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David,
You wrote:

> I imagine there's also the problem of different authorities assigning 
negative numbers to different directions.  SHA increases to the west while 
right ascension (e.g. 
> 2102-D star finder) increases to the east.  Avoiding negative numbers can 
help avoid headaches when using references from different publishers.

There is a very fundamental reason for this.
The whole celestial sphere (currying stars, planets, Sun and Moon with it) 
rotates clockwise (daily rotation), if you observe from the Northern 
hemisphere.
That is E to W.  That is the direction of increase of hour angle. This is why 
the hands of our watches rotate this way.

With respect to this sphere, everything (planets, Sun, Moon) rotate 
counterclockwise (this comes from the yearly rotation of the Earth and 
planets around sun,
and Moon around the Earth.) So they choose this as positive directions in all 
coordinate systems which do not depend on the daily rotation.

These choices were made in antiquity, exactly with the purpose to minimize or 
avoid using negative numbers:-)

Alex.

   

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