Hello Helmut,
Thank you, Helmut, for this graphical demonstration on the problems with
tropical sundials on certain dates. The hours cannot be discriminated. The
declination line and hour circle can coincide. Height of the gnomon and
length of the shadow are of no help as analemmatic sundials are are two
dimensional so height doesn't matter. This is why I developed and
distributed a spreadsheet and gave a presentation in 1999 on "How Long is my
Shadow: The Use of Declination Lines in the Design of Analemmatic Sundials".
After that we successfully collaborated on improving the software and you
developed an excellent stand alone design programs that easily provides
information like your pdf sketch. Now all sundial designers can review how
parameters interact. Sometimes analemmatic sundials don't work in the
tropics.
Regards, Roger Bailey
--------------------------------------------------
From: "Helmut Sonderegger (Tele2)" <h.sondereg...@utanet.at>
Sent: Saturday, January 12, 2013 2:32 PM
To: <sundial@uni-koeln.de>
Subject: Re: Analemmatic sundial
This as a printout with my software for an analemmatic sundial at
Northern latitude 20 deg and with shadow path of a person on Jun 1st. I
hope the attachment comes trough (52 kB).
Best regards
Hlemut
Am 12.01.2013 19:11, schrieb Roger Bailey:
Perhaps this is why there are so few analemmatic sundials in the
tropics. I do not know of any except for the one I built years ago at
20.4°, or 3° into the tropics. This dial was just markings in the sand
on a beach, as impermanent as all messages on a beach.
For similar reasons there are few vertical sundials in the tropics.
The shadows are too long from the gnomon and any overhanging eaves.
The reversal mid-day to the north side in the spring and summer is
another design complexity.
In any case it is interesting to explore, as Frank has, how latitude
changes the designs.
Regards,
Roger Bailey
from the polar side of temperate zone, N 48.6
--------------------------------------------------
From: "Frank King" <frank.k...@cl.cam.ac.uk>
Sent: Saturday, January 12, 2013 3:14 AM
To: "Donald Christensen" <dchristensen...@gmail.com>
Cc: <vk...@optusnet.com.au>; "Sundial Mailing Mailing List"
<sundial@uni-koeln.de>
Subject: Re: Analemmatic sundial
Dear Donald,
You wrote...
Brilliant idea Roderick!
I put both animations on my website.
Each one has a label under it stating
which hemisphere it's for...
Whoa! Hold on a moment!
The fun has only just begun...
Have you thought what happens in the
tropics?
Someone living at 20 degrees north (well
into the northern hemisphere) will not
be impressed by your northern hemisphere
animation around the summer solstice.
This is what happens:
1. The sun rises somewhat to the
north of due east (no surprise
so far).
2. It heads south for a while and
therefore goes round clockwise
(still no surprise). Then...
3. Suddenly it reverses direction
and goes ANTI-clockwise, and it
stays running that way...
4. ...through noon and...
5. well into the afternoon. Then...
6. Suddenly it reverses direction
again and goes CLOCKwise until...
7. Sunset.
Phew! Quite a day, eh?
You get a hint of what's going on once
you draw out an analemmatic sundial for
20 deg. north. You will see that the
date line is LONGER than the minor axis
of the hour-point ellipse.
There are two times of day when the line
from the summer solstice point (say) makes
a tangent to the ellipse. These are the
times when the direction reverses.
I wonder how many readers think that
I am kidding :-))
Life can get tough when you start thinking
about special cases!
Frank King
Cambridge, U.K.
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