----- 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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