That's unfortunate, because it is not at all difficult to ensure that your garden sundial will be an accurate timepiece, provided, of course that sun is shining.

But that will be covered in another article. For now, let's see what a sundial is, and what it is capable of.

Sundial History

We forget in this modern age that accurate, affordable watches and clocks have been around for much less than 200 years.

Before then, sundials were one of few ways to tell time with reasonable accuracy.

Shadow clocks dated at 1500BC are known from Egypt, but first dials appear to have been Babylonian. The Greeks adapted idea, Romans developed it further, and by about 100BC had perfected horizontal sundial (and placed it in their gardens).

Even in ancient days some people had schedules to keep, and both agriculture and religion required knowledge of seasons and movement of sun to determine planting and timing of ceremonies.

The sundial was an important means of providing that information, and considerable advances in mathematics, geometry and astronomy were made while it was perfected. The knowledge gained forms part of foundations of modern science.

Types of Sundial

There are four reasonably common types of sundial.

They all have two things in common. Each consists of a raised structure, called gnomon (silent "g") which casts a shadow onto a plate called dial. The dial is divided into hourly or shorter time divisions and may also show other information.The part of gnomon whose shadow indicates time on dial is called style.

The most abundant form is horizontal sundial, happily sitting on its pedestal or column and adding beauty and interest to home garden.

Related is equatorial sundial, with its dial oriented at same angle as latitude. It works slightly differently, and is easier to use when properly calibrated.

Thirdly, there is vertical sundial, ideally located on a wall facing due south in northern hemisphere, and north in southern hemisphere. The principle is much same, but sundial only occupies a semicircular area. Vertical sundials displayed time to public, and were used to correct unreliable public clocks.

The chances are very good if you can make one yourself, or get one custom made for your location. But both these alternatives will take either time or money.

The reason for this is that apart from adjustments for time zones and daylight saving , covered in another article, there are three things that must be done.

Compensating for Earth's Path Around Sun

If earth followed a perfect circle as it revolved around sun, life with sundials would be easy. But its path is elliptical or oval, and this causes errors of up to 16 minutes in sundial time at some times of year. The corrections are straightforward, and can be made using a table, or from a figure 8 line called Analemma, often seen on old globes and sundials.

Correcting for Latitude

Let's consider components of a sundial for a moment. It consists of a dial on which time divisions, and sometimes other information is marked, and a triangular piece which sits vertically on dial. It is called gnomon, pronounced nomon, and part which casts shadow onto dial is called style.

For more information on how sundials work, including how to design your own, go to Google and type "how + sundial", without quotes but with +, in search box.

To be accurate, angle between triangular part of gnomon and horizontal must be same as latitude of place it is to be placed in (You can find latitude - and longitude - of your home from any topographic map or good atlas).The arrangement and distance between hour markings on dial must also be correct for latitude.

Hmmmm! This means that unless you are very lucky, that elegant sundial in your local garden supply shop will probably not show time particularly well. It may be calibrated for an average latitude (commonly 45 degrees), which is good if your latitude is not too different. Or it may be purely ornamental and will really only be useful around noon.

Now of course this doesn't matter at all if you are looking for something pleasing to eye, and don't mind answering inevitable question "Does it tell time?" But if you'd like your sundial to be more useful, make sure you find out which latitude it is calibrated to.