Pole - style Sundials

A pole-style sundial has a gnomon which casts a line-shaped shadow on a set of hour lines. Pole-style dials use the hour angle, that is the direction of the Sun in the equatorial plane, which is perpendicular to the gnomon. The tip of the pole-style points to the north celestial pole on the northern hemisphere. Types belonging to this class include the equatorial, vertical and horizontal dials.

Nodal Sundial

In a nodal sundial, the time is read from the shadow of a fixed point or node. It may be the tip of a pin or gnomon. The node may also be a knob on a rod or a notch in an edge. Sometimes the node is an aperture in a plate, casting a spot of light on the dial face.

A combination of pole-style and nodal dials also exist. In this case, the pole-style usually comes with a marker, such as a knob or a notch.

Example

sundial of Catholic University
Figure 19: Catholic University, Nijmegen

An example of a nodal sundial is the sundial in Catholic University, Nijmegan. The gnomon is a tetrahedron. The shadow of the tip of the gnomon indicates the time and the date. The dial works on the principles of the analemma.

Remember that in the section on analemma, we mentioned that if we record the position of the Sun in the sky at the same time everyday, the Sun would take the path of the analemma. In this sundial, the path that the Sun takes every half hourly is computed and drawn on the dial plates. Note that for clarity, the analemmas, which is the path that the Sun takes, have been split and divided over two dial plates. The one on the right is for January to June and the one on the left is for July to December. Date lines are drawn for the first day of each month and for the solstices. The analemmas and times are in red, the date lines and dates in blue. (The colours in Figure 20 are not distinctly shown.)

right dial plate of nodal sundial Figure 20: Right dial plate

The dial for the first half of the year has time lines from 8 to 18 o'clock, of which the last three have not been marked. The date line for the winter solstice has been omitted, as it would almost coincide with the line for January 1.

left dial plate of nodal sundial
Figure 21: Left dial plate

The dial face for the second half of the year has time lines from 8 to 17 o'clock, of which the last two have not been marked.