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] <mailto:[email protected]> > Envoyé : lundi 29 juin 2020 19:31 À : Ross Sinclair Caldwell <[email protected] <mailto:[email protected]> > Cc : sundial list sundials <[email protected] <mailto:[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] <mailto:[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] <mailto:[email protected]> > Envoyé : lundi 29 juin 2020 18:39 À : Ross Sinclair Caldwell <[email protected] <mailto:[email protected]> > Cc : sundial list sundials <[email protected] <mailto:[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] <mailto:[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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