Re: Discs + holes, Sundiale near Cph.

Sat, 20 May 2006 02:28:03 -0700

Hi Thyge,
Thank you for your contribution to this interesting debate. Your sundial is stunning. Trust the Danes to show the rest of the world how to apply good modern design ideas to old products.
 
Yes, we had realised that a circular disk parallel to the wall would cast a circular shadow. The issue was what happens, especially to the image of the circular hole in the middle of the disk, when the angle of incidence to the wall is very small. Your dial overcomes this, very sensibly, by limiting the width of each dial but having one on each face. Thus the sun is only on each dial for about 3 hours either side of when it is directly in front of the dial. So you never have the problem that John Shepherd described.
 
But imagine if your East-facing dial went back much further towards the shopping centre. Then, towards noon the shadow would become very long and indistinct. Also, the hole in the middle would become noticeably elliptical. John's point is that you are then better off with two intersecting lines. I'm inclined to agree with him, though I am somewhat surprised at the poor shadow he photographed just three hours before noon. In my experience a disk is quite usable over a longer time than that. Your photo of your east-facing dial on 3rd May at 11:02 shows the disk very clearly, even though the sun's azimuth was only 43 degrees east of south.
 
On of my favourite dials is the bifilar, but I had doubted whether people would understand and accept reading the intersection of two shadows. John encourages me to think they will.
 
Best wishes
Chris
51.4N, 1.3W
 
----- Original Message -----
Sent: Saturday, May 20, 2006 12:05 AM
Subject: Discs + holes, Sundiale near Cph.

Dear all members.

If I understand you right, you want gnomon like a light-spot.

On my sundial. I had the discs parallel with the wall, and if the discs are exact parallel the shadows will be a circles too.

And the hole (d=15mm) will be a circle too.

Look at my homepage:

http://www.123hjemmeside.dk/tl-solur

And the Danish Newspaper “Jyllandsposten”:

http://www.jp.dk/kbh/artikel:aiid=3730350:img=0/

 

And from Rødovre centrum:

http://www.rodovrecentrum.dk/default.asp?id=264

 

Best regards and good Weekend.

 

Thyge.

 


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