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
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