Most of us regard a sundial as an attractive ornament for a park or garden. Their effectiveness as time keepers is highly variable

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 the 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 the few ways to tell the time with reasonable accuracy.

Shadow clocks dated at 1500BC are known from Egypt, but the first dials appear to have been Babylonian. The Greeks adapted the idea, the Romans developed it further, and by about 100BC had perfected the 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 the seasons and the movement of the sun to determine planting and the 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 the 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 the gnomon (silent "g") which casts a shadow onto a plate called the dial. The dial is divided into hourly or shorter time divisions and may also show other information.The part of the gnomon whose shadow indicates the time on the dial is called the style.

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

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

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

And the most elegant of all, the portable sundial. George Washington had one - at that time pocket watches were most unreliable. Modern examples can be a work of art. They combine a compass with an adjustable dial. The dial is tilted to correspond to local latitude, and the compass defines north. Pretty neat!