Hi! I'm doing research for a possible sci-fi novel and need to check something. Aren't our seasons caused mainly by the fact Earth's axis is tilted and not parallel to that of the sun? I realize Earth is at varying distances from the sun as it orbits the sun, but it seems the axis tilt appears to be the main reason for seasonal changes. Is this a correct assumption? Thanks for any info!!!

That's correct.

The Earth always points at an angle of 23-degrees towards Polaris (aka the North Star) as it completes an orbit around the Sun. The Earth gets closer and further from the Sun during its orbit, but that doesn't really account for the temperature changes.

The reason we have seasons is because during the Summer, the Northern Hemisphere receives the sun's rays coming straight through the atmosphere. During Winter, the pole is pointed away from the Sun, so the light comes in at an angle.

One experiment you can do is take a flashlight and shine it straight at the wall. You'll get a nice circle of light. Then hold the flashlight near the wall and point it at a steep angle so it forms a big oval of light. The same amount of light is coming out of the flashlight, but it's taking up a much larger surface area. In other words, each chunk of wall is receiving less energy.

If you're writing a science fiction novel with some astronomy in it, I highly recommend you read Bad Astronomy, by Phil Plait. It covers that, and many other topics, and focuses on how the media (tv, books, news, etc) always screws it up.

__________________
Fraser Cain
Publisher
Universe Today - Free space news delivered by email every weekday.

Why then isn't one hemisphere's summer much hotter (or conversely much colder in winter) given that the Earth is at perhelion during one hemisphere's summer and at aphelion for the other?? Is it to do with disproportionate surface areas of the oceans? Or airstream currents? Or something else entirely??? please and thank you.

__________________
1·618033988749894848204586834365638117720309179805 76286213544862270526046281890244970720720418939113 74

The Earth's orbital eccentricity is rather small, so Earth's varying distance from the Sun doesn't much affect the seasons.

Earth is CLOSEST to the Sun in January and FARTHEST in July. Logically, therefore, you'd think that the southern seasons would be more extreme. In fact, however, the abundance of water in the southern hemisphere moderates the southern seasons so that they are actually milder than their northern counterparts.

I would like to add that an important point to remember is that sunlight reaches our earth because light travels in a straight beam, therefore this also is contributing factor for the reasons of the seasons. Proof of direct sunlight hitting the surface of the Earth can be seen; for if you look at the ground where beams of sunlight are shining through trees you can see oval shapes of sunlight. This proves that sunlight travels in a straight line which helps give our seasons as the Earth rotates at a tilt on its axis on its yearly trex around the sun.

Why do oval shapes prove that light moves in a straight line? :unsure:

Dips

__________________
"The stars are my home"
"I've seen things you people wouldn't believe... Attack ships on fire off the shoulder of Orion... I've watched c-beams glitter in the dark, near the Tanhauser Gate... all those moments will be lost, in time... like tears in the rain..."

Its Correct.
Thats why, even though earth is near to Sun in January, it is winter in nothern hemisphere and in July when earth is (little) away from Sun, it is summer.
But on the other hand, nearness with tilt effect makes summer hotter in January in southern hemisphere and distance with same time tilt effect make winter nore severe in July in southern hemisphere.

what is the latest in the annual sunspot cycle influencing climate on Terra? With the highs in the cycle corresponding to heat in the northern hemispher, and dips corresponging to the southern hemisphere? Anyone know?

No, that last bit is not true. The abundance of water in the southern hemisphere makes southern seasons milder than their northern counterparts.

If you look at a globe map, you can see that the axis does not go straight up and down, the globe is tilted. If you rotate the stand, the tilt will rotate also. This changes where north is pointed.

That is what happens to earth. That tilt rotates in a 26,000-year cycle. So the North Star keeps changing. In about 22,000 years Thuban will be the North Star again.

I started to think about whether the seasons make a 26,000-year cycle also. It is the tilt of the earth that gives us our seasons. So the rotating the tilt should make the seasons rotate.

Any ideas or data on this?

Bud

This is called the Earth's procession.

I'm pretty sure that's correct. Over the course of 13,000 years, the seasons will switch around. December-February will be the hot months in the Northern hemisphere. It doesn't make much of a difference in our lifetime, but it would make a big difference over thousands of years. I wonder if archeologists take it into account when they say that the pyramids, etc are pointed directly at various stars and constellations. Not only that, but the stars are moving in sky and would look very different thousands of years ago.

__________________
Fraser Cain
Publisher
Universe Today - Free space news delivered by email every weekday.

Thanks for plugging the book, Fraser, but I have a (free) webpage answering this question, too. :P

__________________
Phil Plait
The Bad Astronomer
http://www.badastronomy.com
badastro@badastronomy.com

Yeah, but if people buy the book, it helps pay your rent. But, uh, check out the free web page too. :-)

__________________
Fraser Cain
Publisher
Universe Today - Free space news delivered by email every weekday.

Wow. That's very interesting. I learn a lot of useful facts on this web site.

That is interesting. I wonder if that's what causes all those ice ages and such?

Locke

__________________
"...and remember, the enemy gate is down."

Not that I'm aware of. One of the disturbing things about ice ages is that they seem to happen really quickly. The Earth can become extremely cold in a matter of decades and then stay that way for thousands of years. So, the procession probably isn't causing it.

__________________
Fraser Cain
Publisher
Universe Today - Free space news delivered by email every weekday.

Actually, the modern calendar prevents this switcheroo from happening. Northern winter solstice will always be in December, northern summer solstice will always be in June, etc.

However, previous calendars did have a drift. It got so bad that in 1582 many parts of Europe deleted ten days from October, so that October 4, 1582, was followed immediately by October 15, 1582. See the chapter "Ten Days Lost Forever" in the book Calendar by David Ewing Duncan. Thus, if someone tells you he was born on October 10, 1582, you should be very suspicious!

Ahah, so the modern calendar is shifting to compensate for procession too? Wow, I had no idea.

__________________
Fraser Cain
Publisher
Universe Today - Free space news delivered by email every weekday.

I don't honestly know if the modern calendar compensates for precession... but I do know anyone claiming to be born on October 10 1582 would, of course, be a liar because that would make them nearly 421 years old... :huh:

__________________
"The stars are my home"
"I've seen things you people wouldn't believe... Attack ships on fire off the shoulder of Orion... I've watched c-beams glitter in the dark, near the Tanhauser Gate... all those moments will be lost, in time... like tears in the rain..."

Fraser;

Even the Egyptians realized it, because the orientation of the pyramids changed enough in a few hundred years that they could see something was happening. They figured out for themselves that it was precession. Pretty sharp those Egyptians!

I believe the 10 days lost was when most of the western world changed to the Gregorian calander, and had nothing to do with precession.

Bud

I don't think this is correct. The procession varies the point in the orbit where perihelion and aphelion occur. In 13,000 years, the earth will be closer to the sun during the northern hemisphere's summer and further in the northern hemisphere's winter intead of the other way around like it is right now. As others have said, the effect on our weather of the distance between the earth and the sun is dominated by the effect of the angle of the axis of the earth's rotation relative to the axis of the sun. So, it will still be summer in June in the north, although theoretically it would be a slightly hotter one.