Hello Steve An accurate sundial has many faces! The theory goes hand in hand with the practical problems. In my experience accuracy in the end result depends very much on the particular path you decide to follow. Each case is its own problem and I am sure there are no perfect solutions.
I have done a number of wall sundials and many gnomons for declining and inclining dials on faceted sundial stones. The starting point has to be measurement of the declination of the wall. You calculate the gnomon and the hour lines from there, as I am sure you already know. But if your wall measurement is in error, it is the first of the errors in building your sundial. As you point out there are other parts of the sundial you can control like precision machining. Wall declination may be contributing your greatest error, it is not easy to measure. I can send you pictures if you like to show you what I mean. And I have a device you can use to help with accurate measurement of declination, if this interests you. Your other question about penumbra is really a separate matter. It is more to do with resolution or discrimination you can observe on the time graduations of your dial rather than the accuracy of the sundial itself. In other words the sundial can be quite inaccurate, for example badly made or not well aligned with true north, but you are still able to observe changes in shadow position that apparently are 1 minute apart or less. You might describe this as bad at absolute time but good at incremental time, or not so good as a watch but quite good as a stopwatch! Depending on your ideas for your design of sundial any penumbra effect may not be the most important. Let me know if you want to follow up with any of this. Kindest wishes Alastair Hunter, Macmillan Hunter Sundials, Edinburgh, Scotland ============================= What accuracy to aim for with a carefully made sundial? Steve Lelievre steve.lelievre.can...@gmail.com via uni-koeln.de Jul 30, 2019, 6:37 PM (2 days ago) to Sundial Hello everyone, I'm planning to make a small vertical west dial, about 1m for the width of the dial face, at my latitude of 49N. It will not use a nodus. The angular width of the sun makes it hard to get a really accurate time reading, but there will also be small errors from mis-positioning of the dial plate when installing (declination and inclination), imprecise positioning of the gnomon or the hour lines, and perhaps other causes too. First, questions directed at those of you who have practical experience of creating vertical sundials: If I'm careful and have a well-machined gnomon, what level of accuracy might be achievable in practice? I assume +/- 5 minutes throughout the day and year is fairly easy to achieve, but what about +/- 2 minutes, or even +/- 1 minute? How well did you do? How did you measure your wall's declination? Second, have there been any studies of how well dial users compensate for a penumbra - by which I mean gathering data from volunteers, studying the spread of errors in time readings taken from a dial versus a reference time source? (without employing a shadow sharpener) Thanks, Steve --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
