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(Phil Plait is an Astronomer and Author who writes the for Discover Magazine. His blog is dedicated to clearing up public misconceptions about astronomy and space science in the media. After working for ten years with the Hubble Space Telescope team at NASA Goddard Space Flight Center he went on to work on astronomy education. Phil has appeared on numerous TV and online astronomy features. Here Phil celebrates Perihelion which is just a few days away, and the point when 23 Degrees started it's project a year ago. Phil can be reached )

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Distance travelled ~ 938'107'200 km

The Earth travels 'round the Sun at the terrifying speed of 30 kilometers per second, making a gigantic circle nearly a billion kilometers in circumference once per year.

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That's a fantastic distance, but in one way it's a bit disappointing. After all, a year after you started this high-velocity trip, you're back where you started.

However, the journey isn't actually a perfect circle. It's off by a tiny amount, so slight that you'd never notice if someone didn't tell you. But in fact, this minor deviation can make a big difference: it means that the Earth's distance from the Sun changes over its orbit to the tune of over 5 million kilometers!

That difference amounts to about 3.4% of the average distance to the Sun, which astronomers, for convenience, call an Astronomical Unit, or AU. In real terms, an AU is 149,597,870 km plus a bit. When the Earth is at aphelion - its farthest point from the Sun - it's about 152,141,000 kilometers from our star. At perihelion, that distance shrinks to 147,055,000 kilometers.

You might expect this change in distance to the Sun would have an effect on the Earth's temperature. It does, in fact, but it's pretty small, only a couple of degrees Celsius. That's swamped by the seasonal change in temperature due to the Earth's tilt, which is what really drives the seasons. Think of it this way: the Earth reaches perihelion in January, which is the dead of winter for the northern hemisphere. While this does make northern winters a tad more clement, the amount is too small to make an appreciable difference.

Oddly, for those in the antipodes of the southern hemisphere, we're closest to the Sun in their summer. You'd expect temperatures then to be higher than average, but in reality they're about the same as their boreal neighbors. Why is that? It's because the southern hemisphere is dominated by the Pacific Ocean, and water is an excellent heat sink. It absorbs the extra heat in the summer, and releases it in the winter, mitigating temperature extremes.

Weather, it turns out, is complicated.

There is one measurable effect, though: the size of the Sun changes by that same 3.4% over the course of half a year. You'd never suspect that by eye - of course, looking at the Sun is not a suggested pastime - but if you carefully take pictures of it over the course of the year you'll see it, as astronomer Anthony Ayiomamitis did in the picture here. Since the change is slow day-by-day it's beneath our notice, but the telescope doesn't lie.

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So when is the next perihelion? Why, it's January 5, 2012, and it will occur at about 01:00 GMT. At that moment, or thereabouts, the Earth will again be as close to the Sun as it will get all through 2012. The 23 Degrees project started on the day of Perihelion for 2011 and we're already on our way back out, and on July 5, at 04:00 GMT, the Earth will reach the apex of its orbit, and oh so slowly start the fall back toward the Sun.

And it will do this, over and over for the next few billion years, until the Sun swells into a red giant and cooks the Earth like a banger in a blowtorch.

So enjoy the ride while you can.

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(Scott Sistek is a meteorologist and producer for KOMONews.com. He has been producing weather reports for broadcast and the web since 1994 and can be found on the weather blog. Keep up to date with Scott via )

Distance travelled ~ 934'891'200 km

The Pacific Northwest is known for its beauty, from the lush greenery to the tranquil waters to the majestic mountains. But no mountain is as iconic to the Northwest as Mt. Rainier, which stands just over 14,000 feet tall about 70 miles southeast of Seattle.

But while the Seattle area is world famous for its rainy, cloudy weather, at times, Mt. Rainier can act like its own paintbrush and using the sky as its canvas, bring a whole new awe-inspiring level to a "cloudy" sky. Thanks to it's status as the tallest peak around and its unique position to catch the moist jet stream, it flows in off the Pacific Ocean - Mt. Rainier can create its own weather patterns.

Perhaps the most dramatic are its frequent lenticular cloud displays. Seen maybe a dozen times a year, it still looks amazing every time it's showcased.

The cloud is formed when warm, moist air runs into the surface of Mt. Rainier. The mountain's topography forces the air upward, which cools and condenses the air -- turning it into a cloud. As the air sinks back on the other side of the mountain, it dries out and the cloud dissipates. That's why it just hangs over or near the summit area. (Although it looks like it is "hanging" over the mountain, air is continually flowing over the summit.)

