I was throwing out a junk CD recently when I was struck by the beauty of the coloured patterns of reflected diffracted sunlight. Being curious I got out a protractor and measured the angles for different colours at different orders of refraction. Revisiting an old Physics text and visiting a few web sites soon refreshed the science behind the phenomenon.
Being a true gnomonisist, I then started to wonder if this phenomenon could be used as a modern sundial to tell time. The basic concept soon evolved: 1. A CD lying horizontal in the sunshine reflecting the diffraction pattern to a fixed viewpoint (screen or eyepiece), 2. Rotation of the viewpoint around the circular CD to adjust for azimuth, 3. Telling time from the altitude measured from the diffraction pattern. The hours would be determined from the refraction orders and minutes by the specific colours. What a wonderful invention! Rube Goldburg (Heath Robinson) strikes again. I have not reduced it to practice. The shadow from a simple stick or the un-refracted reflection on the ceiling would provide more accurate monochromatic data reducible to time. The classic Shepherd's Cylinder does the job very well. But the challenge is there. Who is going to build the first technicolour sundial utilizing the low cost diffraction gratings available on old CDs? For background start with <http://129.82.166.181/CD_Spectroscope.html> for the University of Colorado's "Little Shop of Physics". Roger Bailey Walking Shadow Designs N 51 W 115
