What causes the seasons?

mycotheology

I figured out the answer to the question in the title. The seasons are due to the earths north to south axis facing towards or away from the sun at different times in the year. I have a couple of other questions:

1.) What does a week signify? A day is how long it takes for the earth to make a full rotation, a month is the time it takes for the moon to orbit the earth and a year, the time it takes for the earth to orbit the sun. What is a week?

2.) Does the earth have a circular or eliptical orbit around the sun?

Bandersnatch

It bears no physical meaning as far as I know. It's just a cultural artifact probably stemming from people giving mystical significance to the number 7. There used to be cultures with different number of days in a week(like the Romans, who before switching to seven, had eight days in a week).
I suppose one could imagine that it's got something to do with the phases of the Moon, them being four, each taking about seven days. Better ask a cultural anthropoligst, I suppose.

It's slightly elliptic. The difference between the point in orbit closest to the sun(aka perihelion, from Greek meaning lit 'near the sun') and the one farthest from it(aphelion) is about 5 million kilometres. It's not that much when compared to the average radius of the orbit, ~150 million kilometres.
Note that the Earth is in perihelion around the first week of January, so it's closest to the sun when it's the coldest time of the year in the northern hemisphere.

mycotheology

Thanks. Does that 5 million km difference make much difference to the temperature of the earth? I reckon it would. Venus, from what I hear, is scorching hot compared to the earth. Not sure how far venus is from earth, my guess (based on the radius you told me) would be 40 million km.

Bandersnatch

You can calculate that!
The sun gives off a certain amount of radiation, let's call it I₀.
How much of it reaches a planet depends on how far away it is. According to the inverse square law, the irradiation will fall with the square of the distance. Two times farther is four times less sunlight, and so on.

(subscripts "a" shall mean "aphelion", "p" = "perihelion")

I_p=I₀/R_p²
I_a=I₀/R_a²

The distance at aphelion is 1.034 times the distance at perihelion, or:

R_a=1.034 * R_p

gives:

I_p=I₀/R_p²
I_a=I₀/(1.034 * R_p)²

Let's divide both equations by each other.

I_p/I_a = 1.034²

So, the irradiation at perihelion is

I_p=1.069156 * I_a

or 7% more received energy than in its farthest point in orbit. Not that much actually.

Try doing the same calculation for Venus and Earth at their average distances, and you'll see that at 0.7 the distance, Venus receives twice as much energy per unit area as Earth does.


It's actually close to 108 million(Venus' orbit even more circular by the way, with barely 1,5 million kilometres difference between aphelion and perihelion.
Wikipedia has all such data handily presented. Type planet name and look at that table on the right.
You'll find peri- and aphelion there, eccentricity(how close to a circle an orbit is, with 0 meaning ideal circle), semi-major axis(for near-circular orbits roughly corresponds to the radius of a circle) and more.

D H

Not really. That the Earth's orbit has an eccentricity of 0.0167 means that insolation varies by about 3.34% over the course of a year. This is a small effect, much much smaller than that caused by obliquity. Look at it this way: It's winter in the northern hemisphere right now even though we're fairly close to perihelion (closest approach to the Sun). Perihelion occurs only a week or two after the northern hemisphere winter solstice. We're closest to the Sun right when winter in the northern hemisphere reaches its peak*.

The opposite occurs in the southern hemisphere. Winter in the southern hemisphere occurs when the Earth is furthest from the Sun and summer when the Earth is closest to the Sun. This would seem to suggest that seasons in the southern hemisphere should be more extreme than those in the northern hemisphere. That's not the case. It's the other way around. There's yet another contributing factor to the variations of the seasons. About 80% of the southern hemisphere is covered by ocean, but it's only about 60% for the northern hemisphere. The contribution of this uneven distribution to climate is stronger by about a factor of two than is insolation.


* Right now, that is. This near coincidence of the December Solstice and perihelion is just a coincidence. It will be the other way around 10,000 years from now.

lpetrich

Week - Wikipedia, the free encyclopedia goes into gory detail about the different week lengths that various societies have had: 3, 4, 5, 6, 7, 8, 9, 10 days. So it's essentially artificial.

nitsuj

nominal

jim mcnamara

With respect to the elliptical orbit and climate (insolation), google for Milankovitch cycles.
This has a very detailed explanation along with some problems associated with this approach.
Or try:

http://en.wikipedia.org/wiki/Milankovitch_cycles