The Veterans Memorial Sundial of DuPage County consists of two parts, the sundial face marked by strips of metal and the upright gnomon. The “gnomon” of a sundial is the part that casts a shadow for marking time. This sundial uses a polar gnomon. That means that the long straight part, called the “style” points up at the north celestial pole – the point about which the entire sky seems to turn. | ![]() |
Hour Lines = Uncorrected Central Standard Time
The long straight lines are numbered in hours. Read the center of the shadow of the style for a close approximation of Central Standard Time. This still needs correction for seasonal changes.
A graph of the minutes to add to the sundial reading is called the “Equation of Time” and is shown here:
The second seasonal correction can be made by using special markings on the dial and using only the tip of the gnomon.
In the horizontal sundial, the plane that receives the shadow is aligned horizontally, rather than being perpendicular to the style as in the equatorial dial. Hence, the line of shadow does not rotate uniformly on the dial face; rather, the hour lines are spaced according to the rule
tanθ = sinλtan(15° x t)
where λ is the sundial’s geographical latitude (and the angle the style makes with horizontal), θ is the angle between a given hour-line and the noon hour-line (which always points towards true North) on the plane, and t is the number of hours before or after noon.
An additional time can be read using the straight lines that cross diagonally across all the rest. When the shadow of the tip of the gnomon crosses one of these lines it tells how many hours remain before sunset.