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Mike.  The d value for a column of a table is the difference between a value and the next value down.  It enables you to calculate intermediate values.  It can have a slightly different meaning depending on the table.  E.g. for the Moon in the daily pages of the Nautical Almanac, d is the difference in the Moon’s declination between two consecutive hours.  As the value of the Moon’s declination roughly follows a sine wave sometimes increasing and sometimes decreasing, then yes, it should be roughly 50% + and 50% minus as the weeks go by.  The + and – isn’t give on the daily page of the Nautical Almanac, so you have to work out if it’s + or - for yourself by inspecting the declination columns.  In HO249/AP3270 Vols 2 and 3, d is the correction to Hc for intermediate values of declination between consecutive whole degrees of declination.  In this case its + or – is indicated at every value of LHA divisible by five (to save the page getting too cluttered???).  DaveP