----- 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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