The Sundial - Garden Ornament or Exotic Timepiece?

Discussion in 'Garden Design' started by Frank, Apr 8, 2005.

  1. Frank

    Frank GardenStew Founder Staff Member Administrator

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    The Sundial - Garden Ornament or Exotic Timepiece? by Graham McClung

    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
    www.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
     
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  3. eileen

    eileen Resident Taxonomist Staff Member Moderator Plants Contributor

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    What a really interesting article!! :D

    We discovered an old stone sundial buried in the ground when we were constructing our new pond. 8) Only problem is it's missing the actual 'dial.' It's beautiful, old weathered stone and we're thinking seriously of going out to see if we can buy a new dial and setting it up in the garden as a feature.

    Must take a piccie when I find a spare minute. :)
     
  4. Frank

    Frank GardenStew Founder Staff Member Administrator

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    I enjoyed that article as well as I never knew there was so much to sundials, but there you go!

    A old, weathered sundial piccie... now that would be nice :)
     
  5. blackrose

    blackrose In Flower

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    well detailed. i know i'd remember those ideas in the future. :idea:
     

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