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    Re: Stellarium and the heavens in the 17th century
    From: Giuseppe Menga
    Date: 2006 Dec 1, 11:25 +0100

    Dear Frank,
    if you are interested I tested Deneb, Aldebaran, and Polaris  on Dec 1� 1770 
    (6 GMT) using my software based on Swiss Ephemeris and Moshier Epehmeris. I 
    used it very satisfactorily for the last three years with continuous 
    I found the following results very close to those of the stellarium:
    Deneb         44d 28' 36"
    Aldebaran    16d 01' 47.4"
    Polaris        88d 05' 16.8"
    Of course if any one would like to try, I will be happy to distribute it
    ----- Original Message ----- 
    From: "Frank Reed" 
    To: "NavList" 
    Sent: Friday, December 01, 2006 9:21 AM
    Subject: [NavList 1797] Re: Stellarium and the heavens in the 17th century
    > 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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