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 < [email protected]> 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 <[email protected]> > *Envoyé :* mardi 30 juin 2020 15:31 > *À :* 'Ross Sinclair Caldwell' <[email protected]>; 'Michael Ossipoff' > <[email protected]> > *Cc :* 'sundial list sundials' <[email protected]> > *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 <[email protected]> *On Behalf Of *Ross > Sinclair Caldwell > *Sent:* Monday, June 29, 2020 2:06 PM > *To:* Michael Ossipoff <[email protected]> > *Cc:* sundial list sundials <[email protected]> > *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 <[email protected]> > *Envoyé :* lundi 29 juin 2020 19:31 > *À :* Ross Sinclair Caldwell <[email protected]> > *Cc :* sundial list sundials <[email protected]> > *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 < > [email protected]> 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 season, but there must have also been a regular schedule of > maintenance for the mechanism. I don't know if an example of such a > schedule survives from any of these early clocks, since Europe generally > moved to the equal-hour 24-hour day starting at midnight in the sixteenth > century. > > > > For sunrise, it is a flat view east of Milan, and the part of the castle > where he is reported to have been born was one of the highest places in the > city. From the top of one of the four corner towers, you would see clear to > the eastern horizon. But it is possible they made a calculation rather than > an observation, and so perhaps it was theoretical rather than observed, > even if they used an hourglass with minutes we would recognize. Even if it > were a cloudy morning, they knew what time the sun rose. > > > > For what value it had, the propaganda, since he was the second son, he was > not expected to inherit the throne, so there was less reason to fudge the > data to make him appear better than he was. The day of birth was a public > announcement; the time was apparently a closely guarded secret, since > astrology could be a political weapon. > > > > Ross > ------------------------------ > > *De :* Michael Ossipoff <[email protected]> > *Envoyé :* lundi 29 juin 2020 18:39 > *À :* Ross Sinclair Caldwell <[email protected]> > *Cc :* sundial list sundials <[email protected]> > *Objet :* Re: Time problem > > > > Of course, even if the Earth's orbit didn't change, no civil calendar > keeps a constant relation between date and ecliptic-longitude. So you'd > have to determine the calendar's date-ecliptic-longitude displacement for > the date of interest. > . > But the Earth's orbit does change. Our orbit's eccentricity, and the > relation between the apsides and the equinoxes have been steadily changing > since the 14th century. ...as has the obliquity of the ecliptic. > . > Might some of the commercially-available planetarium-programs disregard > that? Sure. At least some of those programs ignore changes in the > precessional-rate, so why expect them to take into account the changing > eccentricity, apsides/equinoxes relation, and obliquity of the ecliptic? > . > 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? > . > Michael Ossipoff > > > > > > On Mon, Jun 29, 2020 at 5:23 AM Ross Sinclair Caldwell < > [email protected]> wrote: > > Hi diallists, > > > > This is not a sundial problem, but a time discrepancy I don't understand > between NOAA sunrise calculations and the results of two reliable > planetarium programs, Stellarium and YourSky (part of HomePlanet). > http://stellarium.org/ https://www.fourmilab.ch/yoursky/ > https://www.fourmilab.ch/homeplanet/ > > > > 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. > > > > First step - get the Gregorian equivalent, and the Julian day. This is 1 > October 1392 Gregorian, which is Julian day 2229751.5 (".5" because Julian > days start on noon, and the .5 represents midnight, the beginning of 23 > September Julian/1 October Gregorian). > > > > Now, both Stellarium and YourSky automatically correct for the change from > Julian calendar to Gregorian. That is, if you look at the sky for 15 > October 1582, and then go back one day, the calendar reads 4 October 1582. > This was the change mandated by Pope Gregory, that Thursday 4 October 1582 > would be followed Friday 15 October 1582. > > > > So, there is no need to use 1 October 1392 for my purposes - both programs > read 23 September as Julian day 2229751.5(etc). > > > > These programs give the sunrise in Milan on that date at 06:00 and 05:59 > respectively. Obviously they use an ideal horizon, but the view east from > Milan is flat, so there is nothing delaying the appearance of the sun. > > > > Now,, when you go to NOAA's Solar Calculator, they use straight Gregorian > dates. That is, you can get sunrise times for 5, 6, 7, etc. up to 14 > October, 1582. So you have to use the Gregorian equivalent of 23 September > 1392, which is 1 October. https://www.esrl.noaa.gov/gmd/grad/solcalc/ > > > > They give the sunrise time as 06:22 on 1 October 1392. If you are in doubt > about the Gregorian/Julian switch, they give the time on 23 September as > 06:12. Neither is in agreement, in any case, with the astronomy programs. > > > > Now, the difference between 1392 and today should be negligible in any > case. We can just as well use this year's 1 October for the time of > sunrise. Of course, it is 06:22 (or 07:22 since in 2020 Italy uses daylight > saving time). > > > > In order to get a sunrise time of 06:22 on Stellarium, I have to push the > date to 11 October. > > > > The problem is that both NOAA and the astronomy programs are right for me > for sunrise and sunset in Béziers today (within a minute). > > > > So, the astronomy programs are apparently wrong for the 1392 date. This is > not really ancient, so I wonder if anyone could suggest to me why it might > be that there is 22 minutes' difference between these programs and the NOAA > data for the same date? > > > > Thank you for any thoughts that anyone might have. > > > > Ross Caldwell > > 43.349399 3.22422981 > > Béziers > > --------------------------------------------------- > https://lists.uni-koeln.de/mailman/listinfo/sundial > >
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