Declination Lines

Declination Lines: lines on a dial showing the sun's declination on a particular date. They are read by observing the shadow of a nodus.

Declination (of the sun): the angular distance of the Sun above or below the celestial equator. Its value follows an annual sine wave like curve varying between 0° at the equinoxes and ±23.4° (approx.) at the solstices. It has positive values when the Sun is above the celestial equator (summer in the Northern hemisphere) and negative when below.

Nodus: a point which casts a shadow to indicate the time and/or date on a dial face. It may take the form of a small sphere or a notch on a polar-pointing gnomon, or it may be the tip of a gnomon with an arbitrary (usually horizontal or vertical ) orientation.

Horizontal Line: a line drawn on a dial plate at its intersection with a plane which passes through the nodus and is parallel to the plane of the horizon.

A declination line traces the path the sun takes during a particular day of the year. They can therefore be used to indicate the date. The solstices form the two extremes for these lines and at the time of the equinoxes the path is a straight line and runs east-west for a number of dials. The declination lines are traced by a nodus. This can be a vertical pin for horizontal and polar dials and a horizontal pin for vertical dials. The shadow point of the nodus will always stay within the two extreme declination lines.

The sundial can be simplified by showing only the three declination lines that indicate the dates of the solstices and equinoxes. If you wish you can include declination lines that show the sun's entrance into the other signs of the zodiac. This results in seven declination as shown in the figures below.

A declination line can be drawn for any day of the year. A sundial can be personalized to indicate a special event such as a birthday, anniversary or any special date.

The following figures show various sundials with the seven declination lines included for a common nodus height. The layout of the declination lines will be affected by the height of the nodus. This is illustrated by figures in the pages "Horizontal Dial" and "Vertical South Dial". 

Figure 1: Horizontal Sundial (SHADOWS)

For a horizontal sundial the summer solstice is indicated by the declination line nearest to the gnomon and the winter solstice by the furthest declination line. A vertical rod nodus would be located at point "A".

Figure 2: Vertical Direct South Sundial (SHADOWS)

For a vertical sundial the summer solstice is indicated by the declination line furthest from the gnomon and the winter solstice by the nearest declination line. A horizontal rod nodus would be located at point "A".

Figure 3: Vertical Direct East and West Sundials (SHADOWS)

Figure 4: Polar Sundial (SHADOWS)

The sundial can be made more useful by including the horizontal line. This line cannot be drawn on a horizontal dial as the plane passes horizontally through the nodus, is parallel to the dial plate and will not intersect it. This line can be drawn on vertical dials and the polar dial. As this line represents the horizon the declination lines do not extend beyond it. Actually, the time of sunrise and sunset can be determined from the points where the declination lines intersect the horizontal line. The following figures show the horizontal line for the dials in the figures above.

Figure 5: Vertical Direct South Sundial (SHADOWS)

Figure 6: Vertical Direct East and West Sundials (SHADOWS)

The vertical direct east dial will show the time of sunrise and the vertical direct west dial will show the time of sunset.

Figure 7: Polar Sundial (SHADOWS)