one of the eeriest things I've ever seen, that.
0:43:44 > 0:43:4820,00 years ago, the entire surface of the world's oceans
0:43:48 > 0:43:53was 120 metres below where it is today.
0:43:53 > 0:43:57And that means if I was standing here 20,000 years ago,
0:43:57 > 0:44:01all of this, including the Blue Hole cave system, would be dry land.
0:44:04 > 0:44:06So where did the ocean go?
0:44:07 > 0:44:11The answer is that it was on land.
0:44:11 > 0:44:14But it wasn't liquid water, it was ice,
0:44:14 > 0:44:18because 20,000 years ago, our planet was in the middle of an ice age.
0:44:21 > 0:44:25The Earth has experienced regular ice ages
0:44:25 > 0:44:28in a cycle going back several million years.
0:44:31 > 0:44:34These dramatic changes to the state of our planet
0:44:34 > 0:44:38are triggered by small changes in the Earth's orbit.
0:44:46 > 0:44:48I've travelled back to Britain
0:44:48 > 0:44:52to uncover the relationship between the Earth's orbit and an ice age.
0:44:55 > 0:45:01Snowdonia's peaks and valleys were carved out in the last ice age.
0:45:03 > 0:45:07It's in mountainous locations like this that an ice age would have begun
0:45:07 > 0:45:10as snow gradually built up.
0:45:16 > 0:45:21When we think of ice ages, we think of extreme cold during the winter.
0:45:21 > 0:45:22But, counterintuitively,
0:45:22 > 0:45:26it's summer temperatures which are important in starting ice ages.
0:45:28 > 0:45:32And the reason for that is, now, ice will build up here during the winter,
0:45:32 > 0:45:35but it will all melt away in the summer.
0:45:35 > 0:45:39But if the summer is a little bit cooler, a layer of ice will be left behind.
0:45:39 > 0:45:41And a series of cool summers
0:45:41 > 0:45:44will leave layer after layer, one on top of the other, building up.
0:45:44 > 0:45:47And here, the ice could have been hundreds of metres high.
0:45:49 > 0:45:52Ice ages always start in the northern hemisphere
0:45:52 > 0:45:56because there's so much more land surface on which ice can build up.
0:46:00 > 0:46:05So the question is, what causes cooler summers in the northern hemisphere?
0:46:06 > 0:46:10The answer comes from small changes in the Earth's orbit,
0:46:10 > 0:46:14caused by the gravitational pull of other planets.
0:46:17 > 0:46:19Our orbit isn't exactly the same every time.
0:46:19 > 0:46:25Aspects of it change just slightly, in cycles lasting thousands of years.
0:46:25 > 0:46:28And when all of those cycles reach their most extreme point
0:46:28 > 0:46:30all at the same time,
0:46:30 > 0:46:36that can change our summer temperatures just enough to tip us into an ice age.
0:46:37 > 0:46:41There are three cycles to do with the Earth's orbit
0:46:41 > 0:46:44that must all coincide to trigger an ice age.
0:46:46 > 0:46:48The first of these cyclical changes
0:46:48 > 0:46:51affects the time of year when perihelion occurs.
0:46:51 > 0:46:55This is the day when the Earth is closest to the sun.
0:46:57 > 0:47:01Today, perihelion is in January,
0:47:01 > 0:47:06but over thousands of years, the date of perihelion changes.
0:47:06 > 0:47:10When perihelion occurs in the northern hemisphere summer,
0:47:10 > 0:47:12it makes summers particularly hot.
0:47:13 > 0:47:16But when it occurs in winter, as it does today,
0:47:16 > 0:47:20then northern hemisphere summers are cooler.
0:47:22 > 0:47:28So at the moment, the perihelion cycle is at the right point to generate an ice age.
0:47:29 > 0:47:34But two other cycles are not in an ice age phase.
0:47:34 > 0:47:37The first is the angle of the Earth's tilt.
0:47:41 > 0:47:46The Earth's tilt is currently at an angle to the vertical of 23.4 degrees.
0:47:46 > 0:47:52But that angle changes between 22 and 24.5 degrees.
0:47:52 > 0:47:57It's only when the angle is at its shallowest - 22 degrees -
0:47:57 > 0:48:01that the seasons become less extreme and the summers cooler.
0:48:03 > 0:48:07Today, the angle of tilt is too great for an ice age.
0:48:11 > 0:48:16The final cycle affecting an ice age is the shape of the Earth's orbit.
