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.

About the Author

Copyright 2005, Graham McClung. A retired geologist, Graham

McClung has had a lifelong interest in the outdoors. And

where there's outdoors there's weather. He is the editor of

Home-Weather-Stations-Guide.com, where you can find reviews

and advice to help you choose and use your own home weather

station. You can contact him by email at

information@home-weather-stations-guide.com