RE: Pub Dials

[Andrew James]

The Chequers at Weston-on-the-Green (1d 13'W, 51d 51'N) had until about 4 or 5 
years ago a dial painted on a wooden board. When I first saw it c. 1995 it had 
lost its gnomon and looked rather sad. Then it was replaced by a very smart 
looking dial in the same place, apparently painted on Perspex or similar 
plastic/acrylic, very neatly done and with a black wrought iron gnomon. 
Unfortunately the delineation is doubtful and the gnomon is certainly at an 
angle of about 60 or 65 degrees from horizontal instead of 52 and it would 
intersect the dial plane some inches above the centre implied by the hour 
lines. I expect it shows the right time now and then. I believe that advice was 
offered but not taken up. Very sad. However I think there are some bright 
lights outside the pub so you can use it equally well at night g. 

The other pub there, about 3/4 mile N, is the Ben Jonson and the stone dial 
looks probably 18th century though there is a possibility that it has been 
moved to its present location over the pub door. From memory it declines.


There is a pub in Pembrokeshire (Dyfed) called The Dial Inn - at Lamphey mear 
Pembroke. Unfortunately it doesn't have a dial outside any more, though there 
presumably once was one; but there is a curious large square copper dial, 
apparently not that old, hanging on the wall in the bar. I couldn't decide 
quite what it was meant to be - anyone passing there?

Regards
Andrew James


-Original Message-
From: Colin Davis [mailto:[EMAIL PROTECTED] 
Sent: 11 August 2003 10:02
To: Sundial List
Subject: Re:Pub Dials


Hi!
  in the village of Weston on the Green on the Northants/Oxford border there 
are two pubs with sundials, one is painted ;the other looks like it is part of 
the original building. Only been able to look as I pass by,must get there and 
have a good look!!! Colin Davis G3VMU 52° 14' N 0°  52' W Northampton

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Re: Poor Man's SGS

[Chris Lusby Taylor]

Dave Bell wrote:
  John,
 
  If the gnomon is outside and the dial face is inside, with a pane of
  glass in between, isn't there a refraction problem?
 
  Mac

 There is a refraction offset introduced, but can't you compensate for
 that? I suppose it would complicate the layout calculations somewhat,
 since the offset varies with incidence angle...

 Dave

My recollection of the laws of physics leads me to believe that the effect
of refraction is much simpler than you would suppose. It is exactly as
though the glass were a thinner 'slice' of air. If you know the thickness
(t) and refractive index (u) of the glass you just imagine the two surfaces
were separated, not by a distance t filled with glass, but a distance t/u
filled with air. (That's t divided by u.)
So, the only practical effect for a normal vertical dial is that the gnomon
must be positioned slightly differently relative to the dial. This can be
checked by eye, as the gnomon must appear, when viewed through the glass, to
be in line with the centre of the dialface (and vice versa).
If the gnomon has a nodus, the scale of the dial must treat it as though its
height is its actual distance from the glass plus t/u.
For a double-glazed window, you can apply the same logic. As someone
mentioned, sealed double-glazed units bow with temperature changes, so the
gnomon should be fixed to the centre of the glass to minimise this effect.

Hope this helps
Chris

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Fw: Double glazing dial

[fer j. de vries]

To All interested in glass sundials and the refraction problem.


Some one year ago I answered to Mike Shaw with a procedure to calculate a
double glass sundial.
This message I forward to the mailing list again.

Playing with the factors for the pin-gnomon and the thickness of the glass
panes
shows the effect of the refraction.
It depends strongly on the thickness of glass in the total thickness.

If you have Deltacad I can send a macro which calculates these double glass
dials and shows the pattern on screen.
Hourlines and datelines for a zodiacal calendar are shown as well as the
horizon line.

The pattern is drawn in two layers, a non-refracted sundial and a refracted
sundial.
Switch between the two layers to see them separate.

You need to change the values for the glass and the gnomon in the macro
yourself.

Send an e-mail if you want the macro to mailto:[EMAIL PROTECTED]


Best wishes, Fer.

Fer J. de Vries

De Zonnewijzerkring
mailto:[EMAIL PROTECTED]
http://www.de-zonnewijzerkring.nl

Home
mailto:[EMAIL PROTECTED]
http://www.iae.nl/users/ferdv/index-fer.htm
Eindhoven, Netherlands
lat.  51:30 N  long.  5:30 E

- Original Message -
From: fer j. de vries [EMAIL PROTECTED]
To: sundial sundial@rrz.uni-koeln.de
Sent: Monday, July 29, 2002 5:01 PM
Subject: Re: Double glazing dial


 Hello Mike,

 Nice idea to make a double glazing sundial.
 I dont know of any program to do the job for you but here you will find a
 solution to calculates such a dial.
 The window can have any declination or inclination as you want.