0:48:18 > 0:48:20The Earth's orbit is an ellipse,
0:48:20 > 0:48:25but over time, it becomes slightly more, and then slightly less, elliptical.
0:48:25 > 0:48:31When the orbit is at its most elliptical, the result is lower summer temperatures.
0:48:34 > 0:48:37At the moment, the Earth is midway through this cycle,
0:48:37 > 0:48:40so again, it's not in an ice age phase.
0:48:45 > 0:48:50It's only when all three of these changes to the Earth's cycle line up together
0:48:50 > 0:48:53that they produce the really cool summers
0:48:53 > 0:48:56in the northern hemisphere that result in ice ages.
0:49:01 > 0:49:05At the moment, those three cycles are all at different positions,
0:49:05 > 0:49:08and so we're still getting enough sun during the summer
0:49:08 > 0:49:10to melt ice and keep us out of an ice age.
0:49:13 > 0:49:17It'll be around 60,000 years before the cycles line up again
0:49:17 > 0:49:19and the next ice age starts.
0:49:24 > 0:49:28It's now the beginning of March and we're nearing the end of our journey.
0:49:29 > 0:49:33In most of the northern hemisphere, spring is on the way.
0:49:35 > 0:49:40But there is still one part of the world that is locked in winter.
0:49:41 > 0:49:43Long after January the 19th,
0:49:43 > 0:49:47on average the coldest day in the northern hemisphere,
0:49:47 > 0:49:50winter still clings on in the Arctic.
0:49:56 > 0:50:01I've come to Greenland, where there's definitely not much sign of spring yet.
0:50:01 > 0:50:07This is Kulusuk. It's a tiny settlement of just 355 people
0:50:07 > 0:50:10perched on the edge of an island in eastern Greenland.
0:50:10 > 0:50:14To the north of here is the Arctic Circle and the Greenland ice cap.
0:50:22 > 0:50:26Kulusuk is surrounded by the Arctic Ocean.
0:50:26 > 0:50:32At this time of year, it's frozen, covered in a thick layer of sea ice.
0:50:33 > 0:50:38I've come here to find out about sea ice - how far it extends
0:50:38 > 0:50:42and why it hasn't melted, despite the fact that it's now March,
0:50:42 > 0:50:47the days are getting longer and the amount of solar energy is increasing.
0:50:50 > 0:50:55In fact, not only is the sea ice not melting, it's still expanding.
0:50:57 > 0:51:00Each year, the extent of the sea ice is different.
0:51:00 > 0:51:03To see how far it reaches this year,
0:51:03 > 0:51:07I need to travel right to the edge of the sea ice.
0:51:09 > 0:51:14But before I can set off, a massive snowstorm hits Kulusuk.
0:51:19 > 0:51:20We can't go anywhere.
0:51:20 > 0:51:24The 140-kilometre-an-hour winds
0:51:24 > 0:51:28make a trip to the local shop a major expedition.
0:51:35 > 0:51:38'By the next morning, the storm has passed.
0:51:41 > 0:51:45'I meet up with my guide, local hunter Gio Utuaq.
0:51:45 > 0:51:49'His hunting grounds lie right at the edge of the sea ice.
0:51:49 > 0:51:54'I'm hitching a lift on the only form of transport that can get us there.'
0:51:54 > 0:51:57She's so keen!
0:51:59 > 0:52:03How far do we have to go to get to the hunting grounds?
0:52:05 > 0:52:0820, maybe 25 kilometres.
0:52:33 > 0:52:38After two hours, we reach a huge expanse of sea ice.
0:52:43 > 0:52:48It's impossible to comprehend that the snow we're travelling across sits on ice,
0:52:48 > 0:52:50which sits on the ocean.
0:52:50 > 0:52:53We're travelling across a frozen sea. And look at this!
0:52:53 > 0:52:57This is an iceberg actually trapped within the sea ice.
0:52:57 > 0:53:03It's the most astonishing landscape, or seascape or ice-scape...
0:53:03 > 0:53:06What do you call it?! ..that I've ever seen!
0:53:06 > 0:53:08It's like another world.
0:53:25 > 0:53:30And then, surprisingly quickly, the edge of the ice comes into view
0:53:30 > 0:53:33and I can see the Arctic Ocean.
0:53:33 > 0:53:35Gio tells me that only days ago,
0:53:35 > 0:53:38the ice extended out for several more kilometres,
0:53:38 > 0:53:41but it seems the storm has broken it up.
