Equatorial 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.

Equinoctial Sundial: another (historical) name for an equatorial sundial.

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

The equatorial sundial is the simplest of all the sundials. It can be made without mathematical calculations and can be used at any latitude. It can be used to determine the hour lines for most other types of sundials when using graphical design techniques.

The sundial has hour lines equally spaced at 15° intervals around the gnomon. Figure 1 illustrates a template for the equatorial sundial and gives 5, 10, 15, 30 ad 60 minute intervals.  Click here if you would like to download this template in PDF format.

Figure 1: Equatorial Sundial Template (CAD)

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 must be positioned so it is at an angle with a horizontal surface equal to the co-latitude (90° - latitude). Figure 2 illustrates how the equatorial sundial must be positioned.

Figure 2: Equatorial Sundial

As the dial plate is parallel to the equator, there are periods of time when the sun is above the dial plate and others when it is below. From the autumnal equinox until the vernal equinox, sunlight will fall on the underside of the dial plate. To use the sundial during this period the gnomon must project below the dial plate as shown in Figure 2. The hour lines must appear on both sides of the dial plate. The numbering on the top of the plate will run clockwise while those on the bottom will run counter-clockwise. The sundial will show the time from sunrise to sunset.

Figure 3 shows the top and bottom dial plates for an equatorial sundial in the Northern Hemisphere. The upper dial plate will show more hours as it is illuminated between the spring and fall equinoxes. The bottom dial plate will show less hours as it is illuminated between the fall and spring equinoxes. For the Southern Hemisphere just reverse the hour numbers.

The sundial plate can be rotated to account for the Equation of Time, longitude and Daylight Saving Time corrections. 

As mentioned previously the equatorial sundial can be used to create many other sundials. Figure 4 illustrates a number of sundials and how, through the projection of the hours lines of an equatorial sundial on to various surfaces, they can be drawn.

How to draw a variety of sundials, with the use of an equatorial sundial, is discussed on the "Long Hand Sundials' page.