First of all, thanks for all your kind responses on shadow
sharpeners. Now some comments on them:
[Edley] The simplest of these is a simple pinhole placed far
enough from the dial surface to focus the image of the gnomon on the surface.
Yes, I knew about these. In Spanish we call them 'disco perforado' and
in French I think it is 'oeilleton'.
My book tells (as a rule of thumb) that its diameter must be around 2.5%
of its distance to the wall. Is that correct?
[Edley] Other more complex shadow
sharpeners are disks inside circles or slits in a complex gnomon. There are several
articles on these in the NASS compendiums. I could clip a few out and send them to
you as pdf files if you like.
Yes, please, send me these pdf files. I'd like to know more about them
and, in general, about the physical principles
which they are based in.
> [Mac] What's needed is for someone to assemble the information from
all those postings into a fairly brief, but accurate explanation
> of how to use and understand shadow sharpeners. Even after all that
been posted on shadow sharpeners, some of us just
> learned some important new things about them.
Mac, thanks for being more straightforward than me, because that was
EXACTLY what I was subreptitiously asking for ;-)
You know, I was afraid of getting an answer like "That's gorgeous,
Anselmo, why don't you do it yourself and post it to the list?" :-DDD
This property of the 'parallel plane ===> round shadow' keeps being
quite surprising to me.
[Patrick] The ones
we have recently been talking about - and the ones we discussed in May
1999 (it's a common toopic on the sundial mail list!)
I know, and I apologize for being redundant. I lost all my e-mail from
last month due to a trojans attack. (by the way,
does anybody know how to get rid of this addiction to the Sundial List?
Does the World Health Organization know about
this place? :-D)
> [Mac] If you search the sundial list archives, you will find lots of
previous messages about shadow sharpeners.
> Go to the NASS website (http://sundials.org)
A lot of thanks, Mac, I was about to send you all an e-mail asking for
these files I couldn't save!
Best regards,
Anselmo
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