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 calibrating outwards from the position of the sun at noon. 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.
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 http://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.co m
How often you mow your lawn will depend on a... Read More
Flowers can be an integral part of cooking. While most... Read More
Adirondack is the name given to a style of furnishings... Read More
Outdoor benches are an excellent way to enhance a garden,... Read More
With an average lifespan of 75 years, teak wood has... Read More
Creating no dig gardens on top of the soil gives... Read More
Today is Thursday the 23rd of June and we're almost... Read More
Gardening is fun and rewarding and may be considered a... Read More
Called the "herb of happiness," Origanum majorana, commonly known as... Read More
Have you noticed you do not see as many butterflies... Read More
Trying to start your lawn mower after a long winter... Read More
Notice any fish disappearing lately? While herons are present year-round,... Read More
Choosing the right bulbs involves more than just selecting colors... Read More
Fruit gardening and vegetable gardening is a very exciting venture.... Read More
Bare Root Roses, what to look for when buyingThe first... Read More
It is important when you start to plan a garden,... Read More
One winter evening I was working on the computer when... Read More
Wooden Garden Furniture can be the ideal solution to various... Read More
Purchased corn, whether on the cob or in a can... Read More
Summer is over, but for many of us - the... Read More
Now we are fast approaching the hottest time of the... Read More
Myths and facts A misconception about bamboo is that they... Read More
Gardening for wildlife has become quite a passion for me,... Read More
A feng shui money tree is an indoor plant used... Read More
Do you love spending time outdoors in your garden creating... Read More
So, you have decided to plant rose bushes in your... Read More
In all likelihood you have heard of teak furniture and... Read More
Do you have clay or layer of hard subsoil problems... Read More
A garden a serves a man's every day need for... Read More
When I was a child, I loved to pick Dandelions.... Read More
February is "Feed the Birds" month in much of North... Read More
Palms Everybody recognises palm trees, they are the... Read More
Tips for winter care of trees and shrubsWith the garden... Read More
A tomato importer, John Nix, decided to challenge the law... Read More
Yes! You can attract hummingbirds to your home. Even if... Read More
Having an outdoor patio offers the opportunity and place to... Read More
Tips for winter care of trees and shrubsWith the garden... Read More
Just like humans need food, water and shelter to survive,... Read More
A fundamental question in vegetable gardening is - what is... Read More
Do you wish you could grow a sustainable vegetable garden?... Read More
Many people believe growing their flowers and vegetables organically is... Read More
Some might ask, "Why grow Italian vegetable seeds?" Why bother... Read More
Worms are not only the gardener's best friend, they are... Read More
One of the best ways to bring together the many... Read More
The areas of your landscape can be divided into several... Read More
Do you ever dream about what your yard could look... Read More
Asparagus is a perennial plant with erect, edible stems and... Read More
Principles refer to standards or prescriptions for working with or... Read More
Here are a few tips to keep your garden and... Read More
Think of your garden like you would a room in... Read More
Noted English clergyman Sydney Smith (1771-1845) is quoted as saying,... Read More
Seeds are the most important ingredient in all gardening. Without... Read More
Flowers are a favorite of many backyard gardeners. Thanks to... Read More
So you want to grow roses? They are a beautiful... Read More
One of the long standing myths that I still hear... Read More
There is nothing more relaxing than kicking back with a... Read More
Sure, it makes sense that there's a buzz about vertical... Read More
Garden lovers around the world love putting fountains among their... Read More
Materials Needed 1 decorative clay or metal planter 1 package... Read More
A patio can be... Read More
Summer is here and it's time to plant and enjoy... Read More
The selection of roses you can grow in your home... Read More
Having an ultra violet pond filter will give you a... Read More
The average home can reduce their water consumption by around... Read More
Roses are the traditional gift given on Valentines Day, but... Read More
Landscape wallpaper can be a great addition to your computer's... Read More
Landscaping & Gardening |