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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