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    Re: Pointer star question
    From: Alexandre Eremenko
    Date: 2013 Feb 2, 16:57 -0500

    Just one remark.
    Stellarium is of course free and nice.
    But not reliable.
    
    I discovered this when I tried to see the recent event of Jupiter passing
    the Moon. Stellarium gave a totally wrong picture.
    I suppose because it had wrong Moon diameter, but did not investigate
    this further.
    
    So I would not rely on Stellarium, but read the coordinates from
    some reliable almanac.
    
    Alex.
    
    P.S. Some distributions of Linux (Fedora) have free software similar to
    Stellarium but much better. I used this 10 years ago when I had Linux on
    my laptop. It was especially useful for star distances. You click
    in a star and drug to another star. And it shows the distance.
    (Not corrected for refraction). It also had bugs though.
    
    A.
    
    > Looks like the distance from Merak to Polaris is 34.0699 degrees or 34°
    > 4' 12". This is the value for today without refraction. You can calculate
    > this if you know the stars' coordinates in RA (or GHA or SHA) and Dec. Let
    > me know if you're interested. You can read out coordinates from the free
    > software Stellarium (available from stellarium.org). While Stellarium has
    > some bugs, you can usually count on things like stellar coordinates. Just
    > click on each star, and the exact coordinates are displayed in the upper
    > left. There is also an angle-measuring tool available in Stellarium and
    > with careful clicking and zooming in and out, you can measure angles
    > between stars with an accuracy of at least 1' of arc.
    >
    > This star-to-star distance will vary slightly during the year due to
    > aberration, and it will vary significantly with the stars' altitudes due
    > to refraction. For example, if you're in latitude 41 N and the Dipper is
    > low in the sky with Merak almost directly below the pole star, Polaris
    > will be lifted by refraction by 1.1 minutes of arc while Merak (at an
    > altitude of about 7 degrees) will be lifted by about 7.3 minutes of arc so
    > the angular separation will be reduced by 6.2' to 33° 58.0'. If instead
    > Merak is high above Polaris, then the distance will be 34° 3.4' reduced
    > by refraction by only 0.8'. In the latter case, we're nearly in the
    > situation where the change in distance due to refraction follows the
    > simple rule "0.1' for every 5 degrees of distance" which applies with good
    > accuracy for any orientation of the pair of stars and any altitude as long
    > as both stars are above 45 degrees altitude.
    >
    > -FER
    >
    >
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