Orbit: Episode Two

    The second instalment of the series follows the Earth's journey from the start of January to the Spring Equinox in March. Available on iplayer. What did you think?

    Kate begins the film on a day with a very significant point in our Earth's journey - Perihelion. Kate climbs Aonach Mor mountain, one of the highest mountains in Scotland, which brings her as close to the Sun as she'll ever be for the entire year.

    This however is not because of where she is but because of the point the Earth has reached in its orbit around the Sun. In fact we kick started our blog on this day just over a year ago, when we explored the elliptical shape of our planet's orbit and how significant this was to our understanding of Earth's climate.

    Later in the film Helen explains how the proximity of the Earth to the Sun doesn't guarantee warmth - which brings us to the tilt of the Earth (23.4 degrees) - a theme we explore in further detail in episode three.

    Throughout this episode Kate and Helen explore the increase in solar radiation and how land and ocean respond to it.

    Kate drives over a frozen lake in Canada with an ice road trucker in one of the coldest places in that region and learns how important this ice formation is to connecting communities.

    In this film we also tackle ice ages and how over time, as Earth has repeated it's annual journey, it's climate has changed.

    Helen dives under water in Belize to discover how sea levels have risen and fallen over time due to ice age - and explores the three cycles that need to be right in order for another ice age to exist.

    What did you think of episode two?

    (There are a total of three episodes in this series)

    Aphelion: shouldn't earth be at its coldest today?

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    Stephen Marsh Stephen Marsh | 15:30 PM, Monday, 4 July 2011

    Distance travelled ~ 475'968'000 km: day 185

    Today July 4 is a rather special day in our annual journey around the Sun. At 15.00 GMT Earth will be at the furthest point away from the Sun it will reach all year. I know it sounds bizarre considering how warm it is today but it's true, and it's all down to our orbit.

    The Earth's orbit around the Sun is not a perfect circle. It's an ellipse and the Sun does not sit at the centre, it's offset to one side. So today we are 5 million kilometres further away from the Sun than when we were closest to the Sun six months ago in January during Perihelion. And if you remember it was pretty cold in January so you might ask why isn't our proximity to the Sun in winter warming us and our increased distance away from the Sun in summer cooling us down?

    sun at aphelion and perihelion

    Image © 2001-2011, Anthony Ayiomamitis

    The truth is our elliptical orbit and our distance from the Sun is not the primary driving force behind our climate. So what is?

    Well number one is the that fact that the Earth is tilted at 23.4 degrees from vertical. During Perihelion the northern hemisphere is tilted away from the Sun, so received less solar radiation and we get winter. In our summer we are tilted towards the Sun so despite being farther away on our orbit we get increased solar radiation. Which is why today in July it's lovely and warm.

    But what about the southern hemisphere? Well it's a bit more complex down there. Earth is closest to the Sun during their summer when they are tilted towards the Sun and this means they get 7% more solar radiation. Therefore you'd expect the southern hemisphere summer to be a lot warmer than the northern summer. But it's not, in fact it's actually colder.

    And now at Aphelion it's winter down south, and the hemisphere is both furthest away and tilted away from the Sun. So you'd think it's winter would be a lot colder, what with the tilt pushing the hemisphere away from the Sun. Well in fact it's warmer than our winters. And that's all because of the ratio of land to oceans in the hemispheres. The northern hemisphere has a lot of land but the southern hemisphere is predominantly water with very little land and it's this that is powering the climate.

    While land reacts very fast to solar heating, it warms up and cools down very quickly. The oceans react very slowly to solar energy. They take a long time to warm up and a long time to cool down. This means that at perihelion in their summer the oceans haven't absorbed enough energy to warm up the atmosphere, they are still cool from the previous winter, so they keep the temperatures cool. In the southern hemisphere winter it's the opposite. The oceans have held onto some of the heat they absorbed during the summer and are keeping the air above warm.

    So even though our elliptical orbit takes us at this point of the year further away from the Sun than it will for the rest of the year, it's warm in the north because we are tilting towards the Sun and a milder winter in the south because of the actions of the oceans.

    Newly discovered asteroid: 2011 MD closest approach 17.03 UT (just above 12,000km)

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    Aira Idris Aira Idris | 17:00 PM, Monday, 27 June 2011

    Distance travelled ~ 457'958'400 km: day 178

    asteroid 2011 MD

    Image courtesy of NASA

    (At closest approach, 2011 MD will pass in broad daylight over the southern Atlantic Ocean near the coast of Antarctica. As the asteroid recedes from Earth, it will pass through the zone of geosynchronous satellites. The chances of a collision with a satellite or manmade space junk are extremely small, albeit not zero.)

