A basic problem with the accuracy of sundials is the Analemma. Due to the tilt of the Earth, the position of the shadow for a given time moves in a "figure-of-eight" shape over the course of the year. Therefore, even if the sundial is very accurately marked and positioned, the shadow will only fall exactly on the hour line twice a year - the winter and summer solstices.
The figure-of eight Analemma is quite often marked for midday (and can serve to give the date as well). On large sundials, the Analemma may also be marked for every hour - but on a smaller dial, this can be visually very confusing! Another solution, used on heliochronometers, is to allow the dial to rotate against a scale marked with the appropriate Analemma offsets according to the date. Best wishes, Patrick On Tue, 30 Jul 2019 at 14:40, <kepler...@gmail.com> wrote: > Hi Steve, > > as I built a large one (https://Kepleruhr.eu with 240m²) and thought some > about getting as accurate as possible here are my readings so far: > > 1) If you go for a sharp edge you will find out that the penumbra is all > the > times about 2 min in width which is the wandering time of all of the sun > diameter: The sun diameter is roughly 0.5° in the sky and it takes roughly > 2 > min for the sun to move this angle. The penumbra in angle does not depend > on > the distance from the gnomon to the face. So I would suggest that the > reading would be +/-2 min for untrained and about +/-1 min for trained > observers. This is valid for sundials using the bypassing shadow of the > Gnomon or the moving flare of any rectangle or circular iris. > > 2) I estimate a reading accuracy of the Kepleruhr by +/-15 sec (at high > noon > only): There is a wandering flare of 2 cm (+/- penumbra) with two side > edges > on a line of 2 cm which increases the reading accuracy. This wandering > flare > is produced by a spherical Nodus with this 2 cm gap southwards. There are > some movies at the concerning YouTube-channel (links given at the website). > > 3) In my case I made the calibration of the sundial by > a) calculate the hour and day line positioning by given parameters > (declination, geometry of gnomon, Nodus, wall) > b) erect the gnomon to the wall firstly without the painting > c) observe the shadow at one of the next fully sunny days - taking > series > of photos, calibrate them with respect to lens distortions, positioning, > etc > d) find the hourly shadow positions by machine vision techniques > e) adjust the above given parameter set as long as the total error of > deviations between the calculated and measured positions got a minimum > f) calculate the lines with the latest parameter set and do the > painting. > g) BINGO - it turned out (observing the sundial since years) that the > lines correctly follow the shadow on time. > > 4) I am on to build a sundial with a second reading of high noon - and did > do the concerning presentations (theory, fulfilled and planned > implementation steps) at sundial conferences in Austria. > > Good luck! > Kurt > > -----Ursprüngliche Nachricht----- > Von: sundial [mailto:sundial-boun...@uni-koeln.de] Im Auftrag von Steve > Lelievre > Gesendet: Dienstag, 30. Juli 2019 19:38 > An: Sundial List <sundial@uni-koeln.de> > Betreff: What accuracy to aim for with a carefully made 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 > >
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