Patrick-- That isn't necesary, because sundials are for telling solar time, not clock time.
If you want Standard-Time, then a conversion table, incorporating EqT & longitude-adjustment, can be used. Michael Osslipoff July 31st Week 32, Wednesday 0540 UTC On Tue, Jul 30, 2019 at 3:17 PM Patrick Vyvyan <[email protected]> wrote: > 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, <[email protected]> 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:[email protected]] Im Auftrag von Steve >> Lelievre >> Gesendet: Dienstag, 30. Juli 2019 19:38 >> An: Sundial List <[email protected]> >> 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 >> >> --------------------------------------------------- > https://lists.uni-koeln.de/mailman/listinfo/sundial > >
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