[EMAIL PROTECTED] (John Carmichael) writes: > Thanks for taking the time to explain the Dali dial to us gnomonistically > challenged dialists. I think I'm beginning to understand it, but will have > to think about it some more. What threw me off was that I was thinking that > a Dali dial would be draped over the edge of a table!
It could be, hence the thread title. Perhaps it's best to start by thinking about a "normal" sundial with a horizontal plate and a paraxial gnomon. Design such a dial to read clock time without any EoT correction. Then design another dial to read clock time plus 5 minutes, another to read clock time plus 10 minutes, and so on for +15 min, -5 min, -10 min, and -15 min (always without any EoT correction). Now, to read the time from such a dial you don't need the whole plate. Any ring will do. So cut a ring out of each of the 7 dials you just made, but of different radii so that the rings do not overlap. Properly position all the rings around a single gnomon. Now if you want a +5 min correction, you just read the time from the +5 min ring. For a -10 min correction, use the -10 min ring. Label each ring with the dates that its correction is valid, and you have a version of my sundial. At any given hour, the shadow will always point in about the same direction, but will be shifted right or left a bit due to the EoT. If you move away from a paraxial gnomon, the concept becomes a bit trickier because a plate with radial hour lines will only be accurate for a particular declination of the sun, that is, a particular date. But, look! You are only using each ring for a particular date anyway, so it doesn't matter. Someplace around here we realize that most of what we have learned about sundials doesn't matter any more either. The gnomon could be bent. The plate could be warped. You just have to make sure that the shadow of the gnomon always crosses the date line at exactly one point, and that this point is never in the shadow of something else (like a fold in the plate). I used this extreme flexibility to propose a sundial with date lines in the shape of Arizona, but only after I checked a map to see that Tucson was conveniently located (somewhat south of the middle) and that no line radiating from Tucson intersects the border more than once. So. I think you have caught on to the concept by now. One thing I like about some dial designs is their inevitability. The plate *must* be horizontal if it is to always be illuminated when the sun is above the horizon. The gnomon *must* be paraxial to minimize the difference to clock time over the course of the seasons. This concept, in contrast, has very few constraints, although it is in a sense more accurate than the conventional form. How can this freedom best be used? Can we require the dial to do something new, that other dials can't? Is there an especially esthetic form? Have fun with it. Art
