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John Denker mentioned declination and right ascension. Being a >50-year-old amateur astronomer, that's the system I grew up with
and that's the system I typically have in mind when I think of the celestial sphere. John is right that there are other coordinate
systems in use. This is expecially true now that telescopes are computerized, and many use alt-azimuth mounts rather than
equatorial mounts. So it is probably a fact that the dec/RA coordinate system on the celestial sphere is waning.
When educated people speak of the effects of precession, there is no doubt they understand that the dec and RA of the stars are
changing over time. Therefore we have no disagreement that these dec/RA celestial coordinates are changing. The question then
becomes whether we are viewing "the celestial sphere" as the coordinate system, or whether "the celestial sphere" exists independent
of the coordinate system.
It is probably now obvious to readers of this thread that I have viewed the celestial sphere as the coordinate system. Although
some ancient astronomers thought the celestial sphere was a literal object, a spherical shell upon which the stars were placed, we
have known for some time that the celestial sphere is just a model. The celestial sphere model is still used because it is a
convenient way to map the positions of objects in the sky using angles to locate objects and also to describe the "distance between"
objects.
Since the celestial sphere is a model that has persisted because it is useful as a mapping device, I view the celestial sphere as
the coordinate system and I think I am not alone in this regard.
Here is a little mind exercise to help describe the differing viewpoints. It is common to have miniature versions of the celestial
sphere in classrooms, observatories, planetariums, and museums. When I teach astronomy lab, one of the things we do is purchase
little globes about an inch in diameter, stick a thin rod through this from south pole to north pole, then position the globe with a
clear plastic spherical shell about 8-inches in diameter. Each student therefore gets a miniature earth suspended within a clear
spherical shell, and the rod through the polar axes holds it all together. The clear shell obviously represents "the celestial
sphere" and the rod is both the earth's axis and the celestial sphere's axis. The students use a fine-tip marker and draw some
stars and constellations on the sphere, as well as the ecliptic and a few dec/RA grid lines.
Now... what did students do a few thousand years ago, and what will they do a few thousand years in the future? It is possible that
they draw the objects on the sphere first. To do this they first locate one star, then a second star at the appropriate angular
separation from the first. Then the third star, fourth star, etc. After locating all the stars they then locate the earth in the
center of this sphere, and they adjust the earth's axis to the appropriate position. That would be in tune with the idea that the
celestial sphere exists independent of the earth.
However, that is not what is usually done. The earth is placed in the sphere first. Why? That's the celestial sphere model.
That's the whole point of the model... to have the sphere centered on the earth and then map the stars on it as seen from earth.
And when I use the word "model" here, I mean not only the little model we hold in our hands, but the also the big model in the sky.
It is, after all, just a model... just a concept.
If the blank celestial sphere with the earth already inside is given to students a few thousand years ago, today, and a few thousand
years in the future... these three groups of students will draw the stars and ecliptic in different positions. The stars were in
different places on the celestial sphere then compared to now, and will be different in the future.
So... are we viewing the celestial sphere as a model onto which we position the stars as we see them from earth. Or... are we
viewing the celestial sphere as something that exists independent of earth. Although I acknowledge people can have the latter view,
I think the former view is historical and in common practice.
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.