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Nicolas, you wrote:
"Does anyone of you know Stellarium (http://www.stellarium.org/) and
how
accurate it is for long gone years? "

First, that is one very fine piece of software --a really beautiful
planetarium simulator. Thanks for bringing it up. This is the first
time I've seen it.

As for Stellarium's accuracy, it does not appear to be very high.
Comparing positions for any date including current dates, I find
typical errors of 30 seconds of arc. For example, at 0600 GMT on
December 1, 2006, Stellarium gives the Moon's Declination as 09d 12'
21" while my Online Nautical Almanac (see my web site at
www.HistoricalAtlas.com/lunars) has 09d 12' 42" (geocentric). I don't
yet see a way in Stellarium to get geocentric coordinates directly so I
extracted the Moon's Dec by travelling to that spot in the Pacific
Ocean where the Moon was in the zenith. That nulls out the refraction
and parallax. I can't think of any way to explain away a discrepancy of
21 seconds of arc. That's a large error by the standards of this type
of software.

For a few other cases, I went to December 1, 1770 at 0600 GMT. I find
these discrepancies in declination:
Object         Stellarium            my online almanac
-------------------------------------------------------------------------
Deneb         44d 28' 36"           44d 28' 23"
Aldebaran    16d 01' 47"           16d 01' 11"
Polaris        88d 05' 16"           88d 05' 16"

I have extensively tested my own almanac output, which is based
internally on the JPL ephemeris data for the Moon and planets and the
Hipparcos positions and proper motions for the stars, and it can be
trusted to the nearest second of arc. Based on these comparisons, it
appears that you could certainly trust the Stellarium positions to one
minute of arc (good enough for the vast majority of historical
navigation problems), but you should not trust it for anything
requiring higher accuracy (like lunars ).

I should add that there is the possibility that the data displayed in
Stellarium refers to some non-standard set of coordinates. So it may
still be "correct" and accurate. The source code of Stellarium is
available in the Linux download. There is one three megabyte block of
code with "VSOP87" in the file name. This is a particular set of
calculational algorithms for planetary positions, generally very
accurate.

If you want really high accuracy (for historical planetary positions in
particular), I highly recommend "Solex" created by Aldo Vitagliano:
http://chemistry.unina.it/~alvitagl/solex/. This is at the other end of
the spectrum from Stellarium. It's extremely accurate over a very long
time range, blowing away all the competition, but it has mostly plain
text output.

-FER


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