Hi Mac:
Yes, on a perpendicular cone or pointed rod gnomon, the slope of the point
or cone could cause a self-shading problem if the cone or point is too wide
at the bottom (a fat cone). But you can easily determine the maximum
allowable width of the cone or point and avoid that design problem if you
look at the nearest solstice line to the nodus. The base of the cone must
be smaller than that distance to avoid self-shading. For my wall dial, the
largest cone possible that would not cause shading would have been just
under 5 inches wide, so the cone had to be skinnier so that it would cast a
shadow on the face at noon on the winter solstice. I made it just two
inches wide at its base, much smaller than the allowable width.
I think fatter cones and points produce easier to read shadows than than the
skinny ones, especially when the shadows are long. So the idea would be to
have the fattest cone or point possible that does not shade itself. (You may
even want to make the cone even skinnier to accomodate artwork around it.
That's why I made my cone much thinner than the allowable so that I could
put the points of a star around its base).
Does this make sense?
----- Original Message -----
From: "Mac Oglesby" <[EMAIL PROTECTED]>
To: "Sundial Mailing List" <sundial@uni-koeln.de>
Sent: Sunday, August 06, 2006 2:28 PM
Subject: Re: Expanded Nodi Shadow Experiment
One problem I've personally encountered with a pointed post gnomon is that
if the post's shadow falls too close to the foot of the post, the shadow
of the tip may be lost in the shadow of the post's shoulder. This was on a
vertical decliner.
Mac Oglesby
http://www.sover.net/~oglesby/Gnomon%20Notes/GnomonShadows3.jpg
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