Wednesday, January 24, 2007

Do Sundials Actually Work?

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. They're also a perfect gift! 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 2º 35'W 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, 10 minutes behind it, 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 1degree you are east or west of your standard meridian. (This can be quite substantial; Vigo in Spain, for example is 8º44'W 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! Our sundials are a classic addition to any yard, and make great gifts! Not only do we have a generous selection of sundials, but also a great choice of bases. Be sure to take a look!

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