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    Re: Pub249 etc
    From: Gary LaPook
    Date: 2007 Sep 09, 23:36 -0700

    A0 accounts for the the cosine of  LHA of Polaris times the distance
    Polaris is from the pole (90� - decl. Polaris).
    
    A1 accounts for "the tilt of the diurnal circle of Polaris with
    respect to the vertical." (Bowditch, article 2105, 1962 ed.)
    
    A2 accounts for the movement of Polaris from its nominal position
    during the year.
    
    To make all the factors positive, constants were added to all three
    and the sum of all the constants is 1� so 1� is subtracted at the end
    of the computation.
    
    The "Q" correction table found in H.O. 249 only corrects for A0 as A1
    is small and irrelevant to the precision possible in aerial celestial
    navigation. The "Q" table strictly is only accurate for the year of
    the  "epoch"  of the table as it is based on the coordinates of
    Polaris on the date of the epoch. Since H.O. 249 is used for a 10 year
    period  a "Precession and Nutation" correction table is provided to
    correct positions obtained for any year 2001 through 2009. This
    correction should also be applied to a Polaris LOP calculated with the
    "Q" table as this will correct for the movement of Polaris from its
    nominal position similar to factor A2.
    
    Since A0 is the largest factor you can compare the N.A. Polaris table
    with the "Q" table by subtracting 1� from A0 and it will be
    approximately the same as the "Q" factor for the same LHA. If you do
    this with a copy of the 2005 N.A. it should end up with the same value
    to the precision of the "Q" table.
    
    Another easy way to derive the correction for a Polaris sight is to
    plot the position of Polaris on a rotating circular plotting board
    such as the Navy Mk 5A or Mk 6A; or the Air Force Polhemus Celestial
    Computer, CPU-41/P; or even on the 2101-D Star Finder. Subtract
    Polaris' current GHA from 360� and then plot Polaris on the plotting
    board on that azimuth and at a distance out from the center of the
    board equivalent to its distance from the pole, currently about 42'.
    (Plotting it this way allows you to use LHA Aries instead of LHA
    Polaris, saving you one step in the computation.) You then only have
    to set the plotting board to LHA Aries and the distance Polaris is
    above or below the center gives you the "Q" correction. You can update
    the position of Polaris from time to time.
    
    
    gl
    
    
    On Sep 9, 5:44 pm, Ron Irwin  wrote:
    > On my 55th birthday my wife suggested I look for an
    > opportunity to exercise my brain. As we get older
    > mental exercise becomes more impotant.  Considering my
    > love of sailing, celestial navigation seemed a natural
    > quest. With the internet I was able to discover this
    > incredible group with members of a caliber and
    > knowledge I am humbled to address. Through Schlereth,
    > Howell, Bowditch and others i have come to a grasp of
    > the process but have a few questions on
    > implementation.
    >
    > My question is in regards to the Polaris tables  in
    > Pub 249.
    >
    > The formula as stated in in tha Almanac  = Ho -1
    > degree + a0 +a1 + a2
    >
    >  I understand A0 and A3, and assume A2 is reflecting
    > the oblation of the earth. But what is the -1 degree
    > about?
    >
    > The further I get into this the more it fascinates me.
    > I especially enjoyed the Shovell  discussions.
    >
    > Thank you,
    >
    > Ron Irwin
    >
    > ____________________________________________________________________________________
    > Take the Internet to Go: Yahoo!Go puts the Internet in your pocket: mail, 
    news, photos & more.http://mobile.yahoo.com/go?refer=1GNXIC
    
    
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