Cross Sundial

Cross Sundial (or cruciform or crucifix dial): a dial in the form of a cross, usually of stone, with the "front" surface of the cross parallel to the equatorial plane, and the top pointing south. The side surfaces of the cross can each form a dial plane, with its gnomon being a corresponding edge of the cross itself. Usually found as churchyard memorials. Rare. Portable cross dials have a long arm parallel to the polar axis and the short arm E-W. 

Figure 1 illustrates a cross sundial. The sundial is independent of the latitude and can be used anywhere. The top face of the cross 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: Cross Sundial (CAD)

Figure 2 illustrates the layout of the hour lines for the cross sundial. This sundial consists of 5 cubes and each of the 6 dial planes will show 3 hours. An extension of one arm is shown as dashed lines. The hour lines are laid out as for either a polar, vertical direct east or vertical direct west sundial.

Figure 2: Cross Sundial Hour Lines (CAD)

The length of the shadow at any hour angle is calculated by taking the tangent of the hour angle. Table 1 provides the shadow lengths for hour angles at 15-minute intervals. As these values are for a cube with an edge length of one unit, the actual shadow lengths are determined by multiplying the actual length of the edge of the cube by each of the values in the table.

Table 1: Cross Sundial Hour Line Distances

For an image complete with shadow click here.