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