Responding to Chris's remark below, I did indeed go 'aargh' when Chris mentioned the simple solution of the circular disk parallel to the wall.
My dial uses a large plate and aperture gnomon and I followed tradition by having the plate 'looking at the sun' which I find logical and aesthetically pleasing. However, I had the idea of using an elliptical plate (and aperture) calculated to cast a circular shadow at the equinox. This it does, and it is surprising how good the eye is at detecting when the shadow is truly circular. As a further refinement the whole ellipse has been rotated to allow for the effect of the glass window declining by 13.5 degrees. The dial is number 16 in the list of etched dials.
This ellipse has the minor advantage that at the summer solstice, the shadow, although elliptical, is much less extended than from a circular plate. The major axis of the shadow is vertical whereas at the winter solstice the major axis of the shallow ellipse is horizontal.
To cast a circular shadow is a useful refinement although I still prefer a plate that is aimed at the sun rather than one that looks to the horizon!
Doug
On May 10, 2006, at 00:20, Chris Lusby Taylor wrote:
Hi John,
It's not obvious perhaps, but I assure you the shadow on a wall of any thin flat object parallel to the wall is exactly the same shape and size as the flat object, regardless of the sun's direction, if you ignore fuzziness.
At low angles of incidence of sunlight, the shadow will be fuzzier but still basically the same size and shape. If the object isn't thin, its thickness will also affect the shadow.
Try it with a CD: hold it parallel to a piece of card and move them around. You should be able to verify that the CD's shadow on the card is always circular. The hole in the middle looks a little bit squished at very low angles, but it isn't bad.
I mentioned this at a BSS conference a few years ago after a talk by Doug Bateman on his etched glass noon dial. Some didn't seem to believe it at first.
Chris
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<x-tad-bigger>From:</x-tad-bigger><x-tad-bigger> </x-tad-bigger><x-tad-bigger>John Carmichael</x-tad-bigger><x-tad-bigger> </x-tad-bigger>
<x-tad-bigger>To:</x-tad-bigger><x-tad-bigger> </x-tad-bigger><x-tad-bigger>Chris Lusby Taylor</x-tad-bigger><x-tad-bigger> </x-tad-bigger>
<x-tad-bigger>Cc:</x-tad-bigger><x-tad-bigger> </x-tad-bigger><x-tad-bigger>Sundial List</x-tad-bigger><x-tad-bigger> </x-tad-bigger>
<x-tad-bigger>Sent:</x-tad-bigger><x-tad-bigger> Tuesday, May 09, 2006 7:54 PM</x-tad-bigger>
<x-tad-bigger>Subject:</x-tad-bigger><x-tad-bigger> Re: Conical Gnomon Advantages</x-tad-bigger>
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
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<x-tad-bigger>To:</x-tad-bigger><x-tad-bigger> </x-tad-bigger><x-tad-bigger>John Carmichael</x-tad-bigger><x-tad-bigger> ; </x-tad-bigger><x-tad-bigger>Sundial List</x-tad-bigger><x-tad-bigger> </x-tad-bigger>
<x-tad-bigger>Sent:</x-tad-bigger><x-tad-bigger> Tuesday, May 09, 2006 11:00 AM</x-tad-bigger>
<x-tad-bigger>Subject:</x-tad-bigger><x-tad-bigger> Re: Conical Gnomon Advantages</x-tad-bigger>
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
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<x-tad-bigger>From:</x-tad-bigger><x-tad-bigger> </x-tad-bigger><x-tad-bigger>John Carmichael</x-tad-bigger><x-tad-bigger> </x-tad-bigger>
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<x-tad-bigger>Sent:</x-tad-bigger><x-tad-bigger> Tuesday, May 09, 2006 5:12 PM</x-tad-bigger>
<x-tad-bigger>Subject:</x-tad-bigger><x-tad-bigger> Conical Gnomon Advantages</x-tad-bigger>
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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