Message text written by "walter.jonckheere" >I have a feeling 1 second could be obtained<
The ability of a conventional sundial to show accurate time is usually compromised by the half degree angle subtended at the earth by the sun's disc. This makes shadows fuzzy and it makes it very difficult to judge the position of the shadow of any edge - like one of the two styles of a thick gnomon of a horizontal dial. It is true that sundials of the conventional horizontal type should have a noon gap (and hence show time using two meridians) and indeed all well made conventional horizontal dials do have this. If one is considering the design of a large dial this fuzziness of the shadow has to be considered. It is usually better therefore to use a cylindrical rod or tube as the gnomon since then time may be better judged from the centre of the shadow. It is not very difficult to get a dial set up to read fairly accurately. One of which I have experience is the dial I designed recently for Amble town square (Amble is on the East coast of Northern England) This dial is about 12m across. The gnomon is very close to being in the right position - the (local apparent) noon shadow on a vertical surface being 'out' by about 75 mm at a distance of around 30m from the dial centre (figs from memory so are approx). However the evening before the opening ceremony I photographed the dial showing time to within 15secs. You can see the dial and this particular photo on http://ourworld.compuserve.com/homepages/Patrick_Powers/amble.htm and clicking to see the opening ceremony shots. For those who might doubt that one can see a movement of the shadow in 15 secs I have another photo not on the web site to show what it looked like 15 secs later. I can up load that to the website if there should be any interest.. Despite this euphoria one must remember that his was one measurement at one specific hour and on one specific day of the year. Of course all the other various and inevitable misalignments present in such a large dial will ensure that it will not necessarily be as accurate as that on other days of the year or even at other times on the same day. I am afraid that in the case of the Amble dial I have not been able to make any long term study of its accuracy. In any case, those local vandals who climb the gnomon on occasion may already have made some adjustments of their own! Patrick
