DuPage County Veterans' Memorial

Reading The Sundial

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.

Seasonal Corrections

1. Daylight Savings Time: Add one hour from the last Sunday in April until the last Sunday in October
2. Because of the way the earth moves around the sun the sundial gets ahead or behind the clock. The clock and the sundial read the same time 4 times a year. For the year 2000 these days are: April 15, June 12, August 31 and December 25. The largest difference is 16 minutes and the average difference is 7 minutes. In other words, reading the time with just the hour lines and not making this correction usually gives the clock time accurate to within seven minutes.

A graph of the minutes to add to the sundial reading is called the “Equation of Time” and is shown here:

Curved Hour Marks

The second seasonal correction can be made by using special markings on the dial and using only the tip of the gnomon.

1. There are two “S” shaped curves for each hour marked by a metal strip. The brass strip should be used from December 21 to June 21. The stainless strip should be used from June 21 to December 21.
   
2. When the shadow of the tip of the gnomon crosses this line it is exactly that hour. The accuracy of your reading is limited only by how well the shadow can be seen.
3. For times between hours, estimate the fraction of the distance the shadow tip has traveled since the last hour. Note that the shadow tip travels along the curved lines that cross the hours. These lines are marked by dates at their ends.

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.

Time Before Sunset

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.