Sometimes if the atmospheric set up is just right, you can get layers upon layers of stacked lenticular clouds that combine to make dramatic shapes -- many times mistaken for UFOs years ago.

Image credit: David Embrey

Locals have used this cloud as a sign that rainy weather is on the way -- many locals might think the cloud is the mountain's version of an umbrella? -- as that cloud usually occurs with west or southwesterly flow in the upper atmosphere, a usual precedent to rainy weather. However, that's not always the case -- especially in the summer. Then, it can just be an indication that we have a good westerly, marine flow and that it won't be too hot anytime soon.

Or on rare occasions, the mountain can have the opposite effect, as seen here:

Image credit: John Meadows

This time, the mountain caused some turbulence that created some sinking air in the vicinity of the mountain peak. Sinking air dries as it does so, in essence "eating" away a hole in the cloud!

Finally, when the mountain isn't creating or destroying clouds, it can just put on a show using the clouds that are already there.

In the autumn and early winter in the Seattle area, the Sun's position on the horizon during sunrise is just in the exact right spot to where Mt. Rainier will cast a shadow against a cloud layer!

Image courtesy of Nick Lippert/YouNews

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So while yes, it rains a lot around here, there are plenty of advantages to living in an area with such unique terrain and meteorology!

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Distance travelled ~ 932'318'400 km

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It鈥檚 very liberating to be completely and utterly soaked by a rainstorm, especially when what鈥檚 falling out of the sky are raindrops the size of large peas.

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I tried to explain this to the director and crew who were huddled beneath enormous umbrellas, missing out on all the fun. They were not convinced. They had not come to India during the monsoon to get wet. That was my job.

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The thing is, weather is fun. We are brought up to hide from it a bit, to carry on (usually with a British stiff upper lip) in spite of it. But it鈥檚 not going away, so I think that we might as well appreciate it. As long as it鈥檚 not giving you hypothermia or sunburn, why not just play with whatever the atmosphere is doing today?

For 23 degrees, we鈥檝e been lucky enough to travel to some fantastic places in our global weather patterns. Different parts of the planet receive different amounts of energy from the Sun, and this is just the start. That energy is carried around the planet by the ocean and atmosphere, and the result is a giant pattern of hot and cold air, dry and moist air and huge swirling wind systems. The pattern is never exactly the same from one year to the next, but there are features that are present all the time (tropical thunderstorms and the jet stream), or that return every year (spring showers and hurricanes).

Two things have stood out for me. The first is how little we appreciate the depth of the atmosphere. I realized this properly while looking at the tornado we found in June.

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It鈥檚 almost certainly the biggest thing I鈥檝e seen whose scale I鈥檝e been able to understand. We know that the clouds are high up, but until you see a single thing joining the clouds to the ground, you have no idea what 鈥渉igh up鈥� means. The atmosphere is big, vertically as well as horizontally.

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The second thing is how little we actually look at the sky, especially in Britain. We鈥檙e too busy getting on with things on the ground, and anyway there are lots of buildings and trees in the way. Above us thousands of tonnes of nitrogen and oxygen are flowing around, carrying water and energy, and all we do is complain about it when it gets uncomfortable down here. But if you look up, you can usually see some of the structure of the atmosphere, and that gives you a hint about the larger scale patterns that cover our continent and our planet. Next time you look up at the sky, imagine how all this is connected to the weather over Iceland and Morocco and Costa Rica.

The last day of filming for this series was on the south coast of England, near Beachy Head. We haven鈥檛 done that many days filming in the UK, and it was as though the weather was determined to prove that it shouldn鈥檛 have been neglected.

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As the day went on, we had incredibly hard rain followed by hail, very strong winds and occasional spells of sunshine. My boots filled up with water and at the end of the day I felt as though I鈥檇 been in a giant washing machine for a few hours. It was impossible not to be impressed by what the atmosphere was up to, even on our own doorstep.

