http://www.msnbc.msn.com/id/3077384/

What causes Earth’s seasons?
Our planet’s tilt dictates cycle of autumn and spring

The seasons are a powerful force in our lives. They affect the activities we do, the foods we crave, the clothes we wear — and quite often, the moods we are in. The seasons officially change once again Monday, with autumn beginning in the Northern Hemisphere and spring starting in the south.

What is it that causes the change in seasons?

The ability to predict the seasons — by tracking the rising and setting points of the sun throughout the year — was key to survival in ancient times. Babylonians, the Maya and other cultures developed complex systems for monitoring seasonal shifts. But it took centuries more to unravel the science behind the seasons.

Nicolai Copernicus (1473-1543) radically changed our understanding of astronomy when he proposed that the sun, not Earth, was the center of the solar system. This led to our modern understanding of the relationship between the sun and Earth.

We now know that Earth orbits the sun elliptically and, at the same time, spins on an axis that is tilted relative to its plane of orbit. This means that different hemispheres are exposed to different amounts of sunlight throughout the year. Because the sun is our source of light, energy and heat, the changing intensity and concentration of its rays give rise to the seasons of winter, spring, summer and fall.

Solstices and equinoxes
The seasons are marked by solstices and equinoxes — astronomical terms that relate to Earth’s tilt.

The solstices mark the points at which the poles are tilted at their maximum toward or away from the sun. This is when the difference between the daylight hours and the nighttime hours is most acute. The solstices occur each year on June 20 or 21 and Dec. 21 or 22, and represent the official start of the summer and winter seasons.

The vernal equinox and autumnal equinox herald the beginning of spring and fall, respectively. At these times of the year, the sun appears to be directly over Earth’s equator, and the lengths of the day and the night are equal over most of the planet.

On March 20 or 21 of each year, the Northern Hemisphere is reaching the vernal equinox and enjoying the signs of spring. At the same time, the winds are turning cold in the Southern Hemisphere as the autumnal equinox sets in.

The year's other equinox occurs on Sept. 22 or 23, when summer fades to fall in the north, and winter’s chill starts giving way to spring in the south.

From year to year, there is always some variability in the equinoxes and solstices because of the way Earth's changing tilt matches up with its orbit around the sun. This year, the precise moment of the September equinox comes at 11:44 a.m. ET Monday.

Effect on climate
Here’s how the seasonal change affects the weather: Around the time of the June solstice, the North Pole is tilted toward the sun and the Northern Hemisphere is starting to enjoy summer. The density of the solar radiation is higher because it's coming from directly overhead — in other words, the sun's rays are concentrated over a smaller surface area. The days are longer, too, meaning that more radiation is absorbed in northern climes during the 24-hour cycle. Another factor that may come into play is that the radiation takes a somewhat shorter path through the energy-absorbing atmosphere before striking the earth.

At the same time that the Northern Hemisphere is entering summer, the South Pole is tilted away from the sun, and the Southern Hemisphere is starting to feel the cold of winter. The sun’s glancing rays are spread over a greater surface area and must travel through more of the atmosphere before reaching the earth. There are also fewer hours of daylight in a 24-hour period.

The situations are reversed in December, when it’s the Southern Hemisphere that basks in the most direct rays of the sun, while the Northern Hemisphere receives less dense solar radiation for shorter periods of time.

Although the solstices represent the pinnacles of summer and winter with respect to the intensity of the sun’s rays, they do not represent the warmest or coldest days. This is because temperature depends not only on the amount of heat the atmosphere receives from the sun, but also on the amount of heat it loses due to the absorption of this heat by the ground and ocean. It is not until the ground and oceans absorb enough heat to reach equilibrium with the temperature of the atmosphere that we feel the coldest days of winter or hottest days of summer.

My favourite season is officially upon us

just got done with almost a 2 hr phone call - plans for tonights ritual

Awwwww yesssss one of my fave seasons

XxAxX said:

It's like someone trying to say Santa Clause isn't real

Steadwood said:

santa IS the sun well, actually he pulls the sun into the sky with his sled. which is why the wintertime is so dark, he only works part time since he has to make all those toys for christmas

XxAxX said:

He makes ALL those toys on his own???

He started early this year... we've had bugger all Sun this year

Steadwood said:

yeah. santa's gettin' to be a bit of a flight hazard up there. it really wasn't so bad back before all the satellites and planes and ultra lights and helicopters and other types of aerial debris began cluttering up the airspace. santy had the skies all to himself in the good old days.

but NOW santy not only has to tug the sun around the earth each day behind his sled but he must also steer the reindeer and make the toys, all while in transit. he must be at terrible risk for deep vein thrombosis, not to mention hypertension and artereosclerosis.

all of which is to say that, i sure don't want to be around when old santy finally blows a gasket. it'll be a big, bad day. no presents, no sunshine and a crazy old man circling the globe with blood in his eye.
[Edited 9/22/08 13:53pm]

XxAxX said:

Santa is George Bush!!!???!!!

Steadwood said:

usually, except when he calls in sick cheney has to take over

XxAxX said:

Steadwood said:

well it has to be him. can you picture condoleeza in a santa suit? can you???!!

XxAxX said:

that's interesting dear thank you for sharing

XxAxX said:

That would be... er.... interesting

BlackAdder7 said:

aw shucks

Steadwood said:

i see

Finally spring here in Australia!

XxAxX said:

Oh... I see

Steadwood said:

well, anyhoo. now that it is autumn we will all have to grow beards and hibernate.

XxAxX said:

Yes ma'am... That'll be quite a spectacle

Steadwood said:

now let's not talk trash about your mum. just because she was bald in places she oughtn't to have been, and hairy in parts where she shouldn't be, well, no one is perfect after all.

XxAxX said:

You knew my Mum???

Steadwood said:

son

XxAxX said:

MUMMY!!!

Steadwood said:

XxAxX said:

edit
[Edited 9/23/08 12:51pm]