Hi John & all,
I believe you absolutely when you told us: " The shadow tests on the conical
gnomon worked great even at low solar angles" and "I love the shadow cast by
a plumbob. It is so dramatic and visible".
What I wondered about is the following. When estimating the position of the
plumbob tip from its shadow on the dial face, wouldn't that point shift
towards the body of the plumbob when the distance gets larger? Intuitively I
would expect the shadow of the tip to be 'eaten' by the fringe rays of the
sun. Exactly the reason why pin-shaped gnomons (for instance the obelisk on
Piazza Montecitorio, Rome) often have a little sphere on top...
Whether the effect on time reading is at all noticeable, may depend on the
specifics of the situation. So perhaps in practice it's not worth worrying?
Another reason to "rush outside to conduct an experiment!"
Regards,
Frans Maes
----- Original Message -----
From: "John Carmichael" <[EMAIL PROTECTED]>
To: "Sundial List" <sundial@uni-koeln.de>
Sent: Wednesday, May 10, 2006 5:32 PM
Subject: Re: Conical Gnomon Advantages
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 -----
From: John Carmichael
To: Chris Lusby Taylor
Cc: Sundial List
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 -----
From: Chris Lusby Taylor
To: John Carmichael ; Sundial List
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 -----
From: John Carmichael
To: Sundial List
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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