The Wide Gnomon

Noon Gap / Gnomon Gap / Split Noon: the gap in the hour scale of a dial to account for the finite thickness of the gnomon. It is positioned on the dial plate where the Sun is in the same plane as the gnomon for horizontal or direct south dials.

Normally the initial design of the sundial's hour lines is performed with the assumption that the gnomon has virtually no thickness. In the case of a horizontal or vertical dial for example, all the lines originate from a single point at the dial centre or origin. This is illustrated in Figure 1.

Figure 1: Horizontal sundial with a narrow gnomon. (ZW2000/CAD)

The construction of a sundial requires the attachment of the gnomon to the dial plate.  As a result the gnomon will now have a finite thickness as illustrated in Figure 2. There will be two dial centres and double noon lines. The double noon lines are spaced a distance equal to the thickness of the gnomon and this space is known as the noon gap. If the thickness of the gnomon is ignored and the hour lines originate from a single dial centre there will be an error introduced into the reading of the sundial. This error will become greater as the thickness of the gnomon is increased. 

Figure 2: The parts of a sundial - Northern & Southern Hemispheres.

Compensating for the thickness of the gnomon is not difficult.  It is necessary to give the dial two centres that are spaced a distance equal to the thickness of the gnomon.  Then, depending upon the hour line being drawn, the hour line will originate from the appropriate dial centre. Figure 3 illustrates the horizontal dial shown in Figure 1 designed to accommodate a wide gnomon.

Figure 3: Horizontal sundial with a wide gnomon. (ZW2000/CAD)

Figure 3 illustrates the fact that the shadow falling on the dial plate from a wide gnomon will originate from a different edge at various times of the day. Before 6:00 a.m. "local apparent time” (LAT), the shadow will be cast from the east edge of the gnomon; between 6:00 a.m. and noon LAT, the shadow will be cast from the west edge of the gnomon; between noon and 6:00 p.m. LAT, the shadow will again be cast from the east edge of the gnomon and finally after 6:00 p.m., the shadow will again be cast from the west edge of the gnomon. Noon LAT will be cast from both edges of the gnomon and, therefore this is where the noon gap is applied.

The correction for a wide gnomon can be applied to other dials such as vertical and polar dials. Figure 4 shows a vertical dial with a wide gnomon but where the hour lines have also been corrected for longitude.

Figure 4: Vertical sundial with a wide gnomon and longitude correction. (ZW2000/CAD)

Figure 5 shows a series of shadows on a horizontal sundial with a typical gnomon over the span of the day. The change from one shadow casting edge to another can be seen.

Figure 5: The Changing of the Edge - Typical Gnomon

It is possible to create a gnomon that does not interfere with the hour lines. This gnomon is illustrated in Figure 6 and its base is located outside of the hour lines. The shadow casting edge for this gnomon is opposite to the one described above as the day passes. This results in some of the hour lines around noon sharing the same space. Rather than having a "noon gap", the sundial has a "noon overlap".

Figure 6: Horizontal sundial with an alternate wide gnomon. (ZW2000/CAD)

Figure 7 shows a series of shadows on a horizontal sundial with the alternate gnomon over the span of the day. The change from one shadow casting edge to another can be seen. The gnomon has been made transparent to show this more clearly.

Figure 7: The Changing of the Edge - Alternate Gnomon

Combining the two gnomons illustrated above in the form of a parallelogram would create two separate styles. This leaves the design open to placing two sets of hour lines on the dial plate with two, actually four, separate dial origins. One could show solar time and the other zonal solar time. Figure 8 illustrates this concept.

Figure 8: The Parallelogram Gnomon

Let's go to a location north of the Arctic Circle or south of the Antarctic Circle. In these regions the sundial has the opportunity to be illuminated by the sun for a period of 24 hours a day during certain periods of the year. Check out the "Sun Charts" page. What happens at midnight if you have a wide gnomon on a horizontal sundial? If the gnomon is the type illustrated in Figure 5 where it has a "noon gap", it will have a "midnight overlap". If it is the type illustrated in Figure 7 where it has a "noon overlap", it will have a "midnight gap". Study the figures and this will become apparent.