I am using Stellarium to perform an analysis of the Azimuth of a number of ancient sites relative to the rising and setting of the Sun and of certain constellations at the Solstices and Equinoxes.
I understand that Stellarium uses the Julian Calendar for dates before 1582, which causes a "drift" in the date of the Equinox. Because of this, the date of the spring Equinox (for instance) would drift away from March 21st. This makes it extremely difficult to use the date/time window to observe the Equinox for years before 1582, unless one knows in which day in the Julian calendar the Equinox would have fallen in each year.
I was wondering if there is any way to overcome this issue, or that allows to easily identify the correct date for the Equinox in any given year.
Is there any way
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After a quick search I found this table: http://ns1763.ca/equinox/vern1452-1811.html
If you read the remarks at the end of the page and search a little deeper, I'm sure you'll find more information.
The times in the table are Universal Time, so take into account the settings in Stellarium for daylight saving time.
You can learn about the calculation of equinoxes and solstices in 'Astronomical Algorithms' by Jean Meeus, chapter 26.
I noticed in the time window, that when the date is set to 1582 Oktober 16 and clicking the down arrow, the day doesn't jump from 15 to 4. When clicking forward from 1582 Oktober 2, the date jumps from 4 to 15. I think this has been mentioned before and it hasn't been fixed. (Bug reported.)
Thanks Kird for your kind reply.
Because I am mostly researching ancient sites, I am looking to even earlier solstice and equinox dates, perhaps as far back as 7,500 BC.
In particular I would be interested to see if, knowing the azimuth, it is possible to determine in which year the solsticial sunrise or sunset would have matched that azimuth: for instance, given an azimuth of 247.5 at a given latitude, I would like to be able to tell in which year (as a consequence of precession) the sunset azimuth at the winter solstice would have had such value?
Most ephemerides aren't accurate over such a long timespan.
The positions of the Sun and the equinoxes in the year 7500 BC calculated by Stellarium may be off by several degrees.
I think that the value of the obliquity of the ecliptic is more important than precession in what you are trying to find out.
The program Julian-Gregorian-Dee Date Calculator at http://www.hermetic.ch/jgdc/jgdc.htm gives the dates and times of the northern spring equinox in both the Julian and the Gregorian calendars for years in the range 1 through 4000.