Can Your Sundial Really Tell The Time?

By Author: Hugh Harris-Evans
Total Articles: 2
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"I am a sundial, and I make a botch
Of what is done far better by a watch"

So wrote Hilaire Belloc, but is this really fair? Sundials are the earliest known form of time-keeping having been used for some five thousand years. The Greek historian Herodotus stated that sundials were first used by the Chaldeans and Sumerians in Babylonia which was part of the modern Iraq. They used vertical rods on their buildings and noted the position of the shadow to record the passing of the hours. The concept was developed by the Greeks and Romans who constructed various different shapes of dial to enable them to tell the time and the season of the year. Usually these were bowl-shaped dials with vertical or horizontal gnomons (shadow-casters) and hour lines marked in the hollow of the bowl. Over the years more elaborate designs were produced until the advent of accurate clocks when the function of the sundial became more decorative than as a reliable means of telling the time.

The question is often asked "Can a sundial really tell the correct time?" to which you will receive the Alice in Wonderland reply that it ...
... depends upon what you mean by "the correct time". Our clocks and watches work on the basis of there being exactly twenty-four hours between one day and the next but, because of the eliptical nature of the earth's orbit around the sun, the time shown on the sundial will vary according to the seasons. In February by the clock the sun is almost fifteen minutes slow, whereas during the spring and summer months it gains and loses between four and six minutes in two cycles. At the other extreme in November the sundial appears to be some seventeen minutes fast. In fact the sundial is accurate on only four days of the year, about April 15, June 14, September 2 and December 25. Some sundials include a table showing the deviation from "clock time" according to the date.

The time indicated by the sun will also vary with the location of the dial. The sun travels across the sky at the rate of fifteen degrees per hour so every degree of longditude represents a difference of four minutes from the standard meridian for the region. The angle of the gnomon also depends on the situation, so to set up your sundial correctly you need to know both the latitude and longditude of its location. For the United States and the United Kingdom this site can provide the information. The gnomon should be set at the angle in degrees which is equal to the latitude of your location. The sundial can then be fixed with the gnomon pointing to the Pole Star. There are various ways of achieving this, the easiest of which, is to use a compass adjusted for the magnetic variation. Further details are beyond the scope of this article, but for those interested look at this site.

If you have read this far you will have discovered that there is a great deal more to the sundial than a mere item of garden decoration. If this has piqued your interest in the subject, then you are not alone. There are Sundial Societies in countries around the world. The North American Sundial Society has details of its objects and activities on its website.

A number of sundial trails have been established. A good example is the Thames Sundial Trail in London, England. This site lists a number of other trails in countries all over the world but only two in the United States. However the North American Sundial Society has a complete list on its website.

Two rather different designs are shown here. The first, which is commonly referred to as a Human Sundial, uses the person's shadow to indicate the time. By standing in the appropriate box for the date the shadow will show the correct time. The second is a