Hi Ross,
Are you making Time Zone adjustments?. When I start Stellarium, by
default it uses my own time zone even if I change location. I have to go
to the Plugins and explicitly set a time zone. I imagine you should be
working in the local time of Milan, which I estimated roughly at (9.2
deg E) or +37 minutes ahead of Greenwich. I can't explain 22 minutes
with this thought process though.
Cheers
Hank
On 29/6/20 6:46 pm, Ross Sinclair Caldwell 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
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