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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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