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I love these discussions that make you want to jump
up from your computer, and rush outside to conduct an experiment!
I used a CD & white foamboard and I indeed saw
the false ellipse of the projected solar image. Holding a 1/16" thick
disk with a 1/4" hole about 3 ft. from the board and parallel to it, the
fuzziness of the CD's perimeter shadow was very pronounced and the
solar projection image was a long ellipse. As I rotated this arrangement,
keeping the CD parallel, the solar image did disappear when the sun was almost
edge on. (due to the thickness of the disk). So at these angles,
the aperature nodus won't function. A conical gnomon won't do
that. I compared the image of a conical gnomon (a plumbob) at the
same distance, and I think it is easier to read.
I love the shadow cast by a plumbob. It is so
dramatic and visible. If you have a plumbob, check it out for
yourself. They are cheap and come in different sizes and
shapes. Almost all of them have a threaded cap for attaching the
string. But you can remove and discard the cap and use the threads to
attach it to a threaded rod which you can mount to the sundial
face. Google: "brass plumb
bobs". Also try: " "weathervane finials"
Many thanks to all that have witten and for the
drawings you sent.
John C.
----- Original Message -----
Sent: Tuesday, May 09, 2006 11:54
AM
Subject: Re: Conical Gnomon
Advantages
Hi Chris:
But I'm not so sure I agree with you about a flat
disk with a small hole. The aperture disk nodus you described
works fine when the sun is not at low angles relative to the face.
example: a direct south vertical wall dial around midday. But if it's
early morning or late afternoon, won't both the shadow of the disk and the
projected solar image elongate into long ellipses? In fact, if the sun
is hitting the disk nearly edge on, I bet the solar image projection might
disappear all together. If you could manually rotate the disk so that it
is perpendicular to the sun, then I would agree with you. But sundial
I'm designing is high above a doorway and can't be touched.
Interested in anybody's comments on
this,
John
----- 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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