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    Re: Stereographic Devices
    From: George Huxtable
    Date: 2010 Apr 20, 09:41 +0100

    About devices using a stereographic projection I had written
    
    "Yes. The oldest instrument of all; the astrolabe uses a stereographic
    projection."
    
    and Wolfgang responded, on 15 April--
    
    "That's true. But the astrolabe was never used to solve the navigational
    triangle. And the Mariner's Astrolabe - which was used in navigation,
    indeed - is not engraved with the stereographic projection."
    
    ===============
    
    I would like Wolfgang to explain a bit further, please. We agree that it's
    the "conventional" astrolabe that is being discussed, not the simplified
    mariners' version.
    
    Such a conventional astrolabe has a serious limitation; that it works for
    only a single latitude. It isn't, then, a navigational instrument, and was
    not used for that purpose, and therefore, as Wolfgang states, "was never
    used to solve the nevigational triangle". But within that limitation, once
    time and date have been set, the pointer corresponding to a marked star,
    such as Sirius, say, is to be found in position against a curved grid which
    allows its altitude and azimuth to be read off directly, within a degree or
    so anyway. Isn't that the equivalent to "solving the navigational
    triangle"?
    
    That was fine for an astronomer-observer, always at a particular spot, but
    not for a traveller. To overcome the latitude problem, some astrolabes were
    provided with a set of interchangeable plates, each engraved for a
    particular latitude. This rather unwieldy solution  was replaced, from the
    16th century, by the universal astrolabe, which could handle a wide range
    of latitudes; some of which, but not all, used a stereographic projection.
    But this is taking me into areas about which I can't speak with knowledge.
    There's rather more in "Astrolabes at Greenwich", van Cleempoel, 2005.
    
    George.
    
    contact George Huxtable, at  george{at}hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    George.
    
    contact George Huxtable, at  george{at}hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
    
    
    
    
    

       
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