Re: North Pole

["John S. Denker" <jsd@MONMOUTH.COM>]

Michael Edmiston wrote:
> I think we need to define orbital precession.

Yes, there are at least four different concepts
being stirred together in this thread

> Earth spins one revolution per day,

That's concept #1 -- spin angular momentum.

> but the precession of the polar axis has a period of 26,000 years.

That's the rate of change of concept #1.

> If my view of precession is what this question is about,
> then wobbles of the instantaneous orbital angular momentum vector

That's concept #2 -- earth's orbital angular momentum.

> I am
> thinking the question is whether there is long scale
> (millions of years) precession of Earth's orbital axis around some reference
> line such as the sun's spin axis, or better, the solar
> system's total angular momentum vector, or better, with respect to distant
> stars.

OK.

> If this is the question,

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....

> I am wondering if we even have ways to determine it.
> Currently we don't know enough about how much mass is
> in the solar system (e.g. mass beyond Pluto) to know
> exactly where the solar system's total angular momentum points, and what
> precession that vector might have with respect to the distant stars.

Uhh, that's mixing in concept #3:  total solar-system
angular momentum.  We can speak of earth-orbit angular momentum
(concept #2) without knowing anything about concept #3.

> Thus, if we felt we did observe some apparent precession of
> Earth's orbital angular momentum with respect to the stars,
> would we know if it is Earth as opposed to the whole solar system that
> is precessing with respect to the distant stars?

The answer seems tautological:  if we observe the precession
of the Earth's orbital angular momentum, then that's what
we observe.  The angular momentum of the whole system is
a separate quantity.  To a first order, concept #1, #2, and
#3 are separately conserved.

BTW concept #4 is the orientation of the moon's orbit, which is
to first order separate from the other three.

Concept #5 (which nobody has mentioned yet) is precession of
the earth's perihelion.  This is a precession _within_ the
ecliptic plane, not precession _of_ the ecliptic plane.

> It seems we first have to know what way the solar-system angular momentum vector
> points,

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).

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

> then ask if the Earth's orbital momentum vector is precessing
> about that axis on some time scale longer than a year.

Wrong question.  The angular momentum vector can't precess
"about" the angular momentum vector.  If it precesses, it
has to precess about some _other_ vector.

This posting is the position of the writer, not that of Wiley, Daffy, or
Bugs.

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