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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
Simon
On Tuesday, November 9, 2021, 12:08:43 PM MST, Dan-George Uza
<[email protected]> 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 <[email protected]> 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
[email protected]
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