I'm attempting to measure the declination of a wall using a method described on this web page of The Sundial Primer https://www.mysundial.ca/tsp/wall_declination.html (also described in "Sundials: Their Theory and Construction" by Albert E Waugh Chapter 10).
Referring to Figure 1 of The Sundial Primer reference, "The direction of the sun relative to the wall, θ, can be determined as follows: θ = arctan( AB / Nail Length)°" If I label the ends of the nail with points C and D (see figure below) then the formula can be understood as θ = arctan( AB / CD )° [image: Figure1Modified.jpg] With my very rudimentary understanding of basic trigonometry, I understand how the formula would work for a simple right triangle existing in a single plane, but not how it works here. It seems to me that AB lies in an XY plane parallel to the wall, but CD lies along the Z axis, perpendicular to the XY plane. The shape described by ABCD is a sort of twisted rectangle and I don't understand how the formula applies. I'm almost certainly thinking about this wrong (it feels like an optical illusion where I can only see the vase and not the faces). [image: image.png] If anyone can help me "see the light" I would appreciate it. Jeff Brewer
--------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
