NavList:

Hi Mike. I assume your looking at Mary Blewitt ‘Celestial Navigation for yachtsmen’.  You don’t really need to know at this stage, but there are actually three volumes to AP3270, volumes 1, 2, and 3.  Known affectionately as the Red, Green and Yellow Band, because they used to have a coloured band around the cover, so you could pick up the correct volume without reading the front in poor light.  You’re looking at one of the pages for Latitude 52N from AP3270 Volume 3, the ‘Yellow Band’.  Most people like to have a table to fill in as a prompt. A good way to tabulate this particular part of your working might be:
Assumed Lat 52N(Mary lived in the UK)
LHA: 014
dec: 18degrees ??’S
Hc 18degrees57’, d -60,  Z 166degrees

Yes, d is the difference in Hc for 52N, LHA014, between declinations 18 degrees and 19 degrees.  In this case d is minus 60, so if the actual declination was 18degrees 30’S you would simply subtract 30’ from Hc.  However, it’s rarely as straight forward as that.  If declination was 18degrees 23’ and d was minus 42, you’d need to be a professor of mental arithmetic at Oxford University to work that one out, so you use Table5 in AP3270 Vol 3 instead.  Going in with d=42 and ‘=23 would give you 16’ to be subtracted from Hc.  The other thing to tell you at this stage is that the original hard back copies of AP3270 came with copies of the tables repeated on loose sheets of thin card, which you could slip in at the main page in use as a book mark and use as a ruler to help you find the correct values from the various tables, so it wasn’t as headachy as you might think .  Yes, you can get + and – d.  When declination is the same name as the latitude, d appears in my complete edition of AP3270 Vol 3 to be positive.  One of our spherical trig experts will no doubt explain why that has to be so.  Keep doing the calcs.  DaveP