Hi Edley:
Well you learn something new every day, especially from the smart people on
this List.
I had no idea that a hazy day would shorten the shadow cast by a cone's
point! In fact, I still don't quite believe it until I get to see it with
my own eye's. It's our rainy season so we might get a hazy day. I will use
my experiment and will test all the gnomons on the next hazy day. I'm
wondering if the shadow shortening is greater with longer shadows when the
sun at a low angle (I bet it is). And by how much? Is it really
significant?
Also, does the width of the cone or point make a difference? Wouldn't a fat
cone's shadow be less susceptible to this effect?
Has anybody else observed this effect?
very interested in this,
John
----- Original Message -----
From: <[EMAIL PROTECTED]>
To: "John Carmichael" <[EMAIL PROTECTED]>; "Sundial List"
<sundial@uni-koeln.de>
Sent: Monday, August 07, 2006 10:26 PM
Subject: Re: Expanded Nodi Shadow Experiment
Hi John,
Since you are in a very dry climate with clear weather you may not
notice that with hazy light the cone image is quite forshortened and
points to an incorrect point. Yes, the cone points out a line very
clearly and yes, it does look quite like a clock hand, but the very
shadow tip position is quite a way off from what it should be. The
spherical nodus and other symetrical ones, although not so
attractive, give a much more accurate spot. The area of untruth may
be mostly in declination rather than hour angle with most dials, such
as your neat painted dial. You could try letting the light shine through
some thin plastic to simulate haziness, or come visit us on the coast
where hazy is very common. Of course, too hazy and nothing works.
for an azimuth dial it might work very well, where for a pillar dial, not
so well.
Thanks for the interesting ideas!
Edley.
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