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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.

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.

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.

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:

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!

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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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(Peter Gibbs is a Ö÷²¥´óÐã weather forecaster and appears as an expert meteorologist on "The Weather Show" for the Ö÷²¥´óÐã 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 - )

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 Ö÷²¥´óÐã 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,

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 ~ 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...)

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

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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