ODial Theory Plot from Equations PDF Plot Back To The Main Page
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The O-shaped sundial requires some mechanical skill for building. The version shown here required drilling and wire bending. The feature is that the time can be read at a glance with more than 10 hours of usable time.

   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.

    The main difficulties with the design are finding a suitable material for the translucent inner circle, and the necessity of having to flip the analemma scale twice a year. The analemma is a folded and flipped plot, described in detail in the lower right link on Theory. As shown here, it is a piece that can be snapped into place on the fixed inside ring. It was made from regular paper plotted with an inkjet printer. The metal shelf was coated with clear epoxy, the plot aligned and smoothed on top, then coated with the clear epoxy. This gives the weatherproof protection. The wire clips were made from galvanized wire and epoxied in place. They snap into holes in the fixed inner ring. The time gnomon is connected to the top movable ring through a hole and epoxied. The wire is bent to move along the surface of the plot to reduce paralax problems. Although probably not necessary, the inner fixed ring was ground to provide a path for the time wire. This allowed the maximum usable shadow around the equinox. Since there is little error at this time, you may not care.
    The round time scale with the gnomon on the outside provides a pleasing, uniform time scale which is easily marked hours or half hours. A circle can be drawn around the inside of the time circle. A perpendicular line at the center of a chord of the circle will pass through the center of the circle. Using a drawing compass set to the radius of the circle will mark off 60 degree segments on the circle circumference. The chords between these segments can be bisected and a line from the circle center through the chord center will give 30 degrees - the hour markings. Continuing, will give half hour markings. ///////These two wires were again brazed in place, but it is possible that adaquate strength might be achieved by drilling a hole and using epoxy. In some ways this would be easier to align since the middle section of the wire could be cut off afterwards. The one side has to be quite strong, however, since it provides the support. The version here was done by carefully measuring the distance from the center of the date shadow wire to this wire circle and clamping before brazing.
    For the summer, the time shown here is about 12:25pm since it would be daylight savings time. Having the dual scale avoids readjustment when the time is changed. The shadow from the date ring above the plot casts a shadow on the gnomon time wire and the date scale. The outer ring is adjusted so the two align. This corrects for the solar time to clock time discrepancy. At each equinox, the scale must be flipped to the other side so that the scale doesn't cast a shadow on the time scale and yet does receive a shadow from the date ring.
    The radius was chosen to be one half of the time gnomon.(??) Again, any radius could have been chosen, but this gives a reasonable plot size and looks attractive. As discussed in the "Analemma Plotting" link, there is no relation between this radius and the time scale lengths.(?) This summer view shows the shadow falling on the front surface of the translucent inner circle. The usefulness of this circle can be seen by the way it leads the eye toward the time scale. Without this it would be necessary to find a small shadow on the outer metal ring. Even if the metal ring is painted white, it is is difficult to see the shadow, especially early and late. The pieces are shown here before assembly. The outer ring simply sits on top of the inner time scale ring. To fit well, the outer ring, probably cut from a similar ring as the inner one, must be bent open slightly. The date wire ring is inserted into small holes drilled in the outer ring and epoxied. Epoxy is certainly useful.
   It is imperative to have a solid support structure. 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. This is another view of the holes in the outer ring. The cut in the circle is to allow clearance for the analemma scale. The drilled notch in the center is not needed but a good mark of the center of the analemma scale. Notice the paint on the top of the ring. Without this, there is not enough friction to keep the outer ring from slipping in the wind. The winter view shows the requirement for the translucent circle. The shadow cast by the time gnomon wire is cast on the back side, and would otherwise be invisible without walking around to the back. For this model, the paper was sprayed with clear acrylic spray. Using an epoxy coating does not give good shadow properties. A good alternative might be a sheet of plastic quilting template. It is not know how this would hold up to UV and weather.