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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).

Nicolàs

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