Rohr mentioned an itinerant sundial maker in Austria, who roamed from village to village making vertical sundials for inns and church walls. I've been trying to imaging how would this person, or others in the same trade, have gone about their business.
The construction part could have been fairly easy. Make a hole in the wall, stick an iron rod in and use plaster or daub to hold it in place. Alternatively, have a couple of supporting struts which are nailed or bolted in place. Paint the metalwork with pitch or grease to stop it rusting away. The hour lines are easily done too - just attach an jig with 15 degree markings and extend a line from it to the wall, once per 15 degrees. Then joint up the contact points to the centre of the dial (as described in Rohr and Waugh I think, and in Claude Hartman's article in the latest NASS journal). Getting the gnomon at the right angle of dip isn't too hard either - just use a plumbline and protractor and adjust until the tilt matches your latitude. But now comes the first difficulty - what is the latitude? Q1) In practice, did they measure it for each location, or did they just follow a general rule like "Austria is at 50N" ? I've read all about finding latitude by sighting Polaris or whatever, but it might take weeks to get a clear night! Q2) How would someone who stays only a day or two in one place go about establishing the latitude using basic tools or home-made devices? I'm not asking how you would do it, but how they did it. Assuming the artisan has worked out the dip angle to use, he then comes to the problem of aligning the style to the meridian. Magnetic compasses wouldn't have been available, or would have been rare and expensive so I image they weren't in the typical toolbox.. I've read of the method of equal shadow lengths for finding south and I image that also works on a vertical due South wall, but I can't see it working for a declining wall. Perhaps he could just wait until the sun is highest in the sky but that is not easy to measure either, and the altitude doesn't change much over the midday period so it wouldn't be accurate. Q3) How did they align their gnomon with their meridian? Or conversely, did they know a way of telling if it is misaligned by watching the movement of the shadow? Steve
