Hi Art ( and the Big Dialers too): No I wasn't kidding, you really do write good answers. Honest!
I've been thinking that, using plywood, I could cut a curved section (20 degrees) of a protractor's scale and tying both ends to the dial's center (w/ two long non-strechy strings), I could rotate this pie-shaped thing, like a drawing compass, about the center to mark the timeline angles. Tom Semadeni did something similar in builing his "Celeste". This rotating protractor section would be very simple and probably easier than the coordinate method. No problems with perpendiculars and measuring tape (which takes two persons). Also the marking of each time lime only involves one measurement instead of two. Thanks to all of you, I think that this thread will be last of my nit-picky questions before I begin designing my big dial. My next step will be to investigate available materials, especially the cable for the gnomon. I will probably end up using high-test stranded stainless steel 1/2"-1" elevator cable. I will also do shadow casting experiments, to determine the maximum size sundial possible using different cable diameters. (The distance from cable to face can't be so large that the umbra of the shadow disappears). I will have to minimize cable sag by using a very heavy counterweight just under the cable's manufacturer's test weight. Once the cable size and the weight of the counterweight are determined, then I can design the metal support structure that will hold the cable and counterweight. And last, but not least, I'll have to find somebody crazy enough who will actually pay me to build this contraption on his property! Thank you all again, John Carmichael http://www.azstarnet.com/~pappas John Carmichael wrote: > >> You know Art, you really write GREAT answers! > >That's probably sarcastic, but my ego refuses to entertain such a thought. >My objective side would advise you to listen to all the advice from others >who have actually built large dials. > >> I thought that I could use either or both of the methods >> described by Mayall >> pg.75-77. I'll call these" the concentric circle method" and" the apparent >> noon method" Wouldn't both of these methods show true north very >> precisely >> if the vertical gnomon was very tall? > >They can. Even here you should consider sources of error. Near the >equinoxes, for example, the change in declination during the course of one >day could be enough to skew your north-south line by a good fraction of a >degree, possibly more than the precision with which your dial can be read. > >> Once I know the true meridian, all of the right angles to it are easy to >> mark using plane geometry. > >In geometry class I learned to construct a perpendicular to a line. The >method can be rigorously proven to be exact, although the lines as they end >up on my paper may not be. I recently learned how carpenters make a large >rectangle: They lay out something that looks about right, then compare the >length of the diagonals, then tweek things until they are really right. The >point is that there may be methods which are not rigorous in the sense of >high school geometry but are easier to use and may even give more >satisfactory results. > >> Well, I will certainly take great pains to see that this does not happen! >> Using water level as a guide, it is possible to produce a perfectly level >> surface. )This is how the egyptians leveled the base of the >> pyramids. If I >> orient the azimuth of my gnomon using Mayall's methods, and I >> know that the >> face is level, then using simple geometry, it is easy to set it at the >> proper angle or height. > >This will work, though it gets complicated if you need a slope for drainage, >your site is deliberately on a hillside, you are installing a dial on a >pre-existing surface, etc. I have given some more thought to the previous >thread involving how to set a sundial using the time method to minimize >errors, but anything like that is probably out of the question for a civil >engineering sized dial. > >> But which day of the year is the best for doing this? Please >> tell me if I'm >> wrong in my thinking, but wouldn't you want a day where EOT=0 because then >> you could quickly mark the timelines by a clock without corrections. (I >> suppose you could also set the clock off by the EOT amount on the day of >> hour line marking). This would give a longitudinally corrected dial. You >> would also want to do it near the solstices when solar declination is >> changing the least. Right? So the best day of the year would be Dec. 25. >> What do you think Art? Is this correct? > >It doesn't seem any harder to me to take measurements at 3 min 46 sec past >the hour rather than on the hour. Why are you worried about the solar >declination? If it's a question of the change of the EOT during the course >of your day of measurements, then December 25 is the worst possible day (15 >sec in 12 hours). > >> >Isn't a giant protractor just a piece of non-stretchy string? >> >> No, a piece of non-strechy string is just drawing compass that hasn't been >> nailed down yet! > >You can tell just how long ago that geometry class was! A curved scale is >hard to make, and I don't see any real advantage over linear measurements. I >would establish a few primary reference points, e.g., with the concentric >circle method or EOT corrected shadow measurements, and then map several >secondary reference points from these using a tape measure. Work down from >there to the smallest scale you need. Always use measurements from at least >three reference points to reduce errors (and also get an idea for how >accurately your work is proceeding). I am presuming that you know enough >analytic geometry to calculate the distances from the reference points and >have a computer to help with the numbers. > >Keep us up to date on any large projects you start! > >--Art Carlson > >
