Hello Fellow Dialists,
Vertical declining dials like those that decorate houses in the alpine
villages of Europe are my favourite style of sundial to design and build.
With programs like Fer de Vries' "Zonwvlak" or Francois Blateyron's
"Shadows", the difficult mathematics are easily solved to produce the basic
design for a dial for any wall declination.
The problem in such a design is to accurately measure the declination of
the wall. Compass measurements are inaccurate. Plumb bob or nail shadow
measurements work but are awkward to do correctly. The math to solve for
solar azimuth is also intimidating. I faced this problem recently when I
designed a dial as a gift for my sister-in-law who lives on the other side
of the country (and Canada is a large country). On a short visit I had to
get the appropriate measurements without anyone suspecting what I was
doing. The usual tools like step ladders supporting plumb bobs, carpenters'
squares etc were unavailable.
The solution was simple. Measure the azimuth of the sun through the window!
The windows in well built modern houses (an oxymoron) are vertically set in
the plane of the wall. The shadow of the window frame on a pad of paper
held horizontally, with one edge pressed on the glass determines the
azimuth of the sun with respect to the plane of the window at that instant.
All you need to do is mark the shadow line on the paper and the exact time.
Take several readings, hours apart, for better accuracy and a check on
measurements. To keep the orientation correct, I also note the rough
heading of north and add to the shadow line an arrow pointing to the sun.
Data reduction is still a problem. You have to correct for clock error,
know the latitude, longitude, equation of time and declination of the sun
and solve the usual spherical trigonometry equations to determine the
azimuth of the sun. This is greatly simplified using "The Dialist's
Companion" a program written by Terwilliger and Sawyer and published by
NASS. Enter the appropriate data; the program solves for azimuth. From that
calculated azimuth compared to the measured azimuth, the declination of the
window (wall) is determined by simple geometry.
The technique works very well. I have used it many times with excellent
results. Generally the declination results agree to within half a degree.
Errors are obvious as outliers.
My sister-in-law was surprised by the gift and amazed by the accuracy of a
dial designed and built thousands of miles from it's unique location. If
you are interested, e-mail me and I will send pictures (106 kb JPG) and
operating instructions for the dial.
Cheers,
Roger Bailey
Walking Shadow Designs
Canmore, Alberta, Canada
