Although the original question referred to a vertical dial, for a
horizontal dial it works very well 6 months in the year on the pole, fairly
well as the latitude moves towards the equator, but does get progressively
less and less accurate.
Simon
On Tue, Nov 9, 2021 at 12:08 PM Dan-George Uza
wrote:
> Dear all,
>
> In his book "Sundials Old and New", A. P. Herbert contends that rotating a
> vertical dial by 15 degrees towards East (i.e. gnomon pointing 15 deg. West
> of North) will switch it to Summer Time. Writing that this is against all
> theory, but works quite well, he calls this "the housewife's trick" (page
> 80). I find it intriguing.
>
> Dan Uza
>
> lun., 8 nov. 2021, 23:50 Francesco Caviglia a
> scris:
>
>> Dear John,
>> attached you will find a graph calculated at latitude 42 degrees N
>> for a vertical sundial mis-aligned by 1 degree toward West.
>> For 52,5 degrees the values are somewhat larger (by about 10 to 30%).
>> With greater mis-alignement (to some degrees) consider
>> that the values are roughtly proportional to the mis-alignement itself.
>> The original declination of the vertical dial has no influence on the
>> error,
>> just he mis-aligment matters.
>>
>> The graphs show the difference:(value indicated by the mis-aligned
>> sundial) - (true value)
>> They refer to the Time (above) and to the Solar declination (below),
>> the value are expressed as a function of the True Time (Ora reale) of the
>> day,
>> at the equinoxes and at the solstices.
>>
>> In any case, to find the errors it is quite simple:
>> just take the classical graph with the curves
>> showing the elevation of the Sun as a function of the Azimuth
>> for a set of Hours and Solar Declinations.
>> Make a transparent copy of the graph,
>> shift it in Azimuth as the considered sundial
>> and paste it on the original graph.
>> On every point in the plane of the graph you can read
>> the values of the real Time and Declination (on the original graph)
>> and those indicated in the same moment by the misaligned sundial (on the
>> shifted graph).
>>
>> Best wishes
>> Francesco Caviglia
>>
>>
>> > 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
>> >
>> >
>> >
>>
>> Francesco CAVIGLIA
>> via SAFFI 21
>> 10138 Torino
>> tel. 011 4333703
>> mob. 3356121207
>> francesco.cavig...@tin.it
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>> https://lists.uni-koeln.de/mailman/listinfo/sundial
>>
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>
>
--
Simon Wheaton-Smith
www.illustratingshadows.com
Phoenix, AZ
W 112.1, N 33.5
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