January 3 - closest point to the sun, can you believe it?
Post categories: northern hemisphere
| 17:07 UK time, Friday, 7 January 2011
d ~ 18'009'600 km: day 7
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You might ask why we chose January 3rd to begin filming. Well believe it or not, it's a very special day. Despite the freezing weather we are getting at the moment, the Earth is at its closest point to the sun it will get all year. It's called Perihelion which is Greek for "near to the sun", and right now the Earth is 5 million kilometres closer to the sun than it will be in July, when it's at its furthest point.
Perihelion highlights how fascinating the relationship is between the earth and its cosmic star. Who would have thought that at the point when we are closest to the sun, we would experience such freezing temperatures? To kick-start our trip, 23 Degrees presenter Kate Humble and the production team visited Aonach Mor - one of the highest mountains in Scotland.
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Perihelion is a result of the slightly strange shape of the Earth's orbit around the sun. It's not a perfect circle, it's an ellipse, and the sun isn't even in the centre, it's slightly off to one side. So at the moment we are just 147.1 million kilometres [91.4 million miles] from the sun. You are probably thinking - if we are closer, then why aren't we warmer? Well, our journey around the sun is complex and there's another factor that shapes our climate that is more powerful than just how close we are to the sun. And that's the tilt of the planet. Earth doesn't sit upright; it's tilted over to one side at an angle of .
The tilt and the Earth's motion around the sun create the seasons. In January the northern hemisphere is tilted away from the sun so the sunlight is spread more thinly over the northern half of the planet so we get our winter. Meanwhile the southern hemisphere which is now tilted towards the sun gets more sunlight and therefore enjoys summer. Gradually the Earth moves around the sun until July when the Northern hemisphere tilts towards the sun, when the reverse is true.
Now – in January – we are in midwinter, but the northern hemisphere is already starting its journey back towards the sun and over the next few months we will get more and more sunlight and slowly the Northern Hemisphere will rise from its slumber and move towards spring.
Kate Humble:
Helen Czerski:
Stephen Marsh:
Aira Idris:
Comment number 1.
At 11th Jan 2011, sportylears wrote:Obviously the prosperity of civilisation in the northern hemisphere and that magic number of 23.5 is no coincidence, but it really leaves one in awe of the smallest of tolerances in nature that work in our favour.
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Comment number 2.
At 11th Jan 2011, RTFishall wrote:At 12:21pm on 11 Jan 2011, sportylears wrote:
Obviously the prosperity of civilisation in the northern hemisphere and that magic number of 23.5 is no coincidence, but it really leaves one in awe of the smallest of tolerances in nature that work in our favour.
Sportylears: Haven't you noticed that two thirds of the earths land mass is in the northern hemisphere? Perhaps that's why most of the "prosperous civilisations" live there! And, actually 23.5 is not the magic number. The Earths tilt is 23.25 lol
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Comment number 3.
At 11th Jan 2011, Andrew Lothian wrote:An easy way to show how the tilt of the Earth affects the warmth we recieve is to use a torch and a piece of card or paper, at least A4 size. The torch must throw a beam, I have torches that give lots of light but no beam. Switch on the torch and shine the beam onto the card so that you can see circle of light on the card. Now tilt the card so that the top is away from the torch, and the bottom nearer, WITHOUT changing the distance of the centreline of the card relative to the torch. The circle is now an ellipse. If you now move the card up and down, at right angles to the beam, the ellipse grows bigger or smaller depending on whether the "landing point" is tilted towards or away from the torch. With the Sun and the Earth, this means more light hence more energy, or less as the case may be, so we have the seasons. Try it, it works. (I should point out that when I was shown this experiment, the demonstration took less time than I took to write this explanation).
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Comment number 4.
At 11th Jan 2011, sportylears wrote:Thanks for that RTFishall. The point I (tried to) make is that most of us take these things for granted from day to day. 23.25 degrees you say? I'll update my notes!;-)
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Comment number 5.
