NavList:

Mike you wrote:  Obviously I understand all the information contained in the NA and AP3270 Vol 1 will have changed since my book was published in 1990 but providing the method has not changed I am pretty happy that the application is pretty well explained it is just that I became curious about the MONTH/YEAR anomaly.

Mike.  You have to remember that AP3270/HO249 was originally designed as a speedy method of pre-computing altitudes and azimuths for use in the air with a bubble sextant.  When post war yachties and eventually Navies realised how useful they were, they pinched them for their own use.

However, when you consider all the other errors affecting a bubble sextant in the air, possibly travelling at 8nm a minute, e.g. acceleration errors and Coriolis acceleration, to name but two biggies, such that a 3nm position line is very good indeed, errors due to minor inaccuracies in the Polaris correction pall into insignificance.  The important thing is to get your fix onto your chart as quickly as possible; otherwise you’ll be overshooting your destination before you’ve plotted your fix.  With a seven shot sandwich fix, you can’t even start working at the fix until at least 5.5 minutes after mid-time, if you’re really good and get the fix on the chart in the next 4.5 minutes, a jet bomber might have travelled 80nm.  You haven’t really time to consider the finer points of P&N and Polaris corrections; you just need quick ball park figures and as few publications to carry around as possible. Hence tables 5, 6, & 7 in AP3270/HO249 Vol 1.  Mariners have more time to think about things, more space to store stuff, and fewer corrections to apply, so they can work to greater accuracy and need the tables such as the a0, a1, a2 tables (which frighten me to death!) to do this.

Don’t worry too much about the age of your editions of yachting books.  Oft times the only thing that seems to change significantly is the colour and style of the diagrams and the glossiness of the cover.  Sometimes they don’t even change the dates of the included almanac daily page (they’d have to change the worked examples if they did).  E.g. the 12^th edition of a particular book published in 2013 still contains the daily page from 1997, which just happens to be the publication date of the 11^th edition.  DaveP