So it snowed yesterday in Vancouver, it's March. I know the cause has nothing to do with what I'm about to ask but it did cause me to ask this question(which I searched for and found wobble, not quite!)

 

Anyway, I'm assuming that the axis of the Earth with respect to it's position around the Sun does change over time. Where over x amount of time the Summers in the Northern/Southern Hemispheres become Winters and vice versa. Over what period of time does this occur?

So it snowed yesterday in Vancouver, it's March. I know the cause has nothing to do with what I'm about to ask but it did cause me to ask this question(which I searched for and found wobble, not quite!)

 

Anyway, I'm assuming that the axis of the Earth with respect to it's position around the Sun does change over time. Where over x amount of time the Summers in the Northern/Southern Hemispheres become Winters and vice versa. Over what period of time does this occur?

 

If I remember right the full cycle takes 26,000 years.

 

Hmmm. I just checked Wikipedia and it says 25,771.5 years, so 26 thousand is about right.

http://en.wikipedia.org/wiki/Precession_(astronomy)

 

So half of one cycle would be about 13,000 years. That would change our northern hemisphere seasons by 6 months. The winter solstice which is now around 21-22 December would then be around 21-22 June, give or take a day or two.

Edited March 17, 200916 yr by Martin

By far the largest effect is the 26,000 year lunisolar precession, which is somewhat analogous to the precession of a spinning top:

The angle between the Earth's rotation axis and the Earth's orbital plane remains roughly constant (about 23.5 degrees or so), but the orientation of the rotation axis rotates.

 

There are lots of motions, collectively called nutation, on top of that big, slow precession.

 

The largest of these results from the 18.6 year lunar orbital precession. Others include the Chandler wobble (about 433 days). After accounting for all the effects that can be explained there remains motion that we cannot yet explain with some causal model. (We don't know enough about the interior of the Earth.) These unmodeled "wobbles" are empirically called polar motion and are determined after-the-fact.

 

For more info, see the International Earth Rotation and Reference Systems Service (IERS) at http://www.iers.org or the US Naval Observatory at http://www.usno.navy.mil.