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    Re: How far is polaris?
    From: Paul Hirose
    Date: 2007 Nov 23, 12:29 -0800

    A similar discussion occurred in May 2006 on the old Nav-L list, and I
    posted this:
    
    [quote]
    
    As already explained by George Huxtable, the month to month variation in
    the declination of Polaris is largely caused by the annual aberration.
    
    Let's first examine the declination with respect to the true equator and
    equinox of date *without* applying aberration. In the table below, the
    first column of declinations shows a practically constant increase of
    17″ per year. This is the effect of precession and nutation, that is,
    the changing orientation of Earth's axis. (For some point elsewhere on
    the celestial sphere the geometry could be quite different, possibly
    resulting in very little change in declination during a year.)
    
    In the second column of declinations I have applied annual aberration
    but not precession and nutation. That is, Earth's axis is held fixed.
    You can see a cyclic fluctuation, with Polaris returning to the same
    place after one year.
    
    The last column combines both effects. The values here essentially
    duplicate the original posting.
    
                 °  '  "     '  "     '  "
    2006 05   +89 17 35   +17 34   +17 34
    2006 06   +89 17 36   +17 24   +17 25
    2006 07   +89 17 38   +17 17   +17 21
    2006 08   +89 17 40   +17 15   +17 20
    2006 09   +89 17 41   +17 18   +17 25
    2006 10   +89 17 42   +17 25   +17 33
    2006 11   +89 17 43   +17 35   +17 44
    2006 12   +89 17 44   +17 45   +17 55
    2007 01   +89 17 46   +17 53   +18 04
    2007 02   +89 17 48   +17 55   +18 08
    2007 03   +89 17 50   +17 52   +18 07
    2007 04   +89 17 51   +17 44   +18 00
    2007 05   +89 17 52   +17 34   +17 51
    
    [end quote]
    
    The table ignores annual parallax, but it's only .008 arc seconds for
    Polaris. That is, its apparent position will shift .008 arc second
    relative to some extremely distant background object, as your viewpoint
    moves from the Sun to Earth.
    
    Like all stars, Polaris also has "proper motion", a slow drift in its
    position relative to a space-fixed coordinate system. It's too tiny to
    be apparent at 1 second precision during one year.
    
    I used the Hipparcos catalog as the basis for the table. The data for
    Polaris may be viewed by entering its Hipparcos Identifier, 11767, on
    this page:
    http://www.rssd.esa.int/index.php?project=HIPPARCOS&page=hipsearch
    
    Note the high quality of the data: the standard errors for position,
    parallax, and annual proper motion are all tenths of a milli arc second!
    
    The original post is here, but the table formatting is messed up, at
    least in my browser:
    http://www.i-DEADLINK-com/lists/navigation/0605/0222.html
    
    -- 
    I block messages that contain attachments or HTML.
    
    
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