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Re: North Pole



I think John Denker and I are mostly using the same language, but not totally. I think his numbering of the concepts is what I was
doing, but I didn't number them.

There is spin angular momentum for each of the earth, sun, moon, and every other planet, moon, rock in the solar system.

There is orbital angular momentum of anything that is orbiting something else.

We can also talk about the total angular momentum of any grouping of objects, including the whole solar system.

Are all of these precessing? Just some of them?

When I began talking about the earth's orbital angular momentum, John said,

The Subject: line speaks of the north pole, which is
related to concept #1. But we can segue to concept #2
now if you want....

But the subject line is miswritten; it does not represent the original question. The original question asked by Jim Green on
Tuesday 10/1/02 was... "Does the orbital axis precess?" Maybe that's not what he meant, but those are the words in the original
posting.

When I began talking about the solar system angular momentum, John said,

Yes, but that's almost trivial to observe. Just watch the motion
of the sun across the sky and keep track of its position relative
to the fixed stars. This defines the ecliptic. Note that the
ecliptic (concept #2) is defined without reference to the north
pole (concept #1).

I don't see this. The fact the ecliptic is tilted with respect to the celestial equator tell us that the earth's orbital momentum
vector is not parallel to the earth's spin momentum vector. I don't see how it shows us anything about the orienation of the vector
that represents the total solar system angular momentum.

Sidebar... I think astronomers use the words "north pole" with respect to anything with angular momentum. The north pole aligns
with the angular momentum vector and the south pole is oppositely directed. The sun has a north pole, the solar system as a north
pole, the milky way has a north pole, etc. It is when we view the solar system from a viewpoint north of the solar system that we
say the planets orbit counterclockwise.

John said,

It is observed that the precession of the ecliptic (#2) is
very slow compared to the precession of the equator (#1).

I don't think so. These the same thing and are not refering to (#1-spin) and (#2-orbit). They are both refering to (#1-spin). The
celestial north pole and earth's north pole are aligned by definition. The ecliptic and celestial equator are circles drawn on the
celestial sphere. Because these poles (a direction in space) precess with a 26,000 year period, anything drawn on the celestial
sphere will show effects of precession (i.e. cycle it's position on the sphere over a 26,000 year period). This includes the
celestial equator and the ecliptic.

Suppose the celestial pole precesses with a 26,000 year period but the ecliptic does not cycle its position on the sphere with this
same period; e.g. suppose its apparent position remains fixed on the sphere. This would mean the apparent precession of our
celestial pole is not caused by precession of our spin axis, but rather because of precession of the orbital axis. To the best of
my knowledge that is not what we think. I believe we think it is the spin axis that is precessing with the 26,000 year period. The
original question of whether the orbital axis is precessing at some rate is not something I have seen discussed anywhere other than
this current discussion. If the orbital axis is not precessing, the ecliptic will cycle it's position on the celestial sphere with
the same periodicity as the spin precession.

Michael D. Edmiston, Ph.D.
Professor of Physics and Chemistry
Chair of Sciences
Bluffton College
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu

This posting is the position of the writer, not that of SUNY-BSC, NAU or the AAPT.