NavList:

Mike Freeman you said: Evident that not everyone is happy with Mary Blewitts method.

Mike
I’m not sure that Mary Blewitt was dogmatic about working from a convenient fixed longitude interval.  The 12^th edition of Celestial for Yachtsmen rather suggests that you could define either longitude or latitude first.  However, if you read her obituary there are hints as to why she might have preferred a fixed longitude interval.  https://www.telegraph.co.uk/news/obituaries/1366813/Mary-Pera.html   First her formative years were spent as an RAF Intelligence Officer.  In WW2, the RAF’s plotting charts were almost all Mercator’s, so many of the little tricks with dividers and protractors so beloved by air navigators were mainly designed for use with Mercator charts; just look at the 1941 edition of AP1234.

Secondly, these were the days before digital calculators, let alone scientific or programmable ones.  The RAF adopted special tables for quick working, e.g. AP1618, the predecessor to AP3270.  The entry to these was a whole degree of latitude and a whole degree of LHA.  This meant your assumed position for celestial could no longer be your DR position.  You had to pick a whole degree of latitude and an assumed longitude which made your LHA a whole number.  It was easy to find a whole degree latitude line but finding a particular value of minutes of longitude would have been easier if the longitude interval was easily markable.

However, you can still use the divider and protractor method with a fixed latitude interval if you wish, like John D Howard says, except he uses a ruler along the sloping line.  Draw your lat & long lines intersection, say 53N & 3W.  Mark your latitude interval to 54N (preferably easily divisible by 60 or better still with minute markings) and draw your sloping latitude angle line from the intersection (preferably to the left if you’re west of Greenwich).  Scribe an arc from 54N 3W to the sloping latitude angle line and drop a vertical from there to the 53N latitude line.  That’s 4W longitude. 

Then suppose you need an assumed position of 53N 3deg 37’W, scribe an arc from 53deg 37’N to the sloping line and drop a vertical.  That’s 53N 3deg 37’W.  

I seem to remember once being issued with a publication called ‘SAC Tables’.  As I recall they gave the length of a minute of longitude for all almost every latitude.  Does anyone remember them or still have a copy?  DaveP