    Will monsoons once again return to the Sahara?

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    Stephen Marsh Stephen Marsh | 12:50 PM, Friday, 24 June 2011

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

    africa's sahara desert

    Image and notes courtesy of NASA

    Image and notes courtesy of NASA
    (On August 25, 2000, the Moderate-resolution Imaging Spectroradiometer (MODIS) acquired this spectacular image of a region in Africa's Sahara Desert, including the southern part of the border between Algeria and Libya. Three large rock massifs appear to be pushing up from beneath red sand dunes: from left to right are the Tassili, Tadrart-Acacus, and Amsak massifs. Different rock types account for rock colors varying from dark brown (Acacus) to the pale tone of Amsak eastern portion (Amsak Mellet means Pale Amsak in the local Tuareg dialect) The dendritic structures of ancient riverbeds are clearly visible in the Acacus-Amsak region.)

    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.

    Sun-Earth-Moon relationship: First total lunar eclipse of 2011

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    Aira Idris Aira Idris | 13:15 PM, Tuesday, 14 June 2011

    Distance travelled ~ 424'512'000 km: day 165

    One of the few significant things Earth and the Moon share is their relationship with the Sun. To an observer on Earth at least - as different portions of both the Earth and Moon's surfaces are illuminated, this signifies the movement of time and the root of our calendar, which is very important.

    On 15th/16th June we will observe a unique point in the Sun-Earth-Moon annual relationship - a total lunar eclipse and the first for this year.

    total lunar eclipse 2000

    Image courtesy of Fred Espenak/NASA

    It takes the Moon 27 1/3 days to orbit the Earth (lunar month 29 ½ days), going through the new Moon, first quarter, full Moon, last quarter and back to new Moon. A total lunar eclipse happens when there's a full Moon and the Moon passes through a part of the Earth's shadow, known as the umbra - an area not directly receiving the Sun's rays.

    Although there is a full Moon every month, we don't get a total lunar eclipse each month because the Moon's orbit is not in the same plane as the Earth's around the Sun (the ecliptic). From the image below we can see that the Moon's orbit goes over and under the Earth's orbital plane around the Sun.

    geometry of a lunar eclipse

    Image courtesy Wikimedia commons

    The inclination of the Moon's orbit is around 5 degrees to the Earth's orbit, and passes through the ecliptic only twice a month at a pair of points called the ascending and descending nodes. This is where the Nodal Axis is aligned with, or pointing at, the Sun.

    The period when the Earth completely blocks the Sun's rays from the Moon is when we experience a total lunar eclipse - known as totality. This moment repeats itself every 6 months.

    For this week's eclipse the best placed observers to see it in it's entirety are those in East Africa, central Asia, Middle East and West Australia, lasting a total of 1 hour and 6 minutes. For Europe and South America we will miss the beginning of the show and places like west Australia will miss the end - check specific times for your location. North America completely misses the total lunar eclipse.

    Penumbral Eclipse Begins: 17:24:34 UT
    Partial Eclipse Begins: 18:22:56 UT
    Total Eclipse Begins: 19:22:30 UT
    Greatest Eclipse: 20:12:37 UT
    Total Eclipse Ends: 21:02:42 UT
    Partial Eclipse Ends: 22:02:15 UT
    Penumbral Eclipse Ends: 23:00:45 UT

    (credit NASA)

    What can you expect to see? The shade of the Moon at eclipse is hard to predict because of the Earth's atmosphere. Although the Earth will block out the Sun during totality, the Sun's rays will still penetrate through the Earth, and mixed with the dust and cloud in the atmosphere the total lunar eclipse may take a variation of different shades. Volcanic ash can also affect the shade of the total lunar eclipse - turning it a darker shade of red. Ash from the recent eruption of the Puyehue volcano in Chile may have placed some sulphur dioxide into the stratosphere, according to atmospheric scientist Richard Keen of the University of Colorado.

    If you plan on observing or photographing the total lunar eclipse of June 15th/16th and would like to share your comments and images with the 23 Degrees team for a possible story or image gallery do get in touch.

    The next long lunar eclipse will be in 2018.

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