Dear Mike, Frans & all, I think this goal is absolutely the most important thing we can do to improve the appreciation of sundials by the general public. Wouldn't it be nice if we could make non-dials unsellable by educating people into what makes a real sundial. At the same time, we can educate makers and sellers to stop making/selling them. Imagine if someone tried to sell a watch with bent hands, a zero at the top, 13 hours around the dial.... I have advised various people on how to select a sundial. My ideas don't seem to duplicate what Mike already has, so I offer themn here. I include instructions for orientation, but I understand Mike's reasons for keeping them separate:
My starting point is always to check that the gnomon is straight and does, or can be made to, point upwards at an angle approximately equal to your latitude. For England, that varies between 50 degrees in the South and 55 degrees in the North. To estimate the gnomon angle while you're in the shop, take off your watch and use its face. Hold it vertically against the side of the gnomon's edge, so 12 o'clock is vertical, and the gnomon slopes up to the right. If the gnomon were an minute hand on your watch, how many minutes would it be until a quarter past? Multiply by 6 to give the latitude. For instance, 9 minutes times 6 is 54, so the sundial is made/set for 54 degrees. If you don't mind setting the hands on your watch, you could set it to 6 minutes past the hour (i.e. 9 minutes until a quarter past), rather than just estimating the hand position, thren check that thre gnomon points in the same direction. Armillary sphere dials can usually be adjusted to almost any latitude by changing the position where the vertical ring is clamped to the base. Check that your latitude can be set. There may be a latitude scale marked on the vertical ring, or you can use the watch trick. Next, you need to check the hour lines on the dial. To do this, get yourself into a position where you can look straight along the edge of the gnomon towards the dial. Now, put your watch against the end of the gnomon, so that 12 o'clock is up, and tilt it so you're looking straight at the watchface. Traditionally, dials have 12 o'clock directly under the gnomon, but some dials these days have 1 o'clock, to show summer time (also called daylight savings time) as this tends to be when the sun shines most and people are out of doors. Either way, when you look past your watch at the dial, the hour lines on the dial should look equally spaced, but there are 24 of them in a day, so every two hours on the dial should correspond to one hour on your watch. Check that the hour lines do appear to be extensions of the hour lines of your watch, all radiating from the foot of the gnomon. Depending on whether you live in the northern or southern hemisphere, and on whether you're looking at a vertical dial or at a horizontal or armillary dial, the numbers on the dial may go clockwise or anticlockwise! When you get it home you must install it so the gnomon points due north-south. In the northern hemisphere, the higher end of the gnomon will point north. Opposite in the southern hemisphere, of course. So, when you look along the gnomon to the dial, are you looking north or south? If you are looking southwards, the numbers on the dial must go clockwise, if northwards they go anticlockwise. So, for instance, a horizontal dial in the northern hemisphere has its gnomon pointing up, so you look down it southwards and the numbers therefore go clockwise. These three tricks work with all horizontal, armillary sphere and vertical sundials you are ever likely to see in a shop anywhere in the world. They don't help with all portable dials, some of which are latitude-dependent. For maximum accuracy, if the gnomon is a thick plate, there should be a gap of the same thickness in the dial markings at noon, so it goes 11:50, 11:55, 12:00, gap, 12:00, 12:05.... Also, the hour lines should appear to radiate from the foot of the gnomon on the side where the shadow edge will be. If the gnomon is a string or a thinnish rod, there is no gap - you read time to the centre of the shadow, which is more accurate but liable to damage. When you get the dial home, you need to set it up so the gnomon points due north-south. For a vertical dial, this means it must be on a south-facing wall (dials can be made specially for other walls, but you won't find them on general sale). For a horizontal dial or armillary sphere the base should be level. The easiest way to set the orientation is by waiting until the sun shines, then turning the dial until it tells the time correctly. But the correct time isn't just the clock time. At different times of the year a sundial is up to a quarter of an hour fast or slow, and your watch may be set to summer time. In any case, your