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(Peter Gibbs is a 主播大秀 weather forecaster and appears as an expert meteorologist on "The Weather Show" for the 主播大秀 News channel. He started his first guest blog post for 23 degrees with 'What would happen if the Earth spun the other way' and provided much food for thought with his post on 'Abundance in fruits indicator to past British weather'. His post on 'What's in a name' cleared some misunderstandings that where flying around the web as remnants of hurricane Katia stirred it's way to the UK, and he provided us with a breakdown of the difference between cyclones, typhoons and hurricanes with his post on Cyclone Yasi. Keep up to date with Peter Gibbs - )

Copyright: United States Government works

Distance travelled ~ 929'745'600 km

It may seem surprising, but weather forecasters need to take a rather parochial view of the world. At an airport, the forecaster has to predict cloud base, visibility, wind speed and direction in great detail over a few hours for a very specific location. Even a forecaster with a national brief will tend to concentrate only on the weather systems moving across that country and give no more than a passing glance to the storm spiralling across neighbouring areas.

One of the advantages of working as a weather broadcaster on 主播大秀 World is that I get to see the whole picture and can begin to understand the interactions of the global weather system with its regular seasonal pulse. A group of thunderstorms produces newsworthy rainfall as it tracks westwards across equatorial Africa, grows into a hurricane over the tropical Atlantic to threaten east coast America, then gets caught up by the jetstream and races across the north Atlantic to bring rain and gales to northwest Europe, passing through several forecast jurisdictions en route.

Other rhythms overlay the annual one. Swings from El Nino to La Nina take place over periods of several years and enhance or diminish normal seasonal features, especially rainfall. 2011 has been mostly a La Nina year,

Copyright: MSP

with unusually warm water washing into the western side of the Pacific. The extra atmospheric moisture this provided was the likely cause of January deluges in Sri Lanka and the Philippines, as well as the extraordinary where an area the size of France and Germany was underwater for a time.

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Continental landmasses tend to produce the biggest temperature contrasts and hence the most violent weather, especially during the transitional periods of spring and autumn. for tornadoes in the USA with an estimated 600, smashing the previous April record of 257 and even beating the all time monthly record of 542, set in May 2003. Arctic air pushed further south than usual, meeting air from the exceptionally warm waters of the Gulf of Mexico and combining with a jetstream pushed unusually far south by La Nina.

As the Atlantic warmed, an active hurricane season was expected and the with a total of 19 named storms, of which seven became hurricanes including three major hurricanes of category 3 or above. Surprising then, that we had to wait for a record eight tropical storms to come and go before our first hurricane. But once formed, hurricane Irene made the biggest impact, passing through the islands of the northern Caribbean before becoming the first landfalling hurricane in the USA since 2008.

La Nina was in the dock again as the likely culprit when weeks of heavy rain produced some of the worst floods on record in . The monsoon season started early and finished late, meaning there were even greater volumes of water than usual flowing from the mountainous north to the low-lying plains of the south.

Having a global perspective makes me even more appreciative of our UK climate. The British Isles are at the crossroads of European weather. Atlantic winds are a moderating influence, while the proximity of continental Europe can provide bigger swings from hot to cold. Last December found me gliding on Nordic skis across the snowfields of Berkshire, while this December the Christmas journeys to friends and family will be easier on roads kept clear of snow and ice by mild westerlies. There is the excitement of the occasional mid-latitude depression or summer thunderstorm, but without the devastation of hurricanes and monster tornadoes.

Meteorological variety without the jeopardy. If you have to be a parochial forecaster, the UK isn't such a bad place to be.

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(Jason Samenow is the weather editor. He founded Capitalweather.com in early 2004, the first weather blog on the web which was absorbed by the Post in 2008. He can be reached via )

Distance travelled ~ 927'172'800 km

A strong case can be made that 2011 was the most extreme weather year on record in the U.S. In addition to the record of at least 12 weather events that produced more than (totaling more than ), never has a larger percent of the country dealt with either extreme drought or abnormally heavy precipitation.

The U.S. contended with virtually every kind of weather hazard including mega snowstorms in the Midwest and Northeast, historic flooding of the Mississippi and Missouri rivers, devastating wind and flood damage from tropical weather systems (Irene and Lee) in the East, and one of the worst spring tornado seasons in memory. The tornado outbreaks that ravaged the central and southern U.S. between April and June resulted in more than 500 deaths, tied second most on record. Several exceptionally strong tornadoes struck densely populated areas including Birmingham and Tuscaloosa in Alabama as well as Joplin, Missouri.

Perhaps the most notable weather to afflict the U.S. was the devastating combination of extreme heat, drought, and wildfires in the South Central U.S. Texas was particularly hard hit. Exceptional drought gripped almost the entire state and groundwater, lake, and reservoir dropped to historic lows. The state suffered its worst wildfire season, with more than 4 million acres burned. In July, neighboring Oklahoma's average temperature was the hottest of any state in 130 years of U.S. weather records, a searing 88.9 degrees.

