The Sundial - Garden Ornament or Exotic Timepiece?

Written by Graham McClung

Continued from page 1

Sundial Accuracy

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

I won't go intorepparttar 113290 mathematics, but on a sundial 16 inches (40cm) in diameter,repparttar 113291 shadow ofrepparttar 113292 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 fromrepparttar 113293 frequent absence of sunlight (Problem 1), all sundials show time by cakibrating outwards fromrepparttar 113294 position ofrepparttar 113295 sun at noon, and if you live east or west of me, your noon is different to mine.

Althoughrepparttar 113296 earth moves aroundrepparttar 113297 sun, we see itrepparttar 113298 other way. The sun appears to move from east to west acrossrepparttar 113299 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 wasrepparttar 113300 development of local time zones. US Railways did this in 1883, but in 1914repparttar 113301 world's governments agreed to dividerepparttar 113302 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 inrepparttar 113303 contiguous 48 states ofrepparttar 113304 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 inrepparttar 113305 zone.

Noon on sundials in places very close to these longitudes will correspond to official noon. For every degree east or west ofrepparttar 113306 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 inrepparttar 113307 earth's path aroundrepparttar 113308 sun - not too difficult to make butrepparttar 113309 theory is beyond this article.

They add torepparttar 113310 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 repparttar 113311 correct time from your sundial.

No reason, that is, provided it has been properly installed in your garden. And that'srepparttar 113312 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, 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

The Perfect Garden Sundial

Written by Graham McClung

Continued from page 1

Finding North

The final essential in sundial installation is to make surerepparttar gnomon is oriented north-south. Sounds easy and, with a little patience, it is.

One way, suitable forrepparttar 113289 northern hemisphere, is to identifyrepparttar 113290 pole star. This is very close torepparttar 113291 projected position ofrepparttar 113292 earth's axis, about whichrepparttar 113293 sun and stars seem to revolve. You could markrepparttar 113294 direction from your sundial's location torepparttar 113295 pole star, but this method isn't quite accurate, and needs to be done inrepparttar 113296 dark. Andrepparttar 113297 southern hemisphere doesn't have a pole star.

Method 2 uses a compass. Sounds pretty simple, doesn't it. But you guessed it - there are some complications.

Firstly,repparttar 113298 needle on a compass points to magnetic north, not true north, which is what we want. The difference betweenrepparttar 113299 two is calledrepparttar 113300 magnetic declination, and is usually shown on good topographic maps. And while a simple addition or subtraction ofrepparttar 113301 difference betweenrepparttar 113302 two norths should give yourepparttar 113303 right direction, there may be some local magnetic effects which can't be compensated for.

The third method goes back torepparttar 113304 ancients - and there were some pretty smart operators around inrepparttar 113305 old days.

You'll need a stick, some paper or board, a marker, a tape measure or long rule, a sunny day, and a bit of time on your hands. Setrepparttar 113306 stick up vertically atrepparttar 113307 location you have chosen for your sundial, so thatrepparttar 113308 top of its shadow falls onrepparttar 113309 sheet of paper or board. If you stand with your back torepparttar 113310 sun, behindrepparttar 113311 pole, setrepparttar 113312 paper up so thatrepparttar 113313 morning shadow falls on its left hand side.

Now markrepparttar 113314 end ofrepparttar 113315 shadow with a permanent marker. Come back throughrepparttar 113316 day and markrepparttar 113317 new positions ofrepparttar 113318 tip ofrepparttar 113319 shadow -repparttar 113320 more oftenrepparttar 113321 better. Asrepparttar 113322 day goes on, you'll noticerepparttar 113323 marks form a curve.

Later inrepparttar 113324 afternoon - any time after three is OK - connectrepparttar 113325 marks you've made intorepparttar 113326 smoothest curve you can manage. Do this whilerepparttar 113327 pole and paper are still in place. Then carefully measurerepparttar 113328 distance betweenrepparttar 113329 base ofrepparttar 113330 pole andrepparttar 113331 curve. The shortest distance corresponds to true north. Mark it in some way, and alignrepparttar 113332 gnomon inrepparttar 113333 same direction when you put your sundial in place.

You can find true north in other ways - again I suggest you try google as suggested above.

Once you have set up your sundial, checkrepparttar 113334 time, compensate for differences with your official time zone, pat yourself onrepparttar 113335 back, and ifrepparttar 113336 sundial tells you it's after midday, pour a glass of your favourite beverage and put your feet up. Your time is now your own.

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, 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

    <Back to Page 1 © 2005
Terms of Use