Hi Fer,
Thanks for your reply;
for a long time I follow with interest and I
appreciate your notes and I substantially find also now correct as you expose,
to pact that in the description of these hourly systems, that I call "of the
ancient ones", the words sunset and sunrise are removed.
I add some observations of mine.
I believe that to follow the historical method is more correct to refer to
the times when the hourly systems have originally been adopted then to those in
which are described.
The ancient gnomonist didn't know the formula of half the daylength; other
motive for which they visually had to refer to the limbs of the Sun.
To directly trace the hourly straight lines on the site, as the ancient
gnomonist had to do, in function of the time spent from the sunset or from the
rising of the Sun is not a computer problem, but simply of time calculation.
To my opinion the reference to the center of the Sun has subsequently been
adopted because it allows a simpler graphic construction; I don't see other
reasons to follow this formulation different from that original; me,
that from a long time deal analiycaly
with the gnomonica, reports me to the superior limb of the Sun.
Jean Meeus for the calculation of the half diurnal arc H0 based on the
superior limb of the Sun and for
including the atmospheric refraction effect proposes the following
formula:
cos H0 = (- 0.1454 - sin phi
* sin decl.) / (cos phi * cos decl.).
Best wishes, Lucio.
Hi Lucio,
Thanks for your note but I don't agree with your
argument.
In all books about gnomonics, old and new,
temporal hours, Italian hours, Babylonian hours and so on are based
on the formula for half the daylength T :
cos T = - tan phi tan decl.
This means that all calculations and
constructions are based on the center of the sun.
Of course it is easy to calculate with respect to
the limb of the sun now we have computers to help us but to my opinion this
isn't usualy done in gnomonics.
In our daily life indeed the sunrise
and sunset is calculated with respect to the limb of the sun. But
gnomonics isn't used in our daily live and I prefer to act in
the historical way.
Best wishes, Fer.
Fer J. de Vries
----- Original Message -----
Sent: Sunday, September 26, 2004 6:15
PM
Subject: R:
I am worthwhile with how much you write, to exception of the
following affirmation:
"Temporal hours are calculated in respect to the center of the sun as we
do for other time systems".
The hours temporarie, italic and babiloniches are reported to the sunset
or to the sunrise; the sunset and rising are, for definition, reported to
the superior limb of the Sun; therefore it is inaccurate to refer to the
center of the Sun and this yeld to an error of about 4 minutes.
Being complicated to establish the transit of the center of the Sun on
the line of the horizon is unlikely, neither it results me, that followed
this method in the antiquity when instead it is simple to refer to the
superior edge.
Best wishes, Lucio
-----Messaggio
originale-----
Da: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED]Per conto di Fer J. de
Vries
Inviato: venerdì 24 settembre 2004 12.22
A: Noam
Kaplan; sundial
Oggetto: Re:
Hi Noam,
Of course the refraction effects the
hourangle of all the time systems we use in dialling.
But also keep in mind the historical meanings
of used systems as the temporal hours.
Temporal hours are calculated in respect to
the center of the sun as we do for other time systems.
The times of sunrise and sunset however are
calculated for the limb of the sun.
Should we care about that
either?
We want to have time in seconds and even in
smaller parts.
But in older days the feelings about time was
much different.
Temporal hours were used as a period of time
peolple was in, not an exact moment of time as we often use.
Calculating the temporal hour lines shows
that the lines aren't straight but sligthly S-shaped which is best visible
with increasing latitude but in older
dials the lines are drawn as straight lines.
It's good to think about the effect of
refraction but in general I should say, forget the refraction in
dialling.
Best wishes, Fer.
Fer J. de Vries
----- Original Message -----
Sent: Friday, September 24, 2004
12:30 AM
Can anyone answer this?
If this is the wrong forum, I
apologize.
I have a calculation to figure out the
atmospheric refraction from Fred Sawyer's article in the NASS
Compendium. It is based on calculations that Meeus brings in his book.
Refraction changes the apparent altitude of the sun, thereby changing
both the apparent declination and apparent hour angle of the sun.
Am I making a mistake when I use the
apparent declination and apparent hour angle for the temporal hour
calculation?
The effect of a few seconds difference
for atmospheric refraction on the hour angle seems to have a much
bigger effect on the temporal hours.
Thanks for any help you can
offer,
Noam
My calculations can be seen on the web at
in the function
temporal(localTime)