Undoubtedly, the moderate La Nina pattern set the stage for the unusually volatile weather conditions across the U.S. It helped fuel the powerful jet stream slicing through the middle of the country, bringing the onslaught of stormy weather. But to the south and southwest of that jet stream, a stifling heat dome blossomed and the moisture abruptly shutoff leading to historic drought.

Although global warming should not be blamed as the root cause of this punishing set of weather conditions, it very likely amplified the sharp contrasts in this pattern. The added heat in the atmosphere presumably juiced up the wet extremes by making more water vapor available, while speeding up evaporation and drying in drought areas.

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Distance travelled ~ 924'600'000 km

23 Degrees is coming to an end, but what a journey it has been and what a year.

Coming into this project we didn't have any expectations, well I didn't anyway. I think we all appreciate that it's hard to predict the weather. The most extreme tornadoes in the US mid West, huge cyclones like Yasi hitting Australia, record snows in the US and the UK. Our mission when we set out on 3rd january 2011 (the day of Perihelion) was to tell the story of Earth's annual journey around the Sun.

So what has this year's weather shown us?. That's a really difficult question.

Making 23 Degrees has had a huge impact on me, I have learnt how our climate and our weather is all generated by our orbit around the Sun. It's all interconnected. It really does feel like a huge single organism where everything is linked to create our extraordinary world. I have always liked James Lovelock's idea of Gaia, the Earth as a single organism and seeing how land , sea and atmosphere all interact powered by the Sun has made me want to look into Gaia more.

It has been a real privilege to work on this series and get a glimpse of our incredible planet's annual journey. I just hope we humans step closer to more action in limiting the affects we have on our climate.

It would be great to hear which weather events of 2011 stand out for you?

(Keep updated - The series will transmit early 2012)

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Distance travelled ~ 919'776'000 km

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Ever wondered why the Moon seems to look different at varying times of the month and sometimes, like today, seems to have totally vanished?

(SORRY SANTA NO FULL MOON TONIGHT...)

(Image courtesy of Dry Icons - )

These are questions that perplexed mankind for centuries but the answer is actually not all that complicated.

Image credit: US Naval Observatory/Astronomical Applications Department.

The first thing to understand is that we see the Moon because it reflects sunlight; turn the Sun off and the Moon would to all intents disappear from view.

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While the Earth is spinning and orbiting around the Sun, the Moon is orbiting around the Earth, completing one orbit in 27.3 days. Its actually more accurate to say that the Moon AND Earth orbit a common centre of gravity called the barycentre which lies inside the Earth but not at its centre. Because the Moon orbits the Earth, and the Earth orbits the Sun its easy to see that the actual angle between the three objects varies throughout the lunar orbit and its this variation that leads to the 'appearance' of the phases of the Moon.

At the start of this blog, I stated that we see the Moon because it reflects sunlight. If the Moon lies opposite the Sun in the sky then we see the fully illuminated portion of the Moon and see a full Moon. If on the other hand, the Moon is between us and the Sun then we see the non-illuminated portion and see a new Moon. Then at various points between we see a varying amount of dark and light portions as the phases change from full to new and back again. which means its in line with the Sun and can't easily be seen without sophisticated equipment.

You might expect that during either a full or new Moon, we should experience a lunar or solar eclipse every month (the Moon blocks sunlight reaching Earth during a solar eclipse and the Earth blocks sunlight reaching the Moon during lunar eclipses) but it turns out that the orbit of the Moon is tilted by about 5 degrees to the orbit of the Earth around the Sun. On most occasions at full or new Moon, the Moon is either just above or just below the Sun or shadow cast by the Earth, making eclipses a little more rare.

Interestingly if you measure the time it takes from one full Moon to the next it takes 29.5 days instead of 27.3! This strange effect is seen because the Earth is independently orbiting the Sun and the Moon has to travel a little further to get back to exactly the same angle as the previous full Moon.

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Distance travelled ~ 917'792'800 km

Who would have thought we'd be closing the submissions for the Flickr group so soon. It's only been a year, right? But that's what it's always been about. Our annual round trip. And sad to say, but 2012 is looking mighty close.

To all who have submitted photos to the pool, the team say a huge thank you! Over two thousand weather and astronomy images have been submitted by a selection of great photographers - photos that will continue to be available to view long after the production ends.

