Re: Precession of the Equinoxes

AK47 wrote:
Quotation from "The Weather Makers: How Man is Changing the Climate and What it Means for Life on Earth" by Tim Flannery (Melbourne, Australia: 2005)
"The …cycle which runs its course every 22,000 [sic] years, concerns the wobble of the Earth on its axis. During the course of this cycle, Earth's axis shifts from pointing at the Pole Star [sic] to pointing at Vega. This affects the intensity of the seasons. When Vega marks true north, winters can be bitterly cold and summers scorchingly hot."
I cannot understand this. I can see how two other long-term cycles of the earth's motion (the axial tilt and the orbital eccentricity) can affect climate, but I fail to see the effect of precession.
Can anyone explain?

I *think* what he's trying to say is this:

Currently, the earth is closest to the sun in early January and farthest
from the sun in early July. This fortunate circumstance means that
winters in the Northern Hemisphere are slightly milder than otherwise,
and summers in the Norther Hemisphere are slightly cooler.

Half a precessional cycle later, things would presumably be reversed,
and we'd be farthest from the sun in northern winter.

Note, of course, that the Southern Hemisphere is currently experiencing
winter at aphelion and summer at perihelion. There's more ice in
Antarctica than in the north polar regions, and I'm told that there
would be even more were it not for all the water moderating things.
Climate swings are more severe in the north.

But there's a little more to the analysis than just the direction of the
north pole. The longitude of perihelion also changes over time. Just
because the north pole will have gone through half a cycle doesn't mean
that perihelion will be in July.

-- Bill Owen
.

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