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!

Sundial Accuracy

A properly designed and installed sundial can be a very accurate means of telling the time, down to intervals of less than a minute.

I won't go into the mathematics, but on a sundial 16 inches (40cm) in diameter, the shadow of the gnomon will move about 1/30th of an inch, or just under 1mm, in a minute. This may be small, it's enough for our eyes to see.

Two Major Problems

Apart from the frequent absence of sunlight (Problem 1), all sundials show time by cakibrating outwards from the position of the sun at noon, and if you live east or west of me, your noon is different to mine.

Although the earth moves around the sun, we see it the other way. The sun appears to move from east to west across the sky, and local noon is when it's vertically overhead. But if you live 100 miles west of me, my noon is still your late morning, and your noon is my early afternoon. This would be inconvenient if we used our sundials to arrange a lunch date, but a real problem if I had a plane to catch in another city.

Solar Time and Official Time

People managed to live with this problem until communications and transport became faster. Imagine calculating train timetables when Boston, New York and Buffalo all worked on different local times.

The answer was the development of local time zones. US Railways did this in 1883, but in 1914 the world's governments agreed to divide the globe into 24 zones, each 15 degrees of longitude in width, and each one hour different in time to its neighbours. Boundaries were altered slightly to account for state and national borders.

There are four time zones in the contiguous 48 states of the USA: Eastern, centred on 75 degrees W longitude; Central, on 90 degrees; Mountain, on 105 degrees; and Pacific, on 120 degrees. Noon was identified astronomically for each of these meridians (now it's done by atomic clocks), and accepted everywhere else in the zone.

Noon on sundials in places very close to these longitudes will correspond to official noon. For every degree east or west of the central meridian, for 7.5 degrees either side, you will need to add or subtract four minutes respectively to correct your sundial.

A few other adjustments are necessary to compensate for irregularities in the earth's path around the sun - not too difficult to make but the theory is beyond this article.

They add to the inconvenience, and that's why sundials have been superceded by more convenient and reliable forms of time keeping. But problems with time zones and orbital paths can be corrected, and there's no reason why you can't find the correct time from your sundial.

No reason, that is, provided it has been properly installed in your garden. And that's the subject of another article.