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--- Begin Message ---It seems as though gnomonicists are like politicians: when asked a question, they’ll always give a reply but generally not an answer to the question that was asked! John’s question relates to declining vertical dials which have been correctly designed but have subsequently been moved to a nearby location but on a wall with a different declination. Regards, John D Dr J Davis Flowton Dials http://www.flowton-dials.co.uk/ BSS Editor http://sundialsoc.org.uk/publications/the-bss-bulletin/ > On 8 Nov 2021, at 18:18, "[email protected]" > <[email protected]> wrote: > > dear John > > there is a precise theoretical procedure to relocate a sundial so that it > works correctly in a new location (for the hour lines). > > In the most general case, the sundial must be placed with a new orientation > so that the style keeps the polar orientation in the new location (thinking > of the stylus and dial as one whole). > This condition provides for many orientations, all the orientations obtained > by rotating the dial around the polar stylus. > The difference caused by these rotations is the meridian to wich the time > marked will refere, that is, as well as orienting the polar style correctly, > a rotation around the axis may be necessary for the marked time system to be > correct (local sun time, time-zone sun time). > > It seems a little bit complicated, indeed it is, but it can be solved. > > If the new location is very close there are not all these complications, you > have only to keep the same orientation. > If the difference are few degrees in declination of the wall, you can place > the sundial not parallel to the wall (keeping polar the stylus), otherwise, > as you write, you have to accept an error. > The error is not constant at the same time, it also changes with the > declination of the sun, that is, it is seasonal. > The answer to your question is a three-dimensional graph, with time and > declination of the sun on x and y axes and the error on the z axis. > The error doesn't depend on latitude but on the angle between the polar axis > and the non-polar stylus. > > Not last, there is a consideration similar to that of Lewis Carroll about the > stopped clocks that strike the right time twice a day: > a non-polar stylus, whatever its orientation, always shows the correct time > once a day, when the sun is in the hourly plane that contains the non-polar > stylus and the polar axis. This plane relates to two times 12 hours apart. > In the other times the error occurs according to the declination of the sun, > cancelling itself every 12 hours, whit the max error about halfway. > The paradox would therefore be that accepting a little approximation in the > direction of the stylus is not important. This is the reason why even in > front of a wrong sundial there are those who can say that they saw it strike > the right time. > If it were not a paradox it would be the end of gnomonics :-( > > I think that few degrees of error in the direction of the stylus (2-3°) might > be acceptable, but it also depends on the size of the sundial and therefore > on the comparison between the accuracy in the reading and the error due to > the non-polar stylus. > > ciao Fabio > > Il 08/11/2021 14:38, John Foad ha scritto: >> If a vertical dial is relocated and now faces a few degrees east or west of >> its designed declination, you might expect it to run a few minutes slow or >> fast. Has anyone ever tabulated the greatest error, and at what times and >> dates it occurs? And does it make sense to think about the average error? >> Clearly the errors depend on the latitude and the design declination, but >> for starters they could be tabulated just for a direct south dial at 52.5 >> degrees N, and for a location move of 1, 2 and 5 degrees either way. As >> the gnomon is no longer polar-pointing I imagine the maths is a bit hairy. >> >> Best wishes, >> >> John Foad >> >> >> >> >> >> --------------------------------------------------- >> https://lists.uni-koeln.de/mailman/listinfo/sundial >> > -- > Fabio Savian > [email protected] > www.nonvedolora.eu > Paderno Dugnano, Milano, Italy > 45° 34' 9'' N, 9° 9' 54'' E, UTC +1 (DST +2) > --------------------------------------------------- > https://lists.uni-koeln.de/mailman/listinfo/sundial >
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