CDial Theory Plot from Equations PDF Plot Back to irthWorks Page
~ click on a small picture for further details ~
The C-shaped sundial requires a lot of patience to build. The versions shown here benefit from brazing but probably could have been done without it. The glass version, especially, looks artistic, but the main feature is that the solar to clock time discrepancy is handled automatically with no intervention by the user, once the dial is positioned. It does require considerable patience and checking to verify the time will be correct. The approximate date can be read off the shadow.

   As in the other designs, this one is centered on 2pm for daylight savings time (1pm standard) to give 8am to 8pm in the summer, but you can set the scale to whatever 12 hour time span you like. This top view shows all the two dimensional features of the analemma. The biggest problem in reading the time is the ambiguity of the seasons - whether the sundial is leading or lagging. To resolve this problem it is necessary to have a non-symmetry between the lead and lag sides. Just the 14 or 16 minute correction is not enough to be able to tell the difference. Further, depending on the time of the year, all of the analemma shadow is not visible on the time scale. For these reasons, the wire only connects to one side, there is a kink in each end on the centerline, and there is a gap in the center. By looking at the analemma and the legend, and with a little practice, it is fairly easy to tell the correct time.
   The glass beads are to mark the months indicated on the legend. This wire shape was made by overlaying the plot. It is easier to make than it would seem. This winter afternoon view indicates 3:31pm using standard time and the laging edge of the analemma, since the sundial leads the clock time in October. The usefulness of the single sided wire support is apparent.
    Having the dual time scale avoids readjustment when the time is changed between standard and daylight savings. The side view of the analemma shows the slight bending of the loop versus the straight center wire. This is an important detail. It must be bent very slightly by trial and error to eliminate the paralax that could give a significant error around December and June. More details will be discussed under the CDial Theory link. The time is clearly shown to be from 9am to 5pm standard or 10am to 6pm daylight time. The end view shows the time scale to date wire relation. The date wire and the glass cylinder have exactly the same radius, using the inside surface of the joint between the glass pieces. Each piece of glass has a 4:1 ratio, in this model exactly 4 inches by 1 inch.
   A notch was made in the glass pieces, before gluing, to allow the wire to pass through each side. It passes through the left side to give better alignment and strength.
   There are 18 glass segments, each a half hour wide. The analemma can be on the inside or outside of the support wire, but it should be chosen so that the analemma falls exactly at the same distance as the inside surface at the joint of the glass. Doing this set the hour and half hour to be exact and a small error of about two or three minutes at 15 and 45 minutes past the hour in the early morning and late afternoon. There is no problem around the center where the shadow is a perpendicular projection. Because of this, fewer segments will have an error that will approach that being corrected. More segments are a nuisance to build.
   The circle was constructed taping all the pieces together on the outside and standing the pieces on edge. This was then aligned on a piece of paper with a circle drawn on it. It was easier to use all 24 pieces and only glue 16 plus the end pieces later. Modeling clay on the paper allows fine adjustment without shifting during the gluing process. Clear, hard epoxy glue was used, a little at a time. The glass legend was hand drawn by placing the glass over a plot of the analemma shape. In this version, the glass was given a coat of stain glass paint then sprayed with clear acrylic for weatherproofing. It should be noted that most materials do a poor job of standing up to the sun and weather. The positioning is unimportant as long as the mental translation from legend to sundial isn't confusing.
   It is imperative to have a solid support structure, like this stump. Remember, the time should be correct after wind, rain, drought, and freezing weather. It is much more fun to watch the months go by accurately without having to constantly make adjustments. The legend to sundial relationship is shown here. The only problem is in the morning and the evening where the shadow begins to wrap around the time scale and there may be confusion whether the shadow should be mentally flipped.
   The straight center wire of the analemma, which is always accurate solar time, can be used to sight the north star for alignment of direction and elevation. Time can be set the next day by rotating around the axis of the time scale cylinder.
    The support is another of the wire hoops. In fact, this is a little marginal on a windy day since the weight of the glass causes the sundial to wobble slightly. The translucent coating is important to be able to view the time as the shadow wraps around the time scale. The coating should be on the inside to avoid reflections and refractions inside the glass. It was found that some fabric paints are a good combination of opaque and clear for this purpose. In this model, stain glass paint was applied first to give a pleasing color. However, the stain glass paint should be underneath since it does not stand up well to weather. The metal version of the analemma has a slightly different approach to the nonsymmetry problem. A notch was cut in the loops and the smaller loop was joined to keep it from falling apart. The other approach seems to be more visible in the shadow. The metal CDial appears easier to build, but it is surprisingly difficult to maintain as accurate a curvature as with the glass version. It tends to flex a lot and brazing would deform the wire hoop. Since the metal is opaque, the time must be viewed from the sides or back. The legend is placed on each side to help with interpretation. The slight bend in the analemma is visible. Again, the center portion must be straight, and gives accurate solar time. At the beginng and the end of the day, the full width of the time scale is not needed. The metal can be trimmed to improve visibility without any loss in funtionality.