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(Jonathan Renouf is an Executive TV Producer making documentaries for the Ö÷²¥´óÐã Science department. His most recent projects are 'Wonders of the Universe' with Brian Cox, history of global warming "Earth: The Climate Wars", the BAFTA nominated "Earth: Power of the Planet" and the highly popular and critically acclaimed 'How Earth Made Us', which transmitted in January and February 2010. Here he shares his insights on making 23 Degrees (working title) due to transmit early 2012.)

Distance travelled ~ 713'844'800 km

A couple of days ago my baby son woke me up early, and even though I got him back to sleep, I was too awake to settle down. Eventually, at about 6.00, I gave up, got out of bed and decided to go out on my bicycle. I live in Cookham, close to a picturesque stretch of the River Thames, so I cycled down the lane towards the river. As soon as I left our suburban close behind, my heart leapt. A mist lay over the fields - just a few metres thick, but dense, and tinged magenta by the dawn light. Down by the river the mist hung over the water, and a grebe drifted into view on the mirror flat surface. In the distance I heard an early train clanking along the branch line towards London. A few last stars flickered above me as the sky lightened. And all around there were cobwebs thick with dew. I settled down to take some photos of the cobwebs, happily absorbed in the task of trying to capture their fragile, jewel-like beauty.



Isolated in my riverside reverie it would be easy to forget that we are hurtling through space, on a planet that is tilted over on its axis, spinning as we go, travelling on an orbit that takes us closer and then further from the Sun. And yet one of the wonderful things about working on 23 degrees is that it has given me a magical new perspective on mornings like this. Intensely local phenomena such as the dawn mist I experienced are also part of a much bigger picture. Dawn mists are a consequence of the longer - and therefore cooler - nights, which in turn relate to our 23 degree tilt and the seasons it creates. 



But the most revealing insight I've gained from the series is the notion that all our weather is driven by gradients - and by the way the Earth seeks to even them out. Gradients are created whenever two (or more) parcels of air (or water or ground) are next to each other, but with different properties - for example, a hot parcel of air next to a cool parcel of air. This means there is a temperature gradient between them, and the Earth system always seeks to even out these differences. The trouble is, there are constant energy inputs creating (or adding to) these differences. Put another way, the climate system is in a constant search for equilibrium, but our journey around the Sun keeps throwing the system out of kilter.



Gradients exist at every scale of the climate system, and crouched down with my camera in the cool, clammy air, I was experiencing one very directly. As the nights lengthen into Autumn, the ground radiates more and more heat back to space. The land cools down, setting up a temperature gradient between the land and the air above. The ground cools the air - attempting to equalise the gradient - until the air reaches its condensation point, forming mist. But then, as the Sun rose, the ground warmed, the air warmed with it, and within a few minutes the mist vanished - the gradient gone.

When I got home almost an hour after leaving I was relieved to find the house still quiet. Just enough time to download this photograph...

Working on 23 Degrees has given me terrific new insights into how our world works, why it is the way it is, and what makes it change. Hopefully when the series is transmitted early next year, you'll get to enjoy those insights too.

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Distance travelled ~ 450'240'000 km: day 175

How did the Sahara once have a monsoon? Well it's all down to the amount of sunlight hitting the region. But not quite in the way you might think. It seems obvious that a hotter Sahara would have less rain, because it's the opposite. Monsoons are not created by less solar energy, they are created by more. To get a Monsoon you need lots of solar energy that heats up the land creating a region of low pressure to suck cool moist air from the oceans towards it. Strange as it may seem 8000 years ago when the cave paintings in Wadi Sora were made the Sahara was getting more sunlight than it is now. And that extra heat helped bring the monsoon rains to this desert. But how did the Sahara get more solar energy?

It's all down to our planets orientation to the Sun and how that changes over time. This orientation, which dictates the amount of sunlight we receive, is controlled by three main factors, Tilt, Precession and Orbit. 8000 years ago when the Sahara was green, these factors were different to what they are today. The science of this is very complex but here's a simple summary of what was going on.

Currently the earth tilts at an angle of 23.4 degrees. But over a 41,000 year period it changes, wobbling between 22.1 and 24.5 degrees. Back when the Sahara was green, the tilt was close to its largest possible angle, 24.2 degrees. Which meant that 8000 years ago the Sun shone more directly, more intensely over the Northern hemisphere.

