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    Re: Stereographic Projection of the Ecliptic
    From: Robin Stuart
    Date: 2021 Feb 15, 05:22 -0800

    David,


            You asked :
    If it looks like a circle, smells like a circle, and goes through the four most important points as a circle would, Can I safely call the whole thing a circle?


    It's a fundamental property of stereographic projection that circles, great and small, on the sphere map to circles on the plane. However the pole of the circle on the sphere is not mapped to the centre of the circle on the plane. To draw the ecliptic you just need to plot the points of the summer and winter solstices then bisect the line connecting them to find the centre. A similar trick can be used in the graphical solution of the double altitude problem discussed here http://fer3.com/arc/m2.aspx/Graphical-Solution-Double-Altitude-Sight-using-Stereographic-Stuart-jun-2010-g13200


    You noted:
    It would be interesting though to see what the Chinese toy manufacturers could make of an astrolabe with their laser perforated push-out plywood toy making technique


    There are a number of companies that provide custom laser cutting and etching services in a variety of materials. I have used Ponoko https://www.ponoko.com for a couple of recent projects. Being very geometrical it would be rather straightforward to produce the required files using readily available CAD of drawing programs or to code it up from scratch in Postscript.


    Regards,
    Robin

       
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