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    Re: star-to-star distances
    From: Alexandre Eremenko
    Date: 2004 Sep 30, 15:21 -0500

    On Thu, 30 Sep 2004, Noyce, Bill wrote:
    
    > Be careful -- the stars' apparent positions,
    > even after correcting for
    > refraction, vary over the course of a year by several tenths of a
    > minute.
    
    Yes, this also should be taken into account.
    I suppose every decent almanach (paper or computer generated)
    does. But with my idea of tabulating,
    an additional correction for the date will be needed.
    I suppose it is easy to arrange.
    
    Also yearly corrections probably have to be made to allow for
    the proper star motion, at least for some stars.
    
    Speaking of the star parallax, it should be ALWAYS negligible
    for our purposes because the distance to the closest star
    is 4.5 light years while the diameter of the Earth orbit is only
    16 light minuites, if I remember correctly.
    The ratio is about 3 million, so you will never detect this
    parallax, with an ordinary sextant, which gives you at most
    5 significant digits.
    This is the reason btw why astronomers discovered this parallax
    so late.
    
    Alex.
    
    > This is called "annual aberration" or "aberration
    > of starlight" or
    > "stellar aberration", and is caused by changes in the direction of
    > earth's
    > orbital motion.
    > A star's apparent position shifts by an amount equal to
    > arctan ( orbital_speed / speed_of_light )
    > toward the direction of the earth's motion.
    >
    > Historical note -- this effect was first discovered by astronomers
    > who were trying to measure parallax of stars (using the earth's orbit
    > as a baseline) to determine how far away they are.  It turns out that
    > parallax is (almost always?) a much smaller effect.
    
    
    

       
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