The date you want to remember for the photography pool is 31st December 2011 - as if you'd forget new years eve :-). Submissions after this date will not be able to be made.

I'll leave you with images of the Aurora Australis, acquired by astronauts on board the International Space Station September 11, 2011 as the ISS orbit pass descended over eastern Australia. Magnificent.

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Wishing you all a Merry xmas!

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Image credit Stu.bloggs/Flickr

Distance travelled ~ 913'772'800 km

Today sees the shortest day in the northern hemisphere and for southern hemisphere dwellers, the longest day! A peculiar idea to get your head round perhaps but it's all actually pretty straightforward. The cause of the changing length of the 'day' (and by this I mean the hours of sunlight, not a 24 hour period) is the same thing that causes the changing of the seasons and we can look to the orbit of the Earth around the Sun for the answer.

Surprisingly perhaps, its not the Earth's distance from the Sun which brings the colder months to the northern hemisphere in fact, the Earth is closer to the Sun during December than it is in July. We need to look at the axis of the Earth's rotation to understand why we experience seasons and changing quantities of sunlight.

The Earth spins once on its axis every day, to be precise once in 23hours, 56minutes and 4seconds which is why stars seem to rise about 4 minutes earlier every day. The axis that the Earth spins around is tilted with respect to its orbital plane around the Sun by just over 23 degrees. During a northern winter, the northern hemisphere is pointing away from the Sun and during the northern summer it's pointing toward it. The opposite is true for the southern hemisphere. This all means that during the northern hemisphere winter, there are less hours when the Sun is above the horizon, indeed around the north pole, there are 6 months of it while the southern pole is basking in 6 months of sunlight.

The exact amount of sunlight we receive at any place on Earth is determined by its latitude. The further away from the equator, the less light is experienced during winter. Even small differences in latitude make a big difference to number of daylight hours for example, London will today experience just 7 hours 49 minutes of sunlight whereas the north of Scotland will experience 6 hours 35 minutes, over an hour of difference. What this means astronomically is that the Sun reaches its southernmost point in the sky today which we call the Winter Solstice, more precisely at 05:30 GMT (or UT - Universal Time). This corresponds to the Sun being overhead at local noon at the Tropic of Capricorn in the Southern Hemisphere.

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Distance travelled ~ 876'413'600 km

The depression is expected to bring gales and heavy rain from the west for Thursday.

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Snow is likely for a time on Thursday morning across parts of central and northern Scotland, particularly affecting higher ground, but with some slushy deposits locally to lower levels, and all parts of the UK will have heavy rain for a time during the day.

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But the main feature of this depression is the gale to storm force winds it will bring. Gusts of 60 to 70 mph likely to become quite widespread across northern and central Britain, but 70 to 80 mph are expected across much of Scotland with 90mph gusts are possible in exposed places, particularly central and southern Scotland. Elsewhere across England and Wales the wind will gusts of 50-60 mph. ()

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As the storm passes through and moves away to the east of the UK on Friday north-westerly winds will bring in cold air which will see a drop in temperatures. This is expected to bring snow to northern and western parts of the UK, giving accumulations to low levels in the north. Accumulations will tend to be confined to higher ground across north Wales and much of northwest England. Untreated surfaces will also become icy at times. Other parts of the country will see a cold night on Friday with widespread frost.

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The forecast for the rest of the month is for the unsettled weather to continue, with spells of wet and windy weather interspersed with brighter, colder periods when we can expect to see frost and snow showers - the heaviest of any snowfalls are expected across higher ground from North Wales northwards, but we could occasionally see some snow cover at lower levels as well.

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Distance travelled ~ 871'482'400 km

Following the news that this , last week saw a return to more normal conditions for the time of year.

It was a very windy, rainy week with heavy rain and gales every day somewhere in the UK. Tuesday saw gusts of up to 76 knots and 65mm of rain in 24 hours in Glasgow, causing major flooding and the closure of some roads. There were also reports of tornadoes in Stockport. On Wednesday the wind and rain continued with a gust of 97 knots recorded in Cairngorm and surface water flooding in Cornwall.

Temperatures were closer to normal than of late, dipping to a low of - 5.4 听掳C at Altnahara on Friday night. Showers turned wintry on higher ground throughout the week in Scotland, and parts of Northern England and Northern Ireland. On Sunday, several inches of snow fell on higher routes in Scotland, and there was a covering of snow as far south as Edinburgh.

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