Azimuth Lines

Azimuth (of the sun) (A, AZ): the angle of the sun, measured in the horizontal plane and from true south. Angles to the west are positive, those to the east, negative. Thus due west is 90°, north is ±180°, east -90°.

Compass rose: a drawing of the compass directions, showing as a bare minimum the cardinal points, but more usually eight, sixteen or thirty-two points.

To show the sun's azimuth or direction on a horizontal or vertical direct south dial is easy. In order to not clutter the dial face, which may lead to confusion in reading the time, a separate portion of the dial plate may be used to indicate the sun's azimuth.

In the case of a horizontal dial, a compass rose can be used to tell the sun's direction. A vertical pin is used to cast a shadow that will indicate the sun's direction. It is located at the center of the compass rose. It is important to note that the azimuth points on the compass rose are labeled in reverse to the actual directions. For example, when the sun is due east, the shadow of the pin will fall to the west. Figure 1 illustrates the layout of a compass rose for a horizontal sundial. Each of the points shown is separated by 22.5°. As mentioned, the compass rose could be located anywhere on the dial and at any appropriate scale. It could be incorporated into the dial face if that was desired. The design of a compass rose is only limited to your imagination and could be a unique feature on your dial.

Figure 1: Compass Rose - Horizontal Sundial (CAD)

For a vertical direct south dial, the sun's direction is shown by a series of parallel lines. When the sun is directly east or west of the dial plate it will be parallel to the plane of the dial plate. The only time the sun can produce a shadow is when it is south of the plane of the dial.

Figure 2 illustrates a device that will show the sun's azimuth on a vertical direct south dial. The drawing shows the view of the dial looking vertically down from the top. A horizontal pin is located on the dial plate. As the sun moves from east to west, the tip of the pin will cast a shadow on the dial plate. In the morning, when the sun is to the east, the shadow will move on the west side of the dial plate. At solar noon, the sun is on the meridian and the shadow will point down. The sun is directly south. In the afternoon the shadow will move along the east side of the dial plate. The distance of the tip of the shadow from the vertical south line for any given azimuth is determined as follows:

Distance = Pin Length x tan (Sun's Azimuth)

Figure 2: Azimuth Device - Vertical Direct South Sundial (CAD)

This distance will be equal for equivalent compass points east and west of the pin. A series of vertical lines will be created to indicate the sun's direction. The spacing of the azimuth lines, assuming a unit pin height, is given by the following table.

 Table 1: Azimuth Lines Spacing

To determine the actual spacing from the south line to any azimuth line just multiply the actual pin length by the "Azimuth Line Spacing" factor.

Figure 3 illustrates the layout of the azimuth lines for a vertical direct south dial. The pin length selected requires some consideration. The length of the shadow will vary and if the pin is too long the tip of the shadow will move off the azimuth scale or perhaps even off the dial plate. 

Figure 3: Azimuth Lines - Vertical Direct South Sundial (CAD)