On Tue, 15 Feb 2000, Steve Lelievre wrote: > From: "John Carmichael" <[EMAIL PROTECTED]> > > >...<cliipped> > >Apparently, ring order and number of > > rings are not important. Am I correct? > > You build your dial with an azimuth scale forming a circular > arc, like a giant protractor. Since the length of the shadow has no bearing > on the azimuth, so the radius of the protractor does not matter. For > convenience, instead of marking the scale with degrees you mark the > corresponding hours. > > But the azimuth for a given hour varies throughout the year so you have to > have lots of individual protractors, each one giving a time scale for one > day.
Very clear and concise, Steve! As this thread wound on a while back, I thought of a dial based on the vertical style design that has been mentioned, but with a single, rather "fat" annular ring, good for one month, with lots of resolution from the inner to outer radius. Manufacture the base with a keyed groove into which the user can drop a single "washer" with the current month's scale. Print front and back faces, so only six scale disks are required. Another option would be to make the base of the (thin, circular cross section) style larger and keyed (isoscles triangle, perhaps?), and tall enough to stack six scale disks. The first of each month, lift the stack, slip the top disk off, invert, and put on the bottom of the "deck", than drop back on the spindle... > We don't have to indicate the date by varying the radius. I have designed a > dial with a vertical style, where the dial face is an upright semi-cylinder. > The date is selected by varying the height from the ground at which the > shadow is read. Then the time is read from the shadow angle. > > Steve I like that! Also leads me back to an old design I worked out some years ago. I imagined four thin, flat, rectangular vanes, crossing at a common center, giving it a '+' cross-section. Tilt it so the long axis is polar, and rotated on that axis, so that one pair of vanes is in the N-S plane. Each edge then casts a straight shadow on one flat surface for a 3-hour period, and the shadow spacings are not too far from linear (as they would be on a flat polar dial, as the hour angle gets much more than 45 deg, Pi/8 rad). >From this Dali Dial work, I realize that there is no good reason to have the axis tilted, and that I could curve the hour lines as in your semi-cylinder dial. However, I can't immediately see how to keep the shadows of straight vane edges from falling off the faces of the (equal-width!) target vanes, when the Sun is ahead or behind mean time. Maybe cutting the vane edges in an analemmic (is that a word?) profile, for straight shadow paths, once again calibrating height vs. date? Or something like that! Dave
