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    Re: Pub 229
    From: Robert Bernecky
    Date: 2017 Dec 10, 07:07 -0800

    As a follow-up to all the helpful messages you have received:   Your numbers from Pub 229 look good.

    Using Lat 44, Lon W 73º 18.7' (i.e. LHA 2), Dec N12º, (Same Name as Lat)   we extract Hc 57º 57.2'   d +60.0'   Z 176.3º  

    The actual declination for the sun is N 12º 12.7' and we either use the interpolation tables (those can be confusing!), a calulator e.g.  dec_inc/60 * d, or in this case, by eyeball (d= +60 means  a dec_inc of 12.7 results in exactly 12.7 as the correction).  In any case, we get

    Hc 57º 57.2' + 12.7' = 58 09.9'.  Zn = 360 - Z (when LHA < 180, as the case here) = 183.7º

    Pub 229 relies on a graphical solution -- once we get these numbers we are only half done.

    We find the intercept:  Ho - Hc  to get 57º 40.0' -  58º 09.9' =  57 40 - 47 69.9 = -29.9   i.e. 30 nm Away.  (or remember  HoMoTo,  Ho Mo(re than Hc), To(ward).  We plot the AP point, draw a line pointing in the direction of the body, mark off 30 nm away from the body, and finally, draw in a Line of Position (LOP), perpendicular.  See the attached.

    In theory, we are near or on the LOP, but to get a fix, we need at least one more LOP that intersects with this one.

    At least for me, plotting can take quite a bit of time.... but if you did this for a living I imagine you learn how to do this accurately and efficiently, all the while bouncing around in an airplane.


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