Hi everyone,
Just to clarify ... my request is for information about the accuracy
that any makers on this list /have actually achieved/.
Thanks, Kurt, for your answer for the KEPLERUHR dial. By the way,
everyone, Kurt's dial is the subject of an article in the next issue of
The North American Sundial Society's journal, The Compendium (September
2019), coming soon.
Alexi, regarding an uneven wall, I'm designing for a plate mounted to a
wall, not for a dial painted directly on the wall. Measurement errors
for declination and inclination of the plate will affect operation but,
yes, my plan relates to a planar surface (or pretty darn close!)
Patrick, regarding the analemma, I'm looking at doing a Local Solar Time
dial, not a Mean Time dial, but I'll probably have an Equation of Time
chart.
Best,
Steve
On 2019-07-30 12:40 p.m., [email protected] wrote:
As I mentioned at my implementation approach I eventually did not take
the parameters I got from maps or measurements by a ruler – I took the
least square error approach by finally parameter adjustment and e.g.
got to the news that the wall is horizontally 0.7° out of angle with
respect to remote imaging/mapping.
Kurt
*Von:*Perit Alexei Pace [mailto:[email protected]]
*Gesendet:* Dienstag, 30. Juli 2019 21:30
*An:* Patrick Vyvyan <[email protected]>
*Cc:* [email protected]; Sundial List <[email protected]>
*Betreff:* Re: What accuracy to aim for with a carefully made sundial?
Another source of error apart from those mentioned in the original
post is how accurate can a wall's declination be measured, say to half
a degree. And what error would half a degree make depends on the size
of your sundial. We are also assuming the wall is planar and built
perfectly plumb!
Alexei
On Tue, 30 Jul 2019 at 21:16, Patrick Vyvyan <[email protected]
<mailto:[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]
<mailto:[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]
<mailto:[email protected]>] Im Auftrag von Steve
Lelievre
Gesendet: Dienstag, 30. Juli 2019 19:38
An: Sundial List <[email protected]
<mailto:[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
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