On Tue, 30 Jul 2002, Peter Tandy wrote: > Mike Shaw's method sounds too wonderfully simple to be true. However, I > would perhaps suggest one small change if this is to apply to distant > 'clients' ordering dials (which is how I seem to remember this thread > starting!). I would supply paper to which was attached a square or thin > strip of reflective 'melinex' or similar plastic. How many people > necessarilly have a plain mirror (or is that plane mirror) to hand or even > know what is meant by the term?
A piece of white card stok or posterboard would probably be better than (thin) paper. I assume "Mylar" is yank for "Melinex" - a thin, metallized, tough plastic film, often used for helium balloons. > One point: I assume one has to look for the reflection at 90 degrees to the > plain of the paper (this may be a naive point to ask, but I'd just like to > be sure) > > Peter Tandy If I understand the method correctly, that's (semi-) automatic: Looking at the card, with the string's shadow on it, you can always move your head until the string lines up with its shadow; this puts your eye in line with the Sun, but not necessarily on the same azimuth as the card surface. Likewise, you can always move your head so the string coincides with its reflection in the mirror; this puts you at the correct azimuth, but of course, not necessarily in line with the Sun. When both criteria are met, at that time, both you and the Sun are lined up with the surface... Should give very good accuracy, for azimuth/dial declination. It does require someone to be there at the moment the Sun is shining full on to the surface, and for the Sun to be shining. A potential client who works full time might need to wait for a weekend day when there are no clouds at just the right time. A method that plots a shadow against a grid or protractor and notes the exact time can be used any time there is clear sunlight on the surface. Can this be extended, to give the surface's inclination, at the same time? Perhaps a small bead or simply a knot, could be lined up in the mirror at the same time. This would put the user's eye on a true normal to the surface, and on the Sun's azimuth. Making a pencil mark at the point where the bead's shadow falls would record the Sun's altitude, relative to the card surface. You would need somehow to ensure the bead is reflected from a know point in the mirror, however. Maybe a pair of crossed lines drawn or scribed on the mirror would work. The user then raises or lowers the plumb line, so the bead and its reflection are centered visually onthe cross mark, and waits for the shadow to advance to match. Dave 37.28N 121.97W -
