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Re: North Pole



I think John and I are visualizing the same physics, but we disagree on word definitions.

John says the celestial sphere is not polar aligned, and I say it is. John says the stars do not change their positions on the
celestial sphere. I say they do. This boils down to the definition of celestial sphere. I teach astronomy even though I am only
an amateur astronomer. I don't say this to prop myself up as an expert; I say it to explain why I have over two dozen astronomy
textbooks on my shelves, and I have read most of all of them. With regard to the celestial sphere they all say the same thing. I
shall quote from Michael Seeds book "Horizons, Exploring the Universe."

* * begin quotations * *

Just as we use the earth's poles and equator as reference marks on the earth, we can use corresponding reference marks on the sky.
The celestial poles and celestial equator are defined by the earth's rotation.
<snip>
In the northern sky, [stars] appear to revolve around a point called the north celestial pole, the point on the sky directly above
the earth's North Pole.
<snip>
In fact, the celestial poles and equator are the basis for a system of precise celestial coordinates much like the system of
latitude and lingitude on the earth, But we must beware. These critical reference marks on the sky are moving, and that can tell
us something new about the motion of the earth.
<snip>
...earth's axis [precesses] in a conical motion, taking about 26,000 years for one cycle. Because the celestial poles and the
equator are defined by earth's rotation, precession changes these reference marks.
<snip>
Over centuries precession has dramatic effects. For example, it makes the celestial poles move across the sky. Egyptian records
show that 4800 years ago the north celestial pole was near the star Thuban (a-Draconis). The pole is now approaching Polaris and
will be closest to it about AD 2100.

* * end quotations * *

That's the definition of "celestial sphere" in all the astronomy books. That's what is being taught by astronomers and used by
astronomers. All the postings I have made about how precession changes things on the celestial sphere were made with this
definition of the celestial sphere in mind. John's definition of "celestial sphere" as he described in his responses to my last
post (see below) is quite contrary to this.

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


=Edmiston
=Denker

<snip>

The celestial sphere is polar defined.

No, the celestial sphere, to follow M.E.'s example, is
the celestial sphere.

The celestial sphere has physical reality quite independent
of what coordinates, if any, somebody imposes on it.

By way of analogy:
-- Vectors are not defined by the coordinate system (if
any) in which somembody (if any) chooses to project out
components.
-- Numbers have a reality independent of the base or font
in which somebody (if any) chooses to write numerals.

I made it clear, I thought, that when I said the ecliptic
was not precessing, it was not precessing relative to the
fixed stars.

Therefore, if the pole
moves with respect to the fixed stars, then the stars
change their locations on the celestial sphere.

No, the fixed stars are fixed. This seems clear. Obvious.
Tautological. Changing your choice of coordinate system
does not cause things to move.

There are techniques for writing down the equations of
motion with respect to a changing coordinate system, but
these are far-from-elementary techniques. If anybody
really wants to get into this, we can get into it.

For a discussion of the methods of (and the importance
of) the coordinate-free approach to doing physics, I
recommend Misner, Thorne, Wheeler _Gravitation_.

And the orbital plane, as you said, stays with the fixed
stars,

and so it does

so that means the ecliptic goes to a new position on the
celestial sphere.

On a couple of occasions they renumbered the area-codes where
I lived. That doesn't mean that my house moved, or that my
phone moved, in any physical sense. Relabeling the coordinate
system is not "motion" or "precession" according to any
physics I've ever heard of.

There are two problems (1) physics definitions, (2) context of our discussion.

(1) The ecliptic is defined as the intersection of the orbital
plane with the celestial sphere.

OK.

The celestial sphere is polar defined.

This is getting repetitive. This is still wrong physics
for the reasons described above.

Therefore the ecliptic is drawn on a sphere that is
oriented with respect to the north pole.

The physics of spheres is not oriented. Your chosen
coordinate system may be oriented in a way of your choosing.
Others may choose coordinate systems that they find more
convenient. The sphere is unchanged by any such choice.

<snip>

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