RE: Time problem

[Schechner, Sara]

>>> In short, I am researching the biography of Filippo Maria Visconti 
>>> (1392-1447), duke of Milan, and you probably know that these Italian 
>>> princes relied heavily on astrology. So, Visconti's time of birth is known 
>>> precisely - "six minutes after sunrise," Monday, 23 September, 1392. His 
>>> natal chart was of course produced and interpreted, but it has been lost. I 
>>> am trying to recreate it as it might have been done by a court astrologer 
>>> of the time.<<<

I have some thoughts about ascertaining the time of "6 minutes after sunrise" 
in 1392 in Milan.

First of all, Milan is one of the earliest towns to have a public tower clock 
in the 14th century, but it would only strike and show hours according to local 
solar time.  It would not be divided into minutes.  It was not reliable enough 
for such a horological chart.

Sundials would be the more commonly used timepiece, but the six-minutes is an 
unusual amount of precision.  My guess is that the court astronomer was using 
an astrolabe, which can be divided into units in the range of 4-6 minutes.  
Many also had arcs for the astrological houses and for both equal and unequal 
hours.  The actual time might have been taken from a bright star still visible 
in the dawn.

It is also worth considering what this 6-minutes after dawn really means.  Is 
the astrologer using unequal hours which were still more common in these early 
days of clocks?  If so, then six minutes would be equal to 1/10 of the first 
hour on that day of the year-i.e., 1/10 of 1/12 of the length of daylight.

Lastly, in reconstructing a horoscope, one needs to know the position of the 
planets to place them on the chart.  Some might be observed, but mostly they 
are taken from a table.  These varied in different manuscript traditions.  Do 
we have a clue what table the astrologer was using?

Good luck with your project.

Sara

Sara J. Schechner, Ph.D.
David P. Wheatland Curator of the Collection of Historical Scientific 
Instruments
Lecturer on the History of Science
Department of the History of Science, Harvard University
Science Center 251c, 1 Oxford Street, Cambridge, MA 02138
Tel: 617-496-9542   |   Fax: 617-495-3344
sche...@fas.harvard.edu  | @SaraSchechner
http://scholar.harvard.edu/saraschechner
http://chsi.harvard.edu/



---
https://lists.uni-koeln.de/mailman/listinfo/sundial

Re: Time problem

[Michael Ossipoff]

According to a graph from Lascar, in 1986, the greater obliquity of the
elcliptic 700 years ago would, even at the Winter-Solstice, only change
Sunrise-time (in local true Solar time) at lat 46 by about half a minute.

In fact, even with the greatest obliquity that ever occurs in the current
cycle, that lat 46 Winter-Solstice Sunrise time would only differ from now
by about 6 minutes.

So evidently one of your sources has simply made a big error of some kind.

On Tue, Jun 30, 2020 at 10:06 AM Ross Sinclair Caldwell <
belmu...@hotmail.com> wrote:

