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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
----- Original Message -----
Sent: Tuesday, May 09, 2006 7:54 PM
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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