>Dear Dialists,
>
>The city of Ghent (Belgium) plans to place 4 sundials on one
>of the chimneys of the town hall. The surfaces of the dials
>will be squares, with sides 1.5 m. They will be placed about 35 m
>above the ground..........
>Herwig Dejonghe
>
>[EMAIL PROTECTED]
Dag, Herwig:
I have some advice based on my experience with a large wall dial.
My dial (images available at
www.phys.washington.edu/Department/tour/sundial) covers an area of about 7
x 10 m, at a mean height of about 15 m above the ground. When designing the
dial, we did many experiments such that it would be readable and look good
from a distance as far away as 60 m.
The gnomon is a 2-m long tube of 5-cm diam with a 17-cm sphere at
its end. The hour lines, solstice and equinox lines, etc. are composed of
polyfluoromer-painted aluminum bars and rods having widths varying from 8
to 1.5 cm. These bars and rods form the sundial pattern and are attached to
the brick wall with about fifty 1.9-cm stainless steel bolts, such that the
various surfaces of the rods and bars are off the wall by distances of 5
to 8 cm. This "depth" means that the rods cast their own shadows when the
sun is oblique, and also limits the acccuracy with which the dial can be
read (but not seriously), but it also gives a pleasing sculptural quality
to the dial, even on rainy days (which are known to happen here in Seattle,
just as they do in Belgium!). Labelling letters and numerals are of height
15-20 cm and in bold Helvetica font.
Your four dials in Ghent are much higher and smaller in size and
you need to pay particular attention to how well they will be able to be
read. You should first establish the likely viewing locations and then do
some experiments viewing, *from the same upward angle and distance*, a
simulated sundial having various widths of rods, colors, materials,
lettering, detail of design, etc. This will teach you a lot about what
works and doesn't work. My *guess* is that 2-cm-wide rods should be visible
(if there's enough contrast between the rod and its background), but 3-5 cm
will be better for the major lines. I also advise against having supporting
struts for the gnomon (but anchor it well!), unless the engineers say they
are absolutely necessary. Finally, at such a height a simple, clean design
will probably be best, without a lot of "furniture" (although you could
have different sides of the chimney indicate different things).
Good luck - keep us informed as the project progresses.
met vriendelijk groeten uit Seattle,
Woody Sullivan
******************************************************************
Prof. Woodruff T. Sullivan, III [EMAIL PROTECTED]
Dept. of Astronomy, Box 351580 tel. 206-543-7773
Univ. of Washington fax 206-685-0403
Seattle, WA 98195 USA