watch is set to a particular time zone, such as Greenwich Mean Time. If you live east or west of the Greenwich Meridian the sun will reach you earlier or later, so your dial will be systematically fast or slow. You need to know your longitude, which you should be able to get off a map. On the BSS Web site (http://www.sundialsoc.org.uk) there is a solar calculator that tells you exactly where the sun is and what your sundial should be reading at your longitude right now. For instance, when I wrote this and having put in my longitude, the BSS calculator told me "The sun is due south today at solar noon at 13:01:38" so the sundial should read 12:00:00 (or 13:00:00 if it's a summer time dial) at 13:01:38. So I should turn my sundial around until it appears to be 1 hour, 1 minute and 38 seconds slow compared with my watch. If I set the dial in this position, it will be as good as it can be, but that still means it will be up to a quarter of an hour fast or slow at different times of year, and it will always be about 5 minutes fast due to my living west of Greenwich I think these ideas supplement what Mike has. If he or anyone else wants to use them / put them on a Web site, please feel free. Piers isn't the only one utterly frustrated and disgusted at the rubbish that's out there. Best wishes Chris 51.4N 1.3W ----- Original Message ----- From: "Frans W. Maes" <[EMAIL PROTECTED]> To: "Mike Shaw" <[EMAIL PROTECTED]>; "Sundial List" <sundial@uni-koeln.de> Sent: Tuesday, May 09, 2006 8:30 AM Subject: Re: Buyer's Guide > Dear Mike & all, > > The most popular garden-center sundial around here is the armillary sphere. > They always read local solar time. Stimulated by questions of (prospective) > buyers, I once tried to write a fail-safe, fool-proof instruction for > correct installation. There are differences with horizontal dials, of > course, but most aspects are common to both. Therefore I would like to tell > my doubts about the feasibility of such instructions. > > I stopped my attempt, when I got frustrated by thoughts like: would any > layman-reader be able to find his latitude, or even know what "latitude" > means? Would he know how to measure and check an angle? Absolutely not > trivial! > > But the most bothering aspect was: how to instruct setting the correct > orientation? That aspect is missing from your 'golden five'. The pole-style, > or gnomon sides in your case, should point to Polaris. Ever tried directly? > The method I would use myself (after having checked the earlier aspects) is: > calculate local solar time from clock time by taking latitude correction, > EoT and DST into account, and turn the sundial until it reads local time. > How to explain this procedure and its rationale unequivocally within 2-3 > sentences? > > Providing insight in the relation between local and clock time is the more > important, as the correctly oriented sundial will henceforth read local > time. In Western Europe the difference with clock time is large, so that it > is useless as a clock to the non-insider. Which makes the artistic quality > the most important property to most people... > > I ended up with only a couple of hints on the Dutch version of my sundial > site: > - Don't take souvenir-type vertical dials from southern Europe back home > - Check if the insertion point of the style is on the 6-18 hr line (for > horizontal dials) > - Are adequate instructions for installing and orienting the dial included? > - Can it be fixed good enough so as not to serve as wind vane or > merry-go-round? > - Is the material weather-proof? > - Doesn't it collect rain that turns into a swamp (except for the bird-bath > type)? > > Have other list members experience with writing general instructions? > > Regards, > Frans Maes > > ----- Original Message ----- > From: "Mike Shaw" <[EMAIL PROTECTED]> > To: "Sundial List" <sundial@uni-koeln.de> > Sent: Friday, May 05, 2006 9:20 AM > Subject: Buyer's Guide > > > Fellow enthusiasts, > > I get really annoyed about the sundials that I see on sale in garden centres > here in the UK. > The people who make them seem to have no conception about how they work. > As they are invariably horizontal dials, I thought I would put a "Buyer's > Guide" to such dials on my web site. > I think there are 5 golden rules - have I missed any? > > Check it out - follow the link from my home page. > Helpful comments welcomed. > > Mike Shaw > > 53.37N > 3.02W > > www.wiz.to/sundials > > > -------------------------------------------------------------------------- -- > ---- > > > > --------------------------------------------------- > > https://lists.uni-koeln.de/mailman/listinfo/sundial > > > > > > --------------------------------------------------- > https://lists.uni-koeln.de/mailman/listinfo/sundial > --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