> Hi Jack,
>
> Thanks for thinking about this problem.
>
> It isn't the clock time, in any system, that matters here. The biographer
> - Pier Candido Decembrio - reports only that it was six minutes after
> sunrise. So all that matters is to determine when sunrise was, by any
> system we can, in order to be able to put the data into an astronomy
> program or a helpful spreadsheet using medieval values, like Lars Gislén's
> "Astromodels" for the Alfonsine Tables, which those astrologers probably
> used. http://home.thep.lu.se/~larsg/Site/download.html
>
> The problem I encounter is that two very apparently reliable sources give
> different times for the sunrise from Milan on that day, once the date is
> corrected to Gregorian and given a Julian day.
>
> The NOAA site gives 06:22 CET, the program Stellarium gives 06:00. On
> Stellarium, today I went back year by year, and noticed that they not only
> automatically switch to Julian calendar before 15 October 1582, but also
> make a change in  times in the year 1847. In both Béziers, where I live,
> and Milan, sunrise for 1 October is 07:22 (what is the historical basis for
> this additional hour?) in 1848, but goes to 06:00 in 1847 and all the years
> thence back to 1583 (within a minute or so, for the quarter days leading to
> a leap year). In 1582, 1 October sunrise in Milan is 06:12, so you have to
> know to change to the Julian calendar date of 23 September to get the right
> sunrise, which is 06:01.
>
> Hank showed from the "old" NOAA Earth System Research Lab page
> https://www.esrl.noaa.gov/gmd/grad/solcalc/sunrise.html that putting in
> the data with the UTC offset at +0.61 for Milan (-0.61 for American users)
> at that longitude produced the "correct" time at 05:58, so with a few more
> decimals it would be within a minute of the Stellarium and YourSky programs
> (which rigorously uses Meeus, I believe).
>
> I am leaning to a 06:00 as the consensus.
>
> Ross
>
>
> --
> *De :* Jack Aubert 
> *Envoyé :* mardi 30 juin 2020 15:31
> *À :* 'Ross Sinclair Caldwell' ; 'Michael Ossipoff'
> 
> *Cc :* 'sundial list sundials' 
> *Objet :* RE: Time problem
>
>
> I have been thinking about this problem but I may not be understanding it
> correctly.  I think you want to find out what time sunrise was on September
> 23 in 1392.  Because of the change from Julian to Gregorian dates, this
> corresponds to our October 1.  On October 1, a real clock in Milan this
> year would not tell quite the same time as a municipal clock in 1392,
> though.
>
>
>
> We can easily correct for daylight saving time.  The second thing to
> consider would be the equation of time.  But it has changed very little
> between 1329 and now, so sunrise on October 1 1329 in Milan should be
> almost the same time as it is now, so if you could transport a modern clock
> to Milan in 1329, it would show sunrise at very close to the same time as
> it does now.  But this would not necessarily be the case in 1392.  At that
> time, clocks would normally not take the equation of time into account at
> all.  Since they were not very accurate over an extended period, they would
> have had to be adjusted frequently using a sundial.  So the municipal clock
> would probably have shown noon at what we would call 12:11.  It is possible
> that a clock used by an astronomer might make the adjustment using a
> contemporaneous equation of time table (which would have been less accurate
> than our calculation) but this seems unlikely.
>
>
>
> The other thing to take into account is Milan's longitude.  At 9.11
> degrees East, Milan is six degrees from the 15 degree time zone center, for
> a clock offset of 24 minutes.   So a calculation for modern civil time at
> that location should include both the longitude and equation of time.  A
> calculation of contemporary civil time would obviously not have included a
> time zone offset, I think, should not have included the equation of time
> either.
>
>
>
> It sounds to me as if the programs may be handling the longitude offset,
> and possibly the equation of time differently.
>
>
>
> Does this make sense?
>
>
>
> Jack Aubert
>
>
>
> *From:* sundial  *On Behalf Of *Ross
> Sinclair Caldwell
> *Sent:* Monday, June 29, 2020 2:06 PM
> *To:* Michael Ossipoff 
> *Cc:* sundial list sundials 
> *Subject:* RE: Time problem
>
>
>
> Yes, but I don't know if any estimation of refraction or diameter would
> account 

RE: Time problem

[Ross Sinclair Caldwell]

Hi Jack,

Thanks for thinking about this problem.

It isn't the clock time, in any system, that matters here. The biographer - 
Pier Candido Decembrio - reports only that it was six minutes after sunrise. So 
all that matters is to determine when sunrise was, by any system we can, in 
order to be able to put the data into an astronomy program or a helpful 
spreadsheet using medieval values, like Lars Gislén's "Astromodels" for the 
Alfonsine Tables, which those astrologers probably used. 
http://home.thep.lu.se/~larsg/Site/download.html

The problem I encounter is that two very apparently reliable sources give 
different times for the sunrise from Milan on that day, once the date is 
corrected to Gregorian and given a Julian day.

