Universal Bifilar Sundial

Bifilar Sundial: invented in 1922 by Hugo Michnik in its horizontal form, although it can be on any plane. The time is indicated by the intersection on the dial plate, of the shadows of two wires (or other lines in space) stretched above and parallel to it. The wires often run E-W and N-S, with their (different) heights above the plane being a function of of the location of the dial. It may have equiangular hour markings, and hence can be delineated to show many kings of hours.

The bifilar sundial with equiangular hour lines can be designed to be universal. With this particular sundial the height "hy" of the north-south wire remains constant. The height "hx" of the east-west wire and the displacement "d" of the east-west wire from the dial centre "C" vary according to the latitude as follows:

hx = hy * sin ø

d = hy * cos ø

where ø is the latitude of location where the sundial is to be used.

The value of "hy" must be chosen carefully in order to maximize the period of time during the year the sundial will indicate the time over the largest range of latitudes. This discussed on the Bifilar Sundial page and depends not only of the height chosen for "hy" but also the the size the dial plate and the starting location of the east-west wire.

Figure 1 shows a drawing of a universal bifilar sundial located in the Northern Hemisphere. Just reverse the numbering of the hour lines for the Southern Hemisphere.

As mentioned above, the height of the north-south wire, once established, does not change. This wire can be mounted on permanent posts on the dial plate. The height and location of the east-west wire varies dependent upon latitude as follows:

• "hx" ranges from 0 at 0º latitude to "hy" at 90º latitude.

• "d" ranges from "hy" at 0º latitude to 0 at 90º latitude.

Figure 2 illustrates the variation in "hx" and "d" with a change in latitude.

Figure 2: Universal Bifilar Sundial East-West Wire Variation

From Figure 2 it can be seen the position of the east-west line follows a semi-circular arc relative to the centre "C" of the sundial. The east-west wire supports must follow this arc to position the wire correctly.