At 11th Jan 2011, Andrew Lothian wrote:May I point out that Stephen Marsh wrote "Earth doesn't sit upright; it's tilted over to one side at an angle of 23.5 degrees.", and the 23.5 degrees is highlighted.
Seriously, does the 0.25 degrees make any difference? Would the planet really notice that difference compared to the small amount of effect of the orbit not being circular?
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Comment number 6.
At 11th Jan 2011, RTFishall wrote:At 2:19pm on 11 Jan 2011, Andrew Lothian wrote:
May I point out that Stephen Marsh wrote "Earth doesn't sit upright; it's tilted over to one side at an angle of 23.5 degrees.", and the 23.5 degrees is highlighted.
Seriously, does the 0.25 degrees make any difference? Would the planet really notice that difference compared to the small amount of effect of the orbit not being circular?
Well, the length of the longest (and shortest) days would change. 0.25degrees is about half the moons diameter in the sky. The sun would be that amount different in its altitude above the horizon and there would almost certainly be a recordable difference in climate of higher latitude countries such as Norway and Northern Canada.
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Comment number 7.
At 13th Jan 2011, sportylears wrote:"The sun would be that amount different in its altitude above the horizon and there would almost certainly be a recordable difference in climate of higher latitude countries such as Norway and Northern Canada."
Recordable undoutbedly, but how significant? .25 degrees is 15 nautical miles? 25 - 30km? Correct me if I am wrong please.
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Comment number 8.
At 13th Jan 2011, Stephen Marsh wrote:I’ve been reading all your posts about the angle of tilt. It’s a very stimulating debate.
What’s interesting to me is that the tilt of the planet isn’t static, it changes over time.
The Earth is currently tilted at 23.44 degrees but over a 41,000 year cycle it will move between 22.1° and 24.5° and back again. These changes affect our climate. When the obliquity increases, summers will get more solar radiation and will be hotter and winters less radiation so will be colder.
We are now on the downward part of the journey and should get to the minimum tilt in around 8,000 years. At that point the Earth will be at its most upright. This reduced angle of tilt will make the winters warmer and summers colder.
But the Earth’s axis doesn’t only wobble up and down – it also spins around itself. I’ll explain what I mean using the pole star. Currently the pole star is the star Polaris in the constellation of Ursa Minor. But it wasn’t always this particular star and it won’t always be in the future.
Around 2700 BCE when the ancient Egyptians looked up at the pole star they didn’t see Polaris they saw star Thuban also known as Alpha Draconis.
In around 12,000 years times, a very brilliant star [the 5th brightest in the sky] in the constellation Lyra will be the closest thing we have to a Pole Star. It’s called Vega or Alpha Lyrae and it will be the brightest pole star ever.
This has nothing to do with changing tilt. It’s called the precession of the equinoxes and it’s a wobble in the direction of the Earth’s axis compared to the galaxy around us.
But how does it work? Andrew Lothian got it spot on. If you imagine that the Earth is a spinning top - as it spins the rotational axis isn’t constant, and appears to move in a circle. The Earth is the same – as it spins the axis follows a circular path. This happens because the Earth isn't a perfect sphere, it actually an 'oblate spheroid', which means it has a bulge at the equator. This bulge is tugged at by gravitational forces from the Sun and Moon, which disturb the planets rotation creating the precession.
Each century the Earth’s axis moves around a half-degree. And the full cycle of precession of the equinoxes takes 25,800 years, so we will have to wait another 25,800 years for Polaris be the pole star again.
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Comment number 9.
At 16th Jan 2011, sportylears wrote:Well, there we have it then. I guess for the purposes of this blog, 23.5° is accurate enough. It is also "interesting" how the numbers given for frequency of the two components of the earth's precession sound very similar to the periods of glacial maximums. Now, yes that is a whole other debate (sort of), yet in the overall context of the subject, it is yet to be fully realised what effect a 3rd relatively new component will have on that. People.
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Comment number 10.
At 20th Jan 2011, Stephen Marsh wrote:sportylears
interesting observation - stay tuned as this is something we are going to be investigating on the blogs and in the TV series.
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