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SUNDIALS
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It is too much to expect that an ornamental sundial will be manufactured to scientific specifications, but if purchasing one there are a few things to look out for.
As mentioned previously the point at which the angled edge of the gnomon (also known as the style) meets the base of the sundial should lie half way along on a straight line between the 6am marking and the 6pm marking. This can be seen more clearly on the flat base experimental paper sundial in the photographs which you can navigate to from the link opposite.
Also the angle of the gnonom (the pointer) should be equivalent to the latitude at which the sundial is being used. For example in London, which lies at a latitude of about 51½º North, the angle of the gnomon should also be 51½º. For Edinburgh, which lies at 56º North, the angle of the gnomon should be (you've guessed it) 56º. This can also be seen in the photographs.
If you are making your own sundial this will be easy to accomplish, but if buying an ornamental one from a garden centre this will be non-negotiable. Usually for sundials sold in the UK the gnomon will be set for a middle compromise latitude for Birmingham of around 52½º. So make sure that this is the case. Often the gnomon will be fixed from underneath the face of the sundial with a couple of small brass screws, so it may be possible to adjust the angle that it sits at by including a brass washer or two between the face-plate and the underside of the gnomon, at either the front or the back accordingly.
The final area of concern is the faceplate itself and the arrangement of the numbers around the edge. The positions are worked out by some serious mathematics, which I don't pretent to fully understand. Suffice to say that as latitude varies, the so does the spacing. At very low latitudes the 5am to 11am positions and the 1pm to 5pm positions get closely bunched up toward the 12 o'clock marking. At higher latitudes the spacing becomes more even around the dial.
It is unreasonable to expect an ornamental sundial to meet these criteria, but if the numerals are reasonably evenly spaced around the face then expect some quite good results. If they are slightly bunched up towards the 12 o'clock marking, then don't worry, that's probably not a bad thing.
There are specialist sundial makers that will produce an excellent sundial with all the correct geometry, so that the time keeping will be spot on for a given latitude. They tend to cost several hundred pounds though, as you might expect.
A garden centre ornament will probably cost from about £50 upwards, and given that it conforms to the basics detailed above will not only be attractive, but also give a reasonable indication of time. In fact, as expected, my ornamental dial is always correct at 6am, Midday and 6pm.
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