Hello Fellow Dialists,
I was pleased to see the overwhelming response to my posting on determining
wall declination through the window. Obviously I was not the only one
frustrated by the usual methods and interested in a simple, one measurement
technique.
I would like to raise some cautions on accuracy.
1. Take readings when the sun is low. My initial data involved readings
when the solar altitude ranged from 0 to 10 degrees. Even if the frame
casting the shadow was not truly vertical, the error would be small.
Applying the technique at noon in the tropics would be a different story.
If in doubt, hang a plumb bob near the glass to get a true vertical. The
shadow of the plumb line cast over the side of the paper pad held against
the window would give an accurate relative azimuth.
2. The paper pad has to be horizontal. If you are lucky, the window sill
will be satisfactory. Otherwise you are back to the rigging that this
technique was to avoid.
3. Remember the declination is the direction the wall faces. This is
perpendicular to the heading along the wall. Yes a picture is worth 1000
words but it is harder to send by e-mail.
I would be interested in feedback on your success with the technique. I
have a series of readings taken in Australia. For the first four
measurements, taken in the morning, the declination results agreed to
within half a degree. The afternoon readings were obvious outliers, 3 to 5
degrees off. I blame the magnified error on the higher solar altitude.
Cheers, Roger Bailey