Precession is even more important. This is not a change in the degree of the tilt, but a sort of lateral wobble, which changes the direction of the tilt. The best way of explaining it is by looking at the stars. Some of you may know about the North Star or Pole Star called Polaris. While other stars move across the sky, Polaris stays fixed just above celestial north. That's certainly true now but when the people painted themselves in the cave of swimmers, Polaris wasn't close to north in fact it was over to the east. Then 8000 years ago Thubon was the North Star. And in 12,000 years, a new star, Vega, will be pointing out due north. The North Star changes because precession makes the earth wobble a bit like a spinning top slowing down and starting to wobble back and forth. This precessional wobble takes 23000 years to complete one cycle, so it will be 23000 years before Polaris will come back round to be our northern star again.

There is one final factor involved. Our annual orbit around the Sun is not a perfect circle - it's an ellipse. Also the Sun does not sit at the centre, it's offset to one side. So there are times when it is closer to the Sun than others. Just like Tilt and Precession the shape of the orbit also changes slowly over time becoming more or less elliptical moving the earth closer or further from the sun. When the Sahara was green, all these orbital factors were in alignment, so summers in the northern hemisphere were hotter than they are now, the Sahara received more sunlight which pulled the monsoon band to northward.

The changes in our orientation to the Sun change all the time and gradually the orbit, tilt and precession changed so that the amount of solar energy hitting the Sahara eventually decreased and with it the monsoons.
Around 6000 years ago, the monsoons failed completely, the rains stopped, the rivers dried up and the land began to turn to desert. But even as you read this the same orbital factors are slowly changing and at some point in the future they will align again and the monsoons should once again return to the Sahara.

But that's not the end of the story because scientists have discovered that the wet period 8000 years ago wasn't a one off, in fact it had happened many times before. Perhaps the most significant greening of the Sahara occurred 120,000 years ago at a particularly important moment in human history.

120,000 years ago homo sapiens, modern humans, emerged from Africa. For thousands of years the Sahara had been an impassable barrier, a bit like it is now. But 120000 years ago the evidence suggests the Sahara was green and criss-crossed with rivers and lakes, and scientists believe that it allowed passage for our ancestors north. They crossed the Sahara travelling along rivers and settled in North Africa, and eventually, Europe and Asia.

It really is incredible to think that such critical moment in our history could have been triggered by changes in our tilt and orbit.

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Distance travelled ~ 429'657'600 km: day 167Ìý

In this blog post we are going to show you how you can calculate the exact tilt of Planet Earth by using your shadow. You can only do this at a certain time of the year, the Summer Solstice, and that moment is fast approaching. Ìý

This year Summer solstice is on 21 June. This is the longest day of the year and also for us here in the northern hemisphere, the time when the Sun is highest in the sky. Astronomers regard it as the start of summer for the northern hemisphere winter for the southern.

So what's the Summer Solstice got to do with measuring the tilt of the Earth, I hear you ask. Well, the orientation of the Earth to the Sun is defined by angles, and on June 21 the physics align so that you can use the position of the Sun in the sky at 1pm BST to accurately measure the tilt of the Earth. It still works around 2 days before and 2 days after so you have a few days to try this out.

Here's a tip - measure from the balls of your feet to the top of your shadow and remember be careful and don't look directly into the Sun.

The first important angle is Solar Zenith. This is the angle between the Sun and straight up and it's marked in green on the figure below. We can find this because the smaller this angle is - or the higher in the sky the Sun is - then the smaller your shadow. To work this out we use . If you know about tan, then your height is the adjacent side and your shadow is the opposite side of a square-angled triangle.

Next we need latitude. This is essentially the angle between the equator and your position on the Earth. You can see from the figure above that if you take the green angles away from the pink angle then you get the yellow angle. This angle between the equator and the place on Earth where the Sun is straight above you is called the "solar declination". But on the solstice it's exactly the same as the tilt of the Earth - convenient.

How to find your latitude?
There are loads of ways of getting your lattitude -a gps device maybe on your phone or satnav, or endless websites can provide you with this. Some are as simple as putting in your postcode.

Most importantly the 23 Degrees team would love to hear how you get on. Send us your calculations, photos or videos of you doing the challenge on the Solstice, or maybe a photo of your shadow. Shadows have long been a key indicator to our Earth's position in it's orbit around the Sun.

Continue the conversation and put #solstice in your tweets!

