Star of David Sundial

Star of David Sundial: a monumental dial similar in concept to the cross dial, except that the six-pointed star with 60° angles gives opportunities for numerous dials. 

Figure 1 illustrates a Star of David sundial. The sundial is independent of the latitude and can be used anywhere. The top face of the star faces north and is tilted back toward the south until it lies in the plane of the equator. The angle of the top face to the horizontal is equal to the latitude. The edges that act as the styles will then be parallel to the earth's axis and point to the north celestial pole.

Figure 1: Star of David Sundial (CAD)

Figure 2 illustrates the layout of the hour lines for the Star of David sundial. This sundial is made of six equilateral triangles set on the sides of a hexagon. The sides of the triangles are equal and the angle between any two sides is 60°. Each side of the star will show the shadow for two hours and as the shadow leaves one side it appears on another. This dial has four blank faces as the sun will will not reach them at lower latitudes. At very northerly latitudes the sun will reach all twelve faces and these faces could have hour lines assigned to them.

Figure 2: Star Sundial Hour Lines (CAD)

The faces of the dial are numbered in the order that the shadow will pass from one face to another, staying on a given face for two hours. Note the following:

• Adjacent faces on the star alternate between morning and afternoon hours.

• Faces that are six intervals apart carry hour lines that are twelve hours apart.

• When the shadow leaves any face it immediately reappears on the face five spaces away in a counterclockwise direction.

• On some faces the shadow starts at the tip and moves inward toward the obtuse angle and on others it moves out toward the tip.

• When the shadow moves inward on any face it moves outward on the two adjacent faces, and vice versa.

Figure 3 will help to illustrate how to determine the distance to the hour lines.  The following is based on equilateral triangles with face dimensions of one unit, i.e. E = 1. The hour line distances start at the inner obtuse angle on any face and move towards the tip of the triangle.

Assume the sun is directly south of the dial. The shadow will be moving on to face 5. The sun will move 30° west before the shadow moves on to face 6. This is equivalent to 2 hours. For the next 30° of movement the sun will cast a shadow on face 6.

Figure 3: Star Sundial Layout (CAD)

The height of the shadow casting edge is 0.866. D is the distance the shadow would move across if it were not for the point of the star. The distance the shadow moves along face 6 during its 2 hour visit can be calculated as follows:

Shadow Length = D - 0.5 = [0.866 x tan (h)] - 0.5

where h is the sun's hour angle.

Table 1 provides the length of the shadow along face E at 15-minute intervals. As these values are for a star with a face length of one unit, the actual shadow lengths are determined by multiplying the actual length of the face of the star by each of the values in the table.

Table 1: Star Sundial Hour Line Distances

For an image complete with shadow click here.