created by Carl Sabanski
Double Gnomon Polar Sundial
Polar Sundial: is a sundial in which the dial plate is set along the East-West direction and inclines so that it is parallel with the polar axis. The standard polar pointing gnomon is thus also parallel to the dial plate.
There are a number of variations of the polar sundial that can be constructed. For clarity, the variations illustrated are drawn with a wide gnomon. Remember that the shadow will be cast by the inner edge of the the gnomon and so this is the line to use when drawing the hour lines. This edge is the style.
The first double gnomon polar sundial is a simple variation of the polar sundial and is illustrated in Figure 1. Rather than having a single gnomon at the centre of the dial plate, there are two gnomons. These are located at each end of the dial plate. The shadow will shift from one gnomon to the other at solar noon. The direction of the shift depends upon whether the sundial is in the Northern or Southern Hemisphere. In this configuration the shadow will cross into the opposite half of the dial plate as 6 a.m. and 6 p.m. are being approached.
Figure 1: Double Gnomon Polar Sundial - Type 1 (CAD)
The second alternate method of construction is called the stacked double gnomon polar sundial and is illustrated in Figure 2. This variation results in a polar sundial half the width of the one shown in Figure 1. This configuration allows you to construct a polar sundial with smaller time increments within a narrower width than would be required with the single gnomon polar sundial. The layout of the hour lines for this sundial is the same as that in Figure 1. If the width of the dial plate is doubled it is possible to include an extra half hour in the morning and evening. It is even possible to carry the hour lines up the gnomons to have the sundial indicate the time from 6 a.m. to 6 p.m.
Figure 2: Double Gnomon Polar Sundial - Type 2 (CAD)
The third double gnomon polar sundial presented here is special in a couple of ways. The first is the sundial's hour lines are carried up the side of the gnomon and therefore allows the sundial to show the time from 6 a.m. to 6 p.m.. The second is, due to the specific relative dimensions of the dial plate width to the gnomon height, there are a number of hour lines that come into alignment. Figure 3 shows a double gnomon polar sundial with a dial plate width "W" and gnomons with heights half the dial plate width or "W/2". The time telling shadow is cast upon the dial plate by the inside edge of the gnomons. This is illustrated in Figure 3 for the right gnomon at 15 minute intervals. Notice how the shadow moves up the left gnomon. Each full hour line is displaced by 15°.
Figure 3: Double Gnomon Polar Sundial - Type 3 Dimensions (CAD)
In Figure 4 the hour lines from the second gnomon are introduced. Notice that the morning and afternoon hour lines coincide at the three points marked "X". The times of coincidence are 8 a.m. / 1 p.m., 9 a.m. / 3 p.m. and 11 a.m. / 4 p.m.
Figure 4: Double Gnomon Polar Sundial - Type 3 Common Hour Lines (CAD)
To lay out the hour lines on a sheet of paper fold down the two gnomons bringing their hour lines with them. This is shown in Figure 5. It is not difficult to lay out this sundial graphically.
Figure 5: Double Gnomon Polar Sundial - Type 3 Layout (CAD)
Figure 6 illustrates this special double gnomon polar sundial. The morning and afternoon hour lines are stacked to make it easier for the time to be read. The times of coincidence are clearly shown in the drawing.
Figure 6: Double Gnomon Polar Sundial - Type 3 (CAD)