Equatorial Ring Sundial

Equatorial Sundial: a dial in which the dial plate is parallel to the equatorial plane and the polar-pointing gnomon is perpendicular to it.

Equatorial Plane: the plane through the Earth defined by the equator.

The equatorial sundial described on the previous page consists of a flat dial plate that lies parallel to the equator. The sun shines on the top of the plate during the summer months and on the bottom during the winter. It requires two sets of hour lines to indicate time throughout the year. If the flat dial plate is replaced with a narrow ring upon which the hour lines are inscribed, it can be easily read throughout the year. This ring represents the equator.

Figure 1 illustrates the template for an equatorial ring dial. The full hour lines are spaced at 15° intervals around the ring. The angle of intermediate hour lines can be easily calculated.

Figure 1: Equatorial Ring Sundial Template (CAD)

If the ring is laid out as a flat rectangular plate, the hour lines are equally spaced apart. Knowing the inside radius "r" of the desired ring dimension the distance between full hour lines is given by (p /12)r. The distance to intermediate hour lines can also be calculated. Once the hour lines and numbers are inscribed the plate can be bent to the correct diameter. The gnomon passes through the centre of and parallel to the ring.

For the equatorial sundial to be used at any latitude the user must only ensure that the gnomon points to the celestial pole. To do this the dial plate, in this case the flat surface of the ring,  must be positioned so it is at an angle with a horizontal surface equal to the latitude.

The sundial can be rotated to account for the Equation of Time, longitude and Daylight Saving Time corrections. To make these corrections the sundial must be rotated around the polar axis, i.e. the gnomon. The gnomon must always point to the celestial pole as the sundial is being rotated. This would result in some awkward mounting positions for the sundial. To avoid this the sundial can be designed in a way the equatorial ring can be rotated around the polar axis independently of the rest of the dial. Such a sundial is permanently mounted in the correct position for its location and the equatorial ring is rotated the required amount to correct for any or all of the above.

If the width of the ring is increased it is possible to include the analemma with the hour lines. The sundial would still be universal but if it were positioned to face south it would indicate local mean time, solar time corrected for the Equation of Time.  It would only require longitude correction to determine clock time. If the dial was rotated it could be corrected for longitude and would indicate clock time directly.

The hour lines can also be adjusted on the dial ring to correct for longitude. With the analemma also inscribed on the hour lines the sundial would indicate clock time when it was positioned correctly. This is no longer a universal dial and is location dependent in its design.

If numerous analemma are placed on the ring the sundial, unless it is very large, becomes crowded and difficult to read. Two rings could be made, each having a half analemma for each half of the year. This would require a design that allowed the rings to be replaced twice a year.

The sundial shown below uses a rod for the gnomon. If the gnomon is a wire, the sundial is called a bowstring equatorial.

For an image complete with shadow click here.