Capricorn: Dec.22-Jan.29 The Sundial Primer
created by Carl Sabanski
Capricorn: Dec.22-Jan.29

The Sundial Primer Index

Noon Sundial

Noon (or Meridian) Dial or Line: a dial which has only one hour line, for noon. It has a nodus rather than a full gnomon. This may be in the form of a small ball on the end of a shaft or, more usually, an aperture in a plate or window opening into a building. Very long meridian lines built into cathedrals were intended for the accurate determination of the equinoxes and solstices. The noon dial (as opposed to noon line) is usually taken to mean a complete noon analemma, possibly including dates.

Noon Mark: a stone, or line marked on a stone, set to receive the noon shadow of a building or other feature.

Nodus: a point which casts a shadow to indicate the time and/or date on a dial face. It may take the form of a small sphere or a notch on a polar-pointing gnomon, or it may be the tip of a gnomon with an arbitrary (usually horizontal or vertical ) orientation.

The noon dial is usually a vertical direct south dial that only indicates one hour of the day, noon. It can also be a horizontal dial. It can be as simple as a single line or may include a detailed analemma that will indicate both time and date. Figure 1 illustrates a noon dial that can be separated into four distinct dials.

  • The red line shows noon in Local Apparent Time or sun time.
  • The blue line shows noon in Zonal Solar Time, i.e. Local Apparent Time with a longitude correction but without the Equation of Time (EoT).
  • The red line with a superimposed analemma (not shown for clarity) shows noon in Local Mean Time, i.e. Local Apparent Time that has been corrected for the EoT but not for latitude.
  • The blue line with a superimposed analemma shows noon in Standard Time, i.e. Local Mean Time at the central meridian of a given time zone. This is clock time.

Figure 1: Noon Dial

Figure 1: Noon Dial (ZW2000/CAD)

A noon mark or line is basically the red line shown in Figure 1 with some shadow casting device. Many examples of these still exist. This line lies in the meridian. To locate the mark correctly, it is necessary to find the true north-south or meridian line. The page "Finding True North" will help you to do this. Knowing this is also required for positioning a noon dial.

To create a noon dial it is necessary to include the analemma. Calculating the analemma is somewhat complex and is not included in this page. A very accurate analemma can be created using the free sundial design software available. Figure 2 shows the top portion of the analemma in Figure 1. The analemma has points indicating day 1, 5, 10, 15, 20 and 25 of each month.

Figure 2: Noon Dial Analemma Details

Figure 2: Noon Dial Analemma Details (ZW2000/CAD)

To trace out the analemma the dial requires a nodus positioned in the correct location. If it is a vertical dial, the gnomon is of the design used for a direct south dial. The desired size of dial will influence how high the nodus will be from the dial plate. The effect of nodus height is illustrated in the figures on the page "Vertical Direct South Dial".

The nodus can be achieved by using a horizontal rod (for a vertical dial) or a rod for the gnomon's style that is cut off at the nodus position. A small ball mounted on the rod and located at the nodus point can also be used. Another type of nodus is a flat circular plate with a small hole drilled at the nodus point that creates a spot of light used to indicate the time. Th

e size of hole that gives the sharpest point of light can be determined by first drilling a number of different sized holes in a piece of material  the same thickness as that being used for the final nodus. Position this test piece above a flat surface at the same distance that it will be from the dial plate and note which one gives the clearest sun spot.

A detailed noon dial could become a focal point in a community and would make a great community project.