----- 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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