Capricorn: Dec.22-Jan.29 The Sundial Primer
created by Carl Sabanski
Capricorn: Dec.22-Jan.29

The Sundial Primer Index

Finding East/West

North: the intersection of the local meridian with the horizon, in the direction of the north celestial pole.

South: one of the cardinal points of the compass, in the direction opposite north, in the direction of the south celestial pole.

East: the point on the horizon 90 (measured clockwise) from the North. The Sun appears to rise from the East point on the equinoxes.

West:  the point on the horizon 90 (measured anti-clockwise) from the North. The Sun appears to set at the West point on the equinoxes.

Meridian: the great circle (or, more usually, half of a great circle) passing through the North and South poles. The same as a line of longitude. The term is sometimes used to mean the meridian line (local) passing through the observer's location, or its representation on the dial face.

The first step to correctly positioning many sundials is to establish the direction of true North in the Northern hemisphere and true South in the Southern Hemisphere. Establishing this meridian line is adequate if you are locating a small sundial such as a horizontal sundial. If you are construction a larger sundial, a monumental sundial for example, it will be necessary to find the east/west line. This line lies perpendicular to the meridian line. It is likely that you will also want to locate it at the centre or origin of the sundial.

If you are fortunate enough to know someone with the necessary surveying equipment the east-west line can be established by turning the angles from the meridian line. If not, there are a couple of methods that can be used to do this.

1. The "3-4-5" Triangle Method.

Figure 1 illustrates the "3-4-5" triangle. The relative dimensions of this triangle makes it simple to calculate the dimensions of the remaining two sides when a value is applied to one. The table in this figure provides calculated values that are ready to be used based on pre-assigned values for the "4" side. The multiplier for the "3" side is 0.75 and for the "4" side 1.25. The lengths can be any dimension type you choose; millimetres, centimetres, inches, etc..

Figure 1: The "3-4-5" Triangle

Figure 1: The "3-4-5" Triangle (CAD)

Figure 2 illustrates how to use the "3-4-5" triangle to lay out the east-west line. By using the 4-step process shown, 4 points will be established along the east-west line. For example, in Step 1 measure up the meridian line a distance equal to the selected "4" side measurement. Using two tapes set the zero of one at this newly established point and the zero of the second at the origin "O". As you move along the east-west line keep these tapes crossed and note their readings. When the tape starting from "O" reads the selected "3" side measurement and the tape from the meridian line reads the selected "5" side value, mark the point A. Repeat this procedure for Steps 2, 3 and 4 but with the triangle rotated to the positions shown. The five points A, B, C, D and O should be located on a straight line.

Figure 2: Finding the East-West Line - "3-4-5" Triangle Method

Figure 2: Finding the East-West Line - "3-4-5" Triangle Method (CAD)

The north-south or meridian line may need to be extended below the east-west line. This would be the case for a horizontal or human (analemmatic) sundial if the sundial is to show times before 6:00 a.m. and after 6:00 p.m.. If the "3-4-5" triangle method is used the two points that will be established should be in line with the existing meridian line. This would make a good check of the layout if you are willing to spend the extra time.

2. The Egyptian Circle Method.

The Egyptians were quite particular in the positioning of their pyramids. They not only established the local meridian line but also the east-west line. They used a simple method that required the drawing of two circles.

Figure 3 shows how to use this method to find the east-west line that passes through the origin "O" located on the north-south or meridian line.

Figure 3: Finding the East-West Line - Egyptian Circle Method

Figure 3: Finding the East-West Line - Egyptian Circle Method (CAD)

Determine the radius "r" of the circle that will be drawn. From "O" measure the distance "r/2" to the north and south and place these points on the meridian line. The two points are the centres of the two circles. Draw a circle of radius "r" from each of the two centre points. The circles will cross at two points. Join these two points with a straight line, which will pass through "O". This is the east west line. This technique can be repeated with various values of "r" to accurately determine the east-west line.