Theoretically it's correct that the projection of a circular disc on
to a flat surface parallel to the disc will be a circle.
Unfortunately the sun's apparent size results in the disc becoming
very blurred when you get a couple of hours off of local noon.
Luckily when we were building the Swensen Sundial
http://www.uwrf.edu/sundial/ I decided to test several gnomon designs
and found the circular disc to be useless. We decided on the double
gnomon.
You are right Bill. That's a great idea! I had never thought about it,
but it's true. A flat circular disk that permanently mounted parallel to
the dial face will always cast a circular shadow on the face. It's such a
simple concept, I wonder why nobody's ever mentioned this before. Do you
know of any dials with this type of nodus? I don't.
Incidentally the Web-Cam on the Swensen Sundial will be shut down on
July 17 due to the fact that the building in which the camera is
housed is to be demolished. I will be trying to get something up as
soon as possible but as I am now retired I don't have much influence.
--
Cheers,
John
-