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d ~ 192'960'000 km: day 75

We're coming up to the spring equinox, the official start of spring for many people, and it really feels to me like the seasons are changing.The seasons on earth are, of course, the result of the 23 degree tilt in our axis of rotation.

But what might our world be like without that tilt, and without its changing seasons? We wouldn't experience the usual swings between summer and winter, obviously, but would we have a permanent spring or autumn climate, or might the lack of axial tilt have different implications for our environment?

The earth hasn't always rotated with a 23 degree tilt. Pretty much nothing about the Earth's climate stays constant if you wait long enough, and that tilt is no exception. It wobbles up and down by a couple of degrees every 41,000 years or so (at the moment the tilt is slowly decreasing), and the strength of the seasons the earth experiences changes with it. When the tilt is greater, summers are warmer and winters are colder, and when the tilt is smaller there's less of a difference in the seasons. These repeating cycles in the strength of the seasons probably play an important role in forcing the huge climate shifts of the glacial cycles that the earth has experienced over the last million years - and that's all with changes of just 2 or 3 degrees in the tilt.

For fun, I set up a relatively simple model to simulate what the climate on an earth with a 0 degree tilt might be like. There are a few details that make this more of a toy than a serious scientific study, but we can still use it to illustrate some of the things that could happen in a 0 degree world. To start with, of course, the seasons disappear: although the weather is still different from day to day, February is much the same as June and October. However, if you guessed that the earth's climate in a 0 degree tilt world would permanently be stuck halfway between our usual summer and winter, you'd be wrong!

A good way of imagining what it would be like to live on the 0 degree tilt world is to see how the ecosystems that we know from our 23 degree world would fare if we and they moved there*. The top panel shows a very simple way to characterise the climate of our 23 degree world in this kind of scheme. Greens show areas predominantly suitable for types of forest, browns are drier areas and grasslands, with grey for tundra, yellow for deserts and barren areas and ice caps in blue. There's a lot of fertile vegetation in this view of our world, with some desert in the hotter, drier areas and tundra and polar ice right up in the north.

The bottom panel shows what our toy simulation of a 0 degree world looks like. This climate is much less suited to our usual types of vegetation, with much larger barren desert areas, and a huge expansion of polar ice over Asia and North America. The
area suitable for vegetation at in the northern hemisphere shrinks dramatically, and northern Europe swaps its forests for tundra. The average temperature here in Britain sinks to a cool 7 degree C all year round, only varying by a couple of degrees warmer or cooler at most. Not everything would change for us, though - we'd still get about as much rain every year in a 0 degree tilt climate as we do now.

So, the earth's 23 degree tilt doesn't just give us the variations of the seasons and all the wonderful things we'll be seeing from this series - it's really important for setting the basic foundations of the environment we take for granted in our part of the world. As you can see, we'd have a very different planet without those 23 degrees.

NCAS-Climate Dept. of Meteorology, University of Reading

* if the earth really had a non-seasonal climate, totally different types of vegetation would certainly evolve, so this is just a simple way of visualising what the different climates would be like.

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d ~ 187'814'400 km: day 73

The short answer to this is yes. But the effects of such a shift are tiny. The Earth's tilt and rotational spin on its axis as it travels around the Sun causes our seasons. The earthquake in Japan moved the axis of rotation by around 16 cm. That might sound like a lot, but it's small compared to the size of the Earth. 1 degree change to the tilt of the axis of the Earth would mean moving it by around 110 km.

But the quake's interference with our axis doesn't stop there. The Japanese landmass was moved around by as much as 4m. This redistribution of mass on the surface changes our moment of inertia. In order to conserve angular momentum, the changes in inertia are compensated by changes in the rate of rotation of the Earth about the axis. After the earthquake it's quite possible that our days will be 1.8 millionths of a second shorter because of this shifting.

We can see differences in the average length of the day due to other changes in the Earth and atmosphere. The plots below show that there is a significant seasonal variation, with the day length (speed of rotation) being shortest (fastest) during the boreal summer. This happens because the northern hemisphere winds slow down in the summer and the momentum they lose - half the momentum of the atmosphere - is transferred to the Earth. This increase in momentum makes the Earth spin faster and our days become slightly shorter by 1-2 milliseconds.

So while the changes brought to our planet by the earthquake are unique and collosal enough to affect the Earth; they aren't big enough that we will notice them any more than we notice the milliseconds we lose each summer.

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