Sundials are devices which tell the time by interpreting the shadow cast as the sun shines upon a fixed dial. They are inaccurate when compared with clocks and watches but are often used as centre pieces for formal gardens. Strangely, sundials seem to be most popular in countries where the sun rarely shines, being hidden behind a cover of dense cloud.

Inaccuracies

Sundials usually tell the wrong time, for three reasons:

• They are set up wrongly.

• They are designed to show 'local time', not official time. Official time is the time used throughout your time zone. It can be up to two hours different from actual local time.

• Due to the complicated motion of the Earth around the Sun, even a correctly set up sundial can be up to 16 minutes fast or slow depending on the time of year.

The first two of these problems can be overcome very easily, giving a sundial which is accurate to within about 15 minutes. The third problem can be corrected by a very elaborate type of sundial, the analemmatic sundial, or by including a correction table with the sundial. This is probably too much trouble for most people.

Setting up a Sundial

To set up a sundial, you must:

• Ensure that the gnomon (the bit that casts the shadow) is parallel to the axis of the earth.

• Make sure that the flat part of the sundial is level.

• Find out your latitude from an atlas of the world. This will be a number of degrees between 0 and 90. For example, Dublin is about 52° north. Make sure the angle between the gnomon and the base is the same number of degrees.

• Point the gnomon towards true north if you live in the northern hemisphere, or towards true south if you live in the southern hemisphere. True north means the North pole, not the magnetic north that is shown by a compass. It is close to magnetic north, but how close depends on where in the world you are. A local map will normally tell you as a footnote how to find the difference between local magnetic and true north.

• You can also find true north at night by finding the pole star, Polaris (alpha Ursae Minoris), and making sure the gnomon is lined up with it. Unfortunately, there is no 'southern pole star', so this method does not work in the southern hemisphere.

Making a Sundial Show the Right Time

Once your gnomon is pointing the right direction, you can mark your sundial so that it shows the right time. In many countries, daylight saving time is used in the summer and clocks are set one hour forward. Since the sun shines most in the summer, it makes sense to set up your sundial to show daylight saving time, rather than the local time that most sundials show. Mark the shadow at exactly 12 o'clock by your watch. Now mark out the other hours on either side, so that 6am and 6pm are at 90° to 12 o'clock. Subdivide the hours into 5 minute intervals.

Seasonal Corrections

You will find that your sundial is accurate to within a couple of minutes on many days of the year, such as 15 April or 2 September. At other times, it will be wildly inaccurate, such as on 4 November, when it will be nearly 17 minutes fast. To overcome this, it is possible to use a correction chart. Some sundials have a chart engraved on them, but consulting a chart and correcting the time is too much trouble for most people.

Mottos

It is traditional to put a motto on sundials, often in Latin. These range from twee to grim. Here are a few good ones:

• Hores non numero, nisi serenas - 'I only count the happy hours'
• Docet umbra - 'The shadow teaches'
• Serius est quam credes - 'It's later than you think'
• Tempus fugit - 'Time flies'
• Tempus edax rerum - 'Time devours all things'