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    Re: help with sun sights
    From: Jeremy C
    Date: 2011 Jan 15, 03:00 EST
    A few hints about the "d" correction in the Nautical Almanac.
     
    1) This is the change per hour of declination.  You can find it at the bottom of the sun, and planet columns.  The moon, being a strange creature of the sky, will have a different correction for each hour.
     
    2) The "sign" is not given but must be deduced based on the trend of the declination over the hour.  For example right now the sun is in the Southern hemisphere moving northward.  Therefore the Declination is labeled South and is getting smaller (heading towards the equator).  The sign is therefore negative.  The biggest wrinkle of this is right near the equinox when the declination changes from South to North or vice versa.  In all cases, the final declination must lie between the two numbers given for the reference hours.
     
    3) Being pretty persnickety with my navigation, I never use the "d" correction at the bottom of the column.  I look at the difference in declination between the two hours and make a quick mental subtraction.  You will notice that when the rate of change of the declination is high (near the equinox), the "d" value listed beneath the column isn't always correct which can lead you to have incorrect declinations just before the top of the hour.  My method avoids this problem.
     
    4) the d-correction in HO 229 reductions is similar but is for interpolating declination in those tables and is not directly related to the "d" in the nautical almanac.
     
    5) The "v" correction is similar to the "d" correction but is applied to the GHA of the body.  You must use the number at the bottom of the column for all non-lunar bodies.
     
    Jeremy
     
    In a message dated 1/14/2011 11:07:39 P.M. Central Asia Standard Tim, goold{at}vwc.edu writes:
    The sky has cleared over this part of the planet and I am trying to move on with my sun sights, taking sights before and after LAN as well as LAN, doing the sight reduction and plotting the LOPs.   One tiny wrinkle holds me up:  I do not understand the d-correction factor.  I have been going back and forth between Burch's Starpath course, Cunliffe's Celestial Navigation and Bowditch (2002) and this has worked reasonable well.  Where I hit a wall with one, I get a hand over it with one of the others.  But here I find no way opening.  Can someone explain this factor to me?

    Burch tells me "look to the bottom of the sun column in the NA to find the 'd-value' and record it..."  The number I find at the bottom of the page for Jan 13, 14 and 15 that has a small case d beside it is 0.4.  Is this what I need?  What do I do with this number?  How is it related to d-corr?  As for d-corr, I am unsure about how to read the increments and corrections table or what to do with the number I get there once I do.  Bowditch has a different way of talking about this, calling the d value a "d Correction Factor" and using it "to enter the corrections table".  This doesn't enlighten me. 

    I know there are people out there who take joy in battling back nescience and confusion about celnav.  Here is your chance.

    Patrick

    --
    Dr. Patrick Goold
    Department of Philosophy
    Virginia Wesleyan College
    Norfolk, VA 23502
    757 455 3357

    Charles Olson: "Love the World -- and stay inside it."

       
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