The NOAA site gives 06:22 CET, the program Stellarium gives 06:00. On 
Stellarium, today I went back year by year, and noticed that they not only 
automatically switch to Julian calendar before 15 October 1582, but also make a 
change in  times in the year 1847. In both Béziers, where I live, and Milan, 
sunrise for 1 October is 07:22 (what is the historical basis for this 
additional hour?) in 1848, but goes to 06:00 in 1847 and all the years thence 
back to 1583 (within a minute or so, for the quarter days leading to a leap 
year). In 1582, 1 October sunrise in Milan is 06:12, so you have to know to 
change to the Julian calendar date of 23 September to get the right sunrise, 
which is 06:01.

Hank showed from the "old" NOAA Earth System Research Lab page 
https://www.esrl.noaa.gov/gmd/grad/solcalc/sunrise.html that putting in the 
data with the UTC offset at +0.61 for Milan (-0.61 for American users) at that 
longitude produced the "correct" time at 05:58, so with a few more decimals it 
would be within a minute of the Stellarium and YourSky programs (which 
rigorously uses Meeus, I believe).

I am leaning to a 06:00 as the consensus.

Ross



De : Jack Aubert 
Envoyé : mardi 30 juin 2020 15:31
À : 'Ross Sinclair Caldwell' ; 'Michael Ossipoff' 

Cc : 'sundial list sundials' 
Objet : RE: Time problem


I have been thinking about this problem but I may not be understanding it 
correctly.  I think you want to find out what time sunrise was on September 23 
in 1392.  Because of the change from Julian to Gregorian dates, this 
corresponds to our October 1.  On October 1, a real clock in Milan this year 
would not tell quite the same time as a municipal clock in 1392, though.



We can easily correct for daylight saving time.  The second thing to consider 
would be the equation of time.  But it has changed very little between 1329 and 
now, so sunrise on October 1 1329 in Milan should be almost the same time as it 
is now, so if you could transport a modern clock to Milan in 1329, it would 
show sunrise at very close to the same time as it does now.  But this would not 
necessarily be the case in 1392.  At that time, clocks would normally not take 
the equation of time into account at all.  Since they were not very accurate 
over an extended period, they would have had to be adjusted frequently using a 
sundial.  So the municipal clock would probably have shown noon at what we 
would call 12:11.  It is possible that a clock used by an astronomer might make 
the adjustment using a contemporaneous equation of time table (which would have 
been less accurate than our calculation) but this seems unlikely.



The other thing to take into account is Milan's longitude.  At 9.11 degrees 
East, Milan is six degrees from the 15 degree time zone center, for a clock 
offset of 24 minutes.   So a calculation for modern civil time at that location 
should include both the longitude and equation of time.  A calculation of 
contemporary civil time would obviously not have included a time zone offset, I 
think, should not have included the equation of time either.



It sounds to me as if the programs may be handling the longitude offset, and 
possibly the equation of time differently.



Does this make sense?



Jack Aubert



From: sundial  On Behalf Of Ross Sinclair Caldwell
Sent: Monday, June 29, 2020 2:06 PM
To: Michael Ossipoff 
Cc: sundial list sundials 
Subject: RE: Time problem



Yes, but I don't know if any estimation of refraction or diameter would account 
for 20 minutes!



In any case, the real time is scarcely relevant - they only wanted to say that 
it was shortly after sunrise, sufficiently so that the Sun  was estimated to be 
clear of the horizon.



The clock they used only matters for the calculation of minutes, which with a 
24-hour clock, however calibrated, would be the same as ours for all practical 
purposes.



The biographer doesn't give the time in clock time, only minutes after sunrise. 
This is why I want to know what that is. The true time of his birth is 
absolutely irrelevant; we only need to know what they believed, and interpreted 
from that belief.



Ross



De : Michael Ossipoff 

RE: Time problem

[Jack Aubert]

I have been thinking about this problem but I may not be understanding it
correctly.  I think you want to find out what time sunrise was on September
23 in 1392.  Because of the change from Julian to Gregorian dates, this
corresponds to our October 1.  On October 1, a real clock in Milan this year
would not tell quite the same time as a municipal clock in 1392, though.  

