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    Interpolation to latitude
    From: Ark Shvetsky
    Date: 2009 Nov 8, 22:30 -0800

    I recently got the book by L. Gray "100 Problems in Celestial Navigation".   
    The very first problem, which had quite a lot of explanations  "how to" for 
    some reasons (?) didn't mention and didn't counted for interpolation for 
    longitude in calculating nautical and civil twilight time.  The interpolation 
    for the latitude was done but not for the longitude.  If I'm not mistaken 
    (correct me please if I'm wrong) but the time of the phenomenon (twilight, 
    f.e) is tabulated to the zone meridian. In the problem 1-1 DR longitude is 50 
    deg.W.  The way I understand it the interpolation should be done for the 
    closest zone meridian which in this case will be 45 deg.W.  I looked in the 
    Appendix but still didn't figure out why interpolaion for Longitude wasn't 
    included.  I made the interpolation and of course the results were different 
    from what were in the book.
    Quite possible the explanation for this can be very trivial:
    1.  I overlooked something in the text
    2.  I didn't understand something which quite obvious for everyone else
    3.  ??? "Oh, sorry, we forgot to include it"  (which I doubt because the book 
    is very well-known and definitely well-proved). 
    So, this is my question: am I mistaken regarding this ?
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