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Original Message -----
Sent:
Tuesday, May 09, 2006 11:00 AM
Subject:
Re: Conical Gnomon Advantages
Hi John et al,
If you want a circular shadow that doesn't
elongate when the sun is at a small angle to the dial face, you can use
a circular disc, held parallel to the plane of the dial. A large disc
with a small circular hole in the middle works very well, too.
The tip of a cone is an excellent idea, but
I imagine it would only be practical for dials that are above head
height, as the tip would seem dangerous otherwise.
Chris
51.4N, 1.3W
-----
Original Message -----
Sent:
Tuesday, May 09, 2006 5:12 PM
Subject:
Conical Gnomon Advantages
Hello All;
I have always been a big fan of
spherical nodi on a perpendicular gnomons. I just like the way they
look and when the shadow is only slightly elliptical, sundials with
them are pretty easy to read.
But in doing some shadow tests
for a new wall dial I am designing, I was greatly dismayed by how much
the shadow elongates when the sun is at a small angle to the dial face.
The shadow ellipse is so stretched along the major axis that the dial
would be very difficult to read.
So, it occurred to me that a
nodus that is the tip of a cone might be better. So instead of
guestimating the center of an ellipse, you look at the shadow of the
point of the cone. (To test this, I attached a brass conical plumbob
to a threaded brass rod). I figured that a cone would work better than
a flat arrow because it has a uniform cross section around it and
therefore would cast a uniform shadow as the sun rotates around it
during the day, unlike a flat arrow.
The shadow tests on the conical
gnomon worked great even at low solar angles.
John
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