On Thu, 3 Jan 2002, John Carmichael wrote: > All of these ideas using lasers, auxiliary sundials and transparencies all > sound like they would help in drawing the sundial face, but I don't think > they will precisely place the dial drawing on the ceiling or help to > correctly align a declining or inclining mirror. > > I think you all are minimizing the importance of the "optical lever" effect. > Very slight errors in placing the transparencies, lasers and auxillarily > sundials can cause the sunspot on the ceiling to be way off.
All quite correct, as you put it, John. However, as I understand how John Lynes suggestion was meant, the mirror becomes an alignment and dial placement tool, not a critical design element: It seems easier to lay out and correctly orient an auxialiary dial, than to design and lay out a ceiling dial. The laser/mirror projection technique was suggested as a way of copying that auxiliary dial onto the ceiling, permitting the designer to place the resulting lines in a pleasing and useful manner by adjusting the mirror before finally fixing it in place. The optical lever effect is certainly a significant problem to consider, and we haven't addressed how to manipulate and eventually mount that little mirror, but I would imagine walking through the dates and times I wanted to see projected, pencilling the major points on the final surface, until I had the layout I wanted. Lock the mirror in place, then run through to re-check the placement. If necessary, shim the mirror, or just use it's final position to re-project the dial points, before painting the lines or whatever... Dave 37.29N 121.97W
