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    Re: Stellarium and the heavens in the 17th century
    From: Frank Reed CT
    Date: 2006 Dec 01, 00:21 -0800

    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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