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Wolfgang,
Thanks for the references. Contrary to my expectations the modern devices do not appear to make use of the special properties of stereographic projection (conformal circle preserving map). This is made clear from Andrew Nikitin�s azimuthal equidistant analogue of ARG-1 (http://fer3.com/arc/m2.aspx?i=109992&y=200909). I have not yet had the opportunity to check all the references you provided. Is Bowditch 1977 available online?

George,
Thanks for pointing out that the astrolabe used stereographic projection. I had not realized that although the circular openings and altitude lines on its face are a dead giveaway. Stereographic projection becomes impractical for an observer near the equator which is a least one reason why modern planispheres/navigational starfinders don�t use it. However I would guess that at the time astrolabes were being made the idea that circles were involved in all things celestial was pretty well entrenched. A desire for circular openings would have singled out stereographic projection and probably been easier to manufacture into the bargain,

Regards,
Robin Stuart

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