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    Re: False Horizons
    From: Nicolàs de Hilster
    Date: 2007 Nov 20, 20:10 +0100

    Isonomia wrote in 3932:
    > Eventually for my younger child  I tried setting up a plank in the
    > garden and tying the sextant up with  string to give a level, but the
    > darn sun moved so quick that it ran off the  end before I had a chance
    > to set it up with any accuracy. I'd appreciate any ideas!
    Then I proposed:
    >> Perhaps making a simple quadrant would do for the kids and instead of
    >> using two visors to look through you could make one with one small open
    >> visor and a larger closed one to cast the smaller visor's shadow onto
    >> the larger (so they do not have to look at the sun). After that exercise
    >> you can explain that a sextant does a similar job using two mirrors.
    Which resulted in Isonomia asking me:
    > The principle sounds a good one for younger children and if the sun
    > ever shines, I'll try out what I think you mean to see how accurate it
    > is. But it would help if I understood what you meant by a "closed
    > visor".
    So I will try to explain here:
    Normally a quadrant is used in a forward manner, like you can see on 
    image 'using a quadrant.jpg'. Facing the sun with the naked eye is not 
    really commendable, so my proposition was to use the quadrant backwards. 
    For this we need a quadrant with an open visor ('A' in the other image) 
    and a closed visor ('B').
    For visor A you could take a screw eye, for B a piece of cardboard with 
    a cross drawn on it. Of course the line through the centre of the screw 
    eye and the intersection of the cross should be parallel with the 
    zero-line of the quadrant.
    Accuracy depends on the diameter of the quadrant, but it is known that 
    astrolabes could produce readings accurate as 20 arc minutes (radius 
    approximately 16cm).
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