Precession of Equinoxes

[gaa_ian (Ian)]

An interesting question has been posed to our club (by mail!) from a budding astronomer.
It is this:
"Given that the Precession of the equinoxes goes through 1 deg, of its 360 deg Arc of 25,920yrs each 72 years, does this mean that the start of summer comes earlier or later each year ?"
Any ideas ?

[[1ponders] (Paul)]

Should be fairly easy to work out Ian. The summer solstice used to be when the sun was in Cancer (northern hemispher speaking). It is no longer in cancer. What constellation is it in. Gemini or Leo?

You could do the same thing with the equinoxes. It used to be in Aries whereas now it is in Pisces I believer. So by that reaconing summer should be getting ealier.

Interesting exercise but in fact it doesn't matter as far as clocks and calendars are concerned, because we (well the powers that be) add a second (a leap second) every few years to the year (after summer) which pushes the next summer back a bit.

So if you want to estimate it by the stars it's getting earlier. But if you want to estimate it by clock it stays the same because of added seconds.

At least that's my story and I'm sticking to it.

[gaa_ian (Ian)]

Thanks Paul, I was forgetting about the added seconds. Its reassuring to know there is someone out there looking afer time for us !
Given this "wobble" on the earths axis of rotation, which must change the angle the sun strikes the earths surface.
I wonder how this effects the cycle of climate on earth ?
I must admit my understanding of the finer details of orbital mechanics is rudimentary

[Volans]

Originally Posted by gaa_ian

Given this "wobble" on the earths axis of rotation, which must change the angle the sun strikes the earths surface.
I wonder how this effects the cycle of climate on earth ?

G'day Ian,

The Earth, as you know is tilted by 23.5deg. from the vertical. This angle can in fact change, anywhere between 21.5 and 24.5 deg. over a 41,000 year period. The main wobble in the Earth's rotation is called precession and this alters the direction of the Earth's tilt...not the angle of the tilt.

So you are correct in saying the angle of sunlight hitting the Earth effects climate but this angle will not change due to precession, so the climate will not change due to precession.

Peter.

[[1ponders] (Paul)]

Originally Posted by Volans

G'day Ian,

The Earth, as you know is tilted by 23.5deg. from the vertical. This angle can in fact change, anywhere between 21.5 and 24.5 deg. over a 41,000 year period. The main wobble in the Earth's rotation is called precession and this alters the direction of the Earth's tilt...not the angle of the tilt.

So you are correct in saying the angle of sunlight hitting the Earth effects climate but this angle will not change due to precession, so the climate will not change due to precession.

Peter.

Hi Peter.

While I agree that our climate as such will not change due to precession (I'll ignore other facts that are currently being debated in another thread), the timing of the seasons will change. If for example we look at the northern Autumnal Equinox, the Axial tilt of the earth is parallel to the direction of the earths orbit, with the north polar region leaning towards the direction of travel. This will lead in three months time to the northern Winter with the northern hemisphere leaning away from the sun.

Now in 6,500 years the earth will have progressed through one quarter of a full procession. Looking down from the north celestial pole the earth appear to precess in clockwise direction. This will mean that at the corresonding time of the year as we experience a northern Autumnal Equinox our future selves will experience a Northern Winter Solstice and our current Northern Winter will become a Spring Equinox

[Volans]

G'day Paul,

I could not have put it better myself! I never doubted that the solstial or equinotical points would alter due to precession and it matters not where the Earth is in its orbit as to when the seasons fall, I think we will all agree that climatically, Summer will be stinking hot and Winter will freeze spherical objects off an alloy of copper and zinc shaped into a monkey.

And all I can add to this is: If I am still around to witness a significant change due to precession then I will be writing a VERY stern letter to the management. I will not be best pleased and I will blame the government for not doing enough about it.

Peter.