Precession of the Equinoxes

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?

In article 36, 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?



precession means a varying axial tilt - hence difference in severity of
seasons for temperate latitudes?

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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

Bill Owen wrote in
:

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

I thought the matter through a little more. Which particular stars the
poles point at has no effect on climate, but if the longitude of the
perihelion revolves around the sun, eventually we will come to a
situation where the southern rather than the northern summer will be
slightly longer.Because of the unbalanced distribution of land and water
areas, this would have an effect on climate, but it is something quite
different from the precession with which Flannery seems to confuse it.

Query: What is the period of revolution for the line of apsides (the line
connecting the perihelion and aphelion)?

In article 36,
AK47 writes:
Which particular stars the
poles point at has no effect on climate, but if the longitude of the
perihelion revolves around the sun, eventually we will come to a
situation where the southern rather than the northern summer will be
slightly longer.Because of the unbalanced distribution of land and water
areas, this would have an effect on climate, but it is something quite
different from the precession with which Flannery seems to confuse it.

Query: What is the period of revolution for the line of apsides (the line
connecting the perihelion and aphelion)?

If you measure with respect to the fixed stars, it's very long
(millions of years, I think, but this is just a guess). However
what's relevant to climate is the *longitude* of perihelion, i.e.,
its location relative to the equinox. That changes very close to the
precession rate.

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
valid Reply-To address to receive an acknowledgement. Commercial
email may be sent to your ISP.)

Steve Willner wrote:
In article 36,
AK47 writes:

Which particular stars the
poles point at has no effect on climate, but if the longitude of the
perihelion revolves around the sun, eventually we will come to a
situation where the southern rather than the northern summer will be
slightly longer.Because of the unbalanced distribution of land and water
areas, this would have an effect on climate, but it is something quite
different from the precession with which Flannery seems to confuse it.

Query: What is the period of revolution for the line of apsides (the line
connecting the perihelion and aphelion)?


If you measure with respect to the fixed stars, it's very long
(millions of years, I think, but this is just a guess). However
what's relevant to climate is the *longitude* of perihelion, i.e.,
its location relative to the equinox. That changes very close to the
precession rate.

Good guess, Steve. A polynomial expansion for the rectangular
components h and k of the earth's eccentricity (J. Laskar, 1990 or
thereabouts) indicates that the longitude of perihelion is currently
advancing at the rate of 0.3225+ degree per century. At the present
rate, the period of revloution for the line of apsides would be
1.116 million years.

This is much longer than the precessional period of 20-odd thousand
years (which itself is somewhat variable). So the "synodic period",
if you will, the time it takes for perihelion to line up with the
solstices, isn't that much shorter than the precessional period itself.

The original poster questioned why (according to one source) there
should be ice ages when the north pole is pointed at Vega but not when
it's pointed at Polaris. My first reply was that one had to account
for the motion of perihelion as well as the motion of the pole. That's
still true, but for a few tens of thousands of years the former won't
make that much difference.

-- Bill Owen