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 HistoryWe 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 SundialThere 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 AccuracyA 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 ProblemsApart 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