TO: [email protected]
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Item Subject: DEJONGHE.TXT
Visibility of sundials on a high chimney.
Dear Dejonghe,
About theoretical determination of the dimensions of gnomon and
hour lines of a sundial, there is my article "Determinacio de
l'amplaria de les linies horaries i altres dimensions d'un
rellotge de sol" (Determination of the width of hour lines and
other sundial dimensions) in LA BUSCA DE PAPER, the bulletin of
the Societat Catalana de Gnomonica (Catalan Society of
Gnomonics), issues 3 (summer 1989) and 4 (autumn 1989).
Unfortunately, the first part (in # 3) is in Catalan only. The
second part (in # 4) is in Catalan, in Spanish and in English (in
a poor English because of a bad translator).
In the article, it is discussed:
- The visual acuity
- Width of hour lines
- Dimensions of the dial
- Reading error
- Lines for a half and a quarter of an hour
- Dimensions of gnomon
- Improvement of precision
This article can perhaps help you. But if you have the sundial
placed 35 m above the ground and you want to read it from a
distance of 35 m, the hypotenuse is about 50 m (distance of
vision). To see a line clearly from a distance of 50 m, its width
must be of 50/1.25 = 40 mm and the same for the shadow of the
gnomon.
It is necessary, in your case, for the width of lines and gnomon
to be the same.
If at the end of the lines, 40 mm is a width equal to 20 minutes
of an hour, the maximum error will be 20 minutes.
This width of 40 mm is also necessary for the lines of the
numerals and letters. (The width of the lines or strokes of
letters not the width of the body of the letters. These will
result larger, of course).
Besides, as Woody Sullivan says, a simple, clean design is
necessary , without any camouflaging of the image of the shadow.
Good luck. Regards.
Josep Maria Vallhonrat
Barcelona - Catalunya (Spain)
