HI all: In the new edition of the BSS Bulletin (vol. 12, Feb. 2000) on page 51, there is a sketch of an azmuthal (Dali) sundial by John Singleton titled: "A Horizontal Dial with Equation of Time Built in".
I see a couple of differences in this drawing and similar dials previously described by Art Carlson and Fer J. DeVires. Note that the order of the 12 concentric date rings, from center to perimeter, is different. In Singleton's version they run from April in the center to March on the perimeter. In Fer's version, they run from December in the center to January on the outside. I might add that in Mayall pg. 181 there are only six date rings with each ring serving for two months with December in the center and January on the outside. Apparently, ring order and number of rings are not important. Am I correct? Also, Singleton mentions that he uses an inclined gnomon equal to the latitude, whereas Art, Fer, and Mayall suggest using a vertical rod of any height with no nodus instead. I can easily understand how the dial works using a polar axis gnomon. But I can not understand how a vertical gnomon would work. Wouldn't a vertical rod have to have a nodus of specific height and be located north of center to work (with the nodus lying on an imaginary polar axis style) Apparently not, as Fer, Art, and Mayall place the rod in the center. I know we discussed the Dali azmuthal dial before, but the type of gnomon one can use still confuses me. How does a vertical rod with no nodus and no set height, function just like a polar axis gnomon? Can anyone explain this so that my thick head will understand? Thanks, John Carmichael