 

We can easily correct for daylight saving time.  The second thing to
consider would be the equation of time.  But it has changed very little
between 1329 and now, so sunrise on October 1 1329 in Milan should be almost
the same time as it is now, so if you could transport a modern clock to
Milan in 1329, it would show sunrise at very close to the same time as it
does now.  But this would not necessarily be the case in 1392.  At that
time, clocks would normally not take the equation of time into account at
all.  Since they were not very accurate over an extended period, they would
have had to be adjusted frequently using a sundial.  So the municipal clock
would probably have shown noon at what we would call 12:11.  It is possible
that a clock used by an astronomer might make the adjustment using a
contemporaneous equation of time table (which would have been less accurate
than our calculation) but this seems unlikely.  

 

The other thing to take into account is Milan's longitude.  At 9.11 degrees
East, Milan is six degrees from the 15 degree time zone center, for a clock
offset of 24 minutes.   So a calculation for modern civil time at that
location should include both the longitude and equation of time.  A
calculation of contemporary civil time would obviously not have included a
time zone offset, I think, should not have included the equation of time
either. 

 

It sounds to me as if the programs may be handling the longitude offset, and
possibly the equation of time differently.   

 

Does this make sense?  

 

Jack Aubert  

 

From: sundial  On Behalf Of Ross Sinclair
Caldwell
Sent: Monday, June 29, 2020 2:06 PM
To: Michael Ossipoff 
Cc: sundial list sundials 
Subject: RE: Time problem

 

Yes, but I don't know if any estimation of refraction or diameter would
account for 20 minutes!

 

In any case, the real time is scarcely relevant - they only wanted to say
that it was shortly after sunrise, sufficiently so that the Sun  was
estimated to be clear of the horizon. 

 

The clock they used only matters for the calculation of minutes, which with
a 24-hour clock, however calibrated, would be the same as ours for all
practical purposes.

 

The biographer doesn't give the time in clock time, only minutes after
sunrise. This is why I want to know what that is. The true time of his birth
is absolutely irrelevant; we only need to know what they believed, and
interpreted from that belief. 

 

Ross

  _  

De : Michael Ossipoff mailto:email9648...@gmail.com> >
Envoyé : lundi 29 juin 2020 19:31
À : Ross Sinclair Caldwell mailto:belmu...@hotmail.com> >
Cc : sundial list sundials mailto:sundial@uni-koeln.de> >
Objet : Re: Time problem 

 

Okay, but there's the inaccuracy of the clocks in those days, and the
importance of that would depend on how they determined Sunrise. I guess they
set the clocks by sundial or noon-mark, but, as you said, it depends on how
often they set them.

 

Anyway, the difference between the NOAA Sunrise-time, and the one calculated
by the planetarium-programs could result from the planetarium-programs not
taking into account the changes in orbit or obliquity.  I'd expect that the
NOAA figure would be more reliable.

 

Sunrise & Sunset times are usually calculated using a standard value for
atmospheric refraction at the horizon. The usual assumption is that the
refraction is 34 minutes and that the Sun's apparent semi-diameter is 16
minutes. Maybe NOAA used a calculated semi-diameter instead of the standard
16 minutes.

 

You don't have sufficiently reliably accurate information for a horoscope
accurate to the minute, and another reason for that is that unusual
atmospheric refractivity could change Sunrise-time by minutes.

 

Michael

 

 

 

On Mon, Jun 29, 2020 at 1:09 PM Ross Sinclair Caldwell mailto:belmu...@hotmail.com> > wrote:

 

Hi Michael,

 

Also, when they said that he was born a certain number of minutes after
Sunrise, how did they determine that? By judging when it seemed to be
Sunrise, when the Sun appeared over the trees, mountains or buildings, or by
calculating Sunrise-time based on a 14th century estimate of Milan's
longiitude?  And were they minutes of equal-hours time, or of
temporary-hours time?

I can answer some of those questions with reasonable certainty. 

 

For minutes, they used an equal-hour 24 hour clock, beginning a half-hour
after sunset the previous day. That is, the clock would strike "1" at, say,
at our 20:45 on that particular day (30 September Gregorian). Of course it
was constantly adjusted, with what frequency I don't know. Obviously it
depended on the