0:53:43 > 0:53:47For obvious reasons, we make the last stretch of the journey on foot.
0:53:53 > 0:53:57Using his traditional spiked tool,
0:53:57 > 0:54:01Gio checks the thickness of the ice at the edge.
0:54:05 > 0:54:07- Are you sure? - SHE CHUCKLES
0:54:07 > 0:54:14There is something very disconcerting
0:54:14 > 0:54:21about walking on sea ice when the open sea is so close.
0:54:22 > 0:54:25- Is it safe? No problem?- No problem.
0:54:25 > 0:54:27Are you sure?
0:54:27 > 0:54:31Yeah, it looks pretty solid. How thick is the ice?
0:54:31 > 0:54:32Like this thick?
0:54:33 > 0:54:35Oh, you can see it.
0:54:37 > 0:54:41Actually, you can, you can see it's like a great cliff of ice
0:54:41 > 0:54:43that goes right down into the water.
0:54:47 > 0:54:52It seems strange to be walking across a frozen sea here in Greenland
0:54:52 > 0:54:56when back at home, the daffodils are beginning to come up.
0:54:56 > 0:54:57But what's even stranger
0:54:57 > 0:55:01is that measurements of the sea ice over the last 50 years
0:55:01 > 0:55:06show that it only reaches its full extent now, in early March.
0:55:06 > 0:55:10So clearly there's a lag between the arrival of the warmth of the sun
0:55:10 > 0:55:13and the melting of the ice. But why?
0:55:16 > 0:55:19It comes down to the properties of water.
0:55:19 > 0:55:22We've already seen that, well into January,
0:55:22 > 0:55:26land continues to lose more heat than it gains.
0:55:26 > 0:55:31Because water radiates heat even more effectively than land,
0:55:31 > 0:55:34the oceans take even longer to start warming up.
0:55:34 > 0:55:38So although the land has been warming since January the 19th,
0:55:38 > 0:55:45the sea is still losing heat and the ice continues to grow.
0:55:45 > 0:55:49Greenland sea ice is at its maximum extent at this time of year, in March.
0:55:49 > 0:55:54But over the next few weeks, the tilt of the Earth towards the sun as it orbits it
0:55:54 > 0:55:58will allow the northern hemisphere to get an increasing amount of solar energy.
0:55:58 > 0:56:00The days will get longer and warmer
0:56:00 > 0:56:03and the sea ice will begin to break up and recede.
0:56:03 > 0:56:06Then the hunting season will be over.
0:56:20 > 0:56:23The existence of the sea ice here in Greenland
0:56:23 > 0:56:28is testament to the complex response our planet has to the sun,
0:56:28 > 0:56:30in whose orbit we travel.
0:56:30 > 0:56:33But it's a very delicate balance
0:56:33 > 0:56:38and no-one is more acutely aware of that than the people who live here.
0:56:41 > 0:56:46Gio tells me that, even before the storm, this year there was less ice
0:56:46 > 0:56:48than in previous years.
0:56:48 > 0:56:52It's part of a trend over the whole of the Arctic.
0:56:52 > 0:56:58The area covered by sea ice has shrunk significantly in the last 20 years.
0:56:58 > 0:56:59A series of warm winters
0:56:59 > 0:57:03have meant that the seas haven't cooled down as much as normal
0:57:03 > 0:57:07so not as much ice has been able to form.
0:57:07 > 0:57:13There's little doubt that the cause of the warmer winters is us.
0:57:15 > 0:57:21Global warming can feel like a myth when, back in the UK,
0:57:21 > 0:57:25we've endured a string of very cold winters.
0:57:25 > 0:57:30But here on the front line, it's a reality.
0:57:37 > 0:57:41Most predictions suggest that the Arctic will continue to warm rapidly
0:57:41 > 0:57:44over the course of this century.
0:57:45 > 0:57:52It could be that we may well prove capable of generating the kind of climate change
0:57:52 > 0:57:56that in the past has been created by changes in the Earth's orbit.
0:58:06 > 0:58:08We've now reached the middle of March
0:58:08 > 0:58:11and we're approaching the spring equinox,
0:58:11 > 0:58:13the end of our journey for now.
0:58:17 > 0:58:20Next time, we'll complete our voyage...
0:58:20 > 0:58:22Wow!
0:58:22 > 0:58:27..travelling from the equinox back to where we started - the summer solstice.
0:58:50 > 0:58:53Subtitles by Red Bee Media Ltd