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A properly designed and installed sundial can be a very accurate means of telling time, down to intervals of less than a minute.
I won't go into mathematics, but on a sundial 16 inches (40cm) in diameter, shadow of 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 frequent absence of sunlight (Problem 1), all sundials show time by cakibrating outwards from position of sun at noon, and if you live east or west of me, your noon is different to mine.
Although earth moves around sun, we see it other way. The sun appears to move from east to west across 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 development of local time zones. US Railways did this in 1883, but in 1914 world's governments agreed to divide 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 contiguous 48 states of 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 zone.
Noon on sundials in places very close to these longitudes will correspond to official noon. For every degree east or west of 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 earth's path around sun - not too difficult to make but theory is beyond this article.
They add to inconvenience, and that's why sundials have been superceded by more convenient and reliable forms of time keeping.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 correct time from your sundial.
No reason, that is, provided it has been properly installed in your garden. And that's 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 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 email@example.com