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    Re: A Nautical Almanac question
    From: David Fleming
    Date: 2016 Nov 24, 12:15 -0800

    Stan K asked

    Is the d value listed at the bottom of the sun and planets columns of the daily pages of the Nautical Almanac the average (or mean) hourly change in the sun's declination for the three days listed (as stated in the Nautical Almanac and other references) or the amount the declination changes between 1200 and 1300 on the middle day of the three shown (as stated in Bowditch)?

    Why do you care?  Why do you doubt the editors of the NA in their description.

    Looking at Sept. 26, 2007 NA for the Sun, d at bottom of page is 1.0 but 1200-1300 d is .9.  That does not answer your question if you are referring to the higher precision numbers that are rounded to produce the table.

    But my intuition tells me that the two answers to your question are identical when rounded to a tenth of a minute.  The d are first differences in tabullated values of dec and are not large.  The difference between your answers depends on the second difference of the values from which the tabulated values are obtained.  The rate of change of dec is small and relatively constant taking a whole year to cycle, so that the second differences are likely orders of magnitude smaller.


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