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Introduction to Sundials
When we were planning Sundials on the Internet, in 1996, I met someone socially for the first time. They were very glad to meet someone interested in sundials. "I have been working for some time on a sundial, but I can't get it to tell the time consistently." He explained that he had a vertical post in his garden, and wanted to make a sundial with it. I thought back to how my own interest in sundials started, when I put a stout baulk of timber vertically in the ground to support one end of a bench, and then thought I might use it for a sundial. So I cast some paving stones with the numbers 9, 10, 11 and so on which I laid around it where the shadow fell. I was so cross when it became more and more inaccurate as the days went by. And it must have been five years later that I came across a book by AP Herbert about sundials which clarified why it hadn't worked. After that, I started making some sundials which did work, and once you start getting interested in sundials, it is very difficult to stop!
These two experiences are a microcosm of the story of sundials from time immemorial. Everyone has noticed shadows moving round during the day. Someone unknown in the past found out that if the shadow was cast by a sloping object pointing to the celestial pole, it would cast a consistent shadow which would be in the same place at the same time every day. Though it has been suggested that this may have been 2000 years ago, it is more likely that it would have been around 500 years ago; before the development of clocks, it would have been difficult to determine what "the same time each day" meant, and anyone such an innovation would probably have been dismissed as impractical and useless.
Practically everybody knows what a sundial is. Most people have a residual idea that, if they had to, they could make one. But most people's practical knowledge of sundials is confined to having seen some standard brass horizontal dials on plinths in gardens. They may even have one in their own garden, or perhaps in their garden shed because they don't know how to set it up.
And there is a widespread - though totally wrong - general impression
that sundials are not very good at telling the time. This has been well
put by Hilaire Belloc who produced a number of sundial
Of what is done far better by a watch.
The poor reputation of sundials is ill-deserved, and has arisen mostly because we have all accepted "watch time" as an absolute standard, without devoting any thought to the nature of the time it is measuring.
Sundials measure time as it is. Noon is when the when the sun is highest in the sky (when it crosses the meridian). Watches measure time as we would like it to be, with noon tomorrow exactly 24 hours, 0 minutes and 0 seconds away from noon today. But noon on 26 December is actually 24 hours, 0 minutes and 29 seconds away from noon on Christmas Day. And noon on 15th September is only 23 hours, 59 minutes and 39 seconds away from noon on the following day.
Mechanical watches obviously cannot be made to run in this way. (Electronic watches could be made to do so, though the manufacturers would probably not find a very large market for them). So Mean Time was invented, an artificial construct in which all days are assumed to be exactly 24 hours long.
The sundials seen above church doors are a reminder of a time when sundials were the standard. Until about 200 years ago, public clocks could not be made sufficiently accurate to run for more than a few days without being reset, and the only way of resetting them was from a sundial.
Until the railways came, there was no particular reason why people in, say, Bristol should keep the same time as people in London. And, of course, at that time there was no practical way of communicating information about time over a distance. When the telegraph made such communication possible, it became necessary for people living in one area to agree that they would not keep their own local time, but would all keep a time based on the local standard meridian. Bristol is at 2º35W of Greenwich, so noon there is just over 10 minutes later than in London.There is still a relic of this change - the clock over the old Corn Exchange in Bristol has two minute hands. One shows Greenwich Mean Time like all the other clocks in England, and the other shows Bristol time!
Later on, another artificial change was made with Summer Time, which arbitrarily adds one hour to all clock times during the summer.
So, in the summer, there are 3 good reasons why your watch will be telling a different time from the sundial. They may be up to 15 minutes different because your watch is assuming that all days are equal in length. Then it will be 4 minutes different for every 1º you are east or west of your standard meridian. (This can be quite substantial; Vigo in Spain, for example is 8º44W of Greenwich, but is on Central European Time, for which the standard meridian is 15ºE of Greenwich, so the correction for longitude in Vigo will be 1 hour 34 minutes and 56 seconds). Lastly, it will be exactly 60 minutes different because your watch, if you live in England, has been arbitrarily altered to tell the time in Prague for the duration of the summer!
With all these artificial difficulties to contend with, it is quite a surprise that interest in sundials continues at all. But interest is in fact growing. Sundial societies exist in Britain and many other countries, and their membership is expanding. You are very welcome to join too.
We hope that Sundials on the Internet will give you some idea of the world of sundials - its complexity and its fascination. Sundials are unique in that Science (in the form of accurate calculations), Art (in the form of pleasing design) and Craft (in the form of good workmanship in the making) all have to come together to create a good sundial. Getting all of these right is quite a challenge! Good luck.
(This page is based on an article "Is that really the time?" by Piers Nicholson which appeared in "The Valuer" of August/September 1992)
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|        first posted April 1997     last revision|
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