 At my web site ( address below ) you may find a method to compute flat
 sundials.
 ( See the link at the site )
 We need the main procedure of that method with some extra routines to
solve
 the problem.
 For definitions and details look at my site.

 I suggest to place the shadow casting point at some distance from the
 outside pane to get a larger dial on your inside pane.
 That distance I call g1
 If you don't want to do this just take g1 = 0

 The thickness of the outside pane is g2
 The space between the two panes is g3
 The thickness of the inside pane is g4

 I assume:
  - the refraction index of glass is   ref   and equal for both panes.
  - the medium between the two panes acts like air with no extra
refraction.
  - the panes are parallel.

 Of course you have to declaire values for latitude phi and for the dial's
 inclination and declination i and d.

 Because (nearly) all the lines on the sundial will become curved, you need
 to calculate a series of points  ( decl, t ) for each line.
 decl is the sun's declination
 t is the hourangle of the wanted point.

 For each pointdecl, tdo as is written below.

 In decl, t

 x0 = sin t . cos decl
 y0 = cos t . cos decl
 z0 = sin decl

 R = 90 - phi
 x1 = x0
 y1 = y0 . cos R - z0 . sin R
 z1 = y0 . sin R + z0 . cos R
 if z1  0 point isn't real: sun is beneath the horizon.

 R = d
 x2 = x1 . cos R - y1 . sin R
 y2 = x1 . sin R + y1 . cos R
 z2 = z1

 R = i
 x3 = x2
 y3 = y2 . cos R - z2 . sin R
 z3 = y2 . sin R + z2 . cos R
 if z3 = 0 point isn't real: sun isn't above the dial.
 These are the coordiantes of the sun relative to the window.

 Add new routine
 x4 = x3 / ref
 y4 = y3 / ref
 z4 = sqrt( 1 - x*x - y*y ) ( square root out of (...) )
 These are the coordinates of the sun corrected with the refraction index
 ref.

 Now we have to calculate the shadowpoints on 4 sundials
 xa = x3 . g1 / z3
 ya = y3 . g1 / z3

 xb = x4 . g2 / z4
 yb = y4 . g2 / z4

 xc = x3 . g3 / z3
 yc = y3 . g3 / z3

 xd = x4 . g4 / z4
 yd = y4 . g4 / z4

 The final coordinates of the point are
 x = -( xa + xb + xc + xd )
 y = ya + yb + yc + yd

 The x value gets a minus sign because you will draw the pattern from the
 opposite side as the gomon is.

 Use the two tests inside the procedure to exclude irrelevant points.
 Also exclude points that are very far away of your sundial.

 Show us what your final dial will be.

 Best wishes, Fer.

 Fer J. de Vries
 mailto:[EMAIL PROTECTED]
 http://www.iae.nl/users/ferdv/
 Eindhoven, Netherlands
 lat.  51:30 N  long.  5:30 E

 - Original Message -
 From: The Shaws [EMAIL PROTECTED]
 To: Sundial list sundial@rrz.uni-koeln.de
 Sent: Sunday, July 28, 2002 8:53 PM
 Subject: Double glazing dial


  I have wondered for a while if it was possible to make use of the two
 panes
  of glass in double glazed units to make a window sundial.
 
  To try out the concept, I made a prototype dial using two panes of
glass,
  clamped 44mm apart (I just happened to have some wood that thickness).
  The pane nearest to the sun carries a double arrow which acts as the
 gnomon
  nodus.  The inner glass carries the dial face.
 
  The dial was designed for a site which declines 76 degrees West of
South -
  the back of my house.
  I used Francois Blateyron's Shadow programme - Version 6.2.1
  I have put a picture on my web page - follow the link to Double Glazing
  Dial - there is a link to Jim Tallman's single glazing 

Re: Poor Man's SGS

[J.Tallman]

Hi John,


 The gnomon would be on the outside of the window and the plastic film on
the
 inside, so drilling a hole is unnecessary if you use adhesive for the
 gnomon.


This arrangement is more complex than it might appear at first thought. Mike
Shaw has been down this road and that is what his double pane dial prototype
demonstrates...the nodus is on the outer pane and the shadowfield is on the
inner pane. Go to his sundial page (referenced on the SGS page) and have a
read...I am sure that he would be happy to help you understand some of the
difficulties he encountered.

One other thing bears mentioning. Every time a ray of light passes through a
pane of glass a certain amount is lost due to incidence reflection at the
surface, especially at low angles. That is how you can see yourself in a
storefront window. The more panes the shadow has to pass through, the dimmer
it will be by the time it gets to the surface of the shadowfield. This
includes any protective coverings as well...


Jim


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