Re: North Pole

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

I wrote:
> > 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).

Then Michael Edmiston wrote:
> I don't see this.

What's specifically is the problem?

>  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.

That's what I said.

> I don't see how it shows us anything about the orienation of the vector
> that represents the total solar system angular momentum.

Indeed it doesn't, as I said.  Specifically I said:

> > 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.

> Sidebar... I think astronomers use the words "north pole" with
> respect to anything with angular momentum.

It depends on context.  Unless context demands otherwise,
"THE north pole" refers to the geographic north pole,
defined by the spin axis.

There are also such things as lunar north pole, solar
north pole, galactic north pole, ecliptic north pole,
but it would be deceptive to abbreviate those to "the"
north pole except in very specialized contexts.

> > 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.

I'm surprised and mystified.

> These the same thing and are not refering to (#1-spin) and
> (#2-orbit).  They are both refering to (#1-spin).

I can't imagine where that's coming from.
When I write "ecliptic (#2)" I don't see how it can
be taken to mean "(#1-spin").  I don't know what
I could do to express myself more clearly.

> The celestial north pole and earth's north pole are
> aligned by definition.

OK.

> The ecliptic and celestial equator are circles drawn on the
> celestial sphere.

OK.  Different circles.  Different physics.

> Because these poles (a direction in space) precess
> with a 26,000 year period,

Because??? That's assuming facts not in evidence.

I've seen no theoretical or observational reason to
believe that the ecliptic precesses significantly
relative to the fixed stars.  If somebody has evidence,
please share it.

> anything ...

Anything???

> 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).

Why anything?  It isn't the sphere that is precessing.
The fixed stars are fixed, to an extreeeemely good
approximation.

> This includes the
> celestial equator and the ecliptic.

Again, I have not seen any reason to believe that the
equator and ecliptic are locked.  Indeed they should
be independent.  The spin angular momentum and the
orbital angular momentum are separately conserved (in
the absence of interactions) and the interactions are
verrrry weak.

> 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;

I am happy to suppose that, because that's what actually
is observed.

> e.g. suppose its apparent position remains fixed on the sphere.

How can it precess and remain fixed, or even "apparently"
fixed.  I'm mystified.

> 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.

That's not what is observed.  That's completely backwards.
The precession of the pole is caused by precession of
the axis, since "pole" and "axis" are practically synonymous.
I don't see what's complicated about this.

> I believe we think it is the spin axis that is precessing
> with the 26,000 year period.

Yes indeed.

>  If the orbital axis is not precessing,

(which indeed it is not)

> the ecliptic will cycle
> it's position on the celestial sphere with
> the same periodicity as the spin precession.

No, it won't.  There is precession of the equinoxes.
There is no significant precession of the ecliptic.

The fixed stars are fixed.  The orbital axis is not
precessing, by hypothesis 12 lines above.  So they
stay together. What is complicated about this?

This posting is the position of the writer, not that of ABC, CBS, or
NBC.

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