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Reading the Sundial
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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.
Reading the Time

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.

    3. A graph of the minutes to add to the sundial reading is called the "Equation of Time" and is shown here:

Equation of Time

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. Curved Hour Marks
  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.

Date lines

The current day of the year can be estimated by using the curved lines having dates marked at their ends. In order to reduce the number of lines, only the 21st of each month is shown and all but two lines are used for two different months. The shadow of the tip of the gnomon moves along these lines on the marked date and parallel to them at other times.

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.


Questions or Comments? Email: Veterans.Memorial@dupageco.org