NavList:

Jim S you asked about the Weems Plath Celestial Computer.  It’s simply a device to allow you to add one value in degrees between 0 and 360 to another value in degrees between 0 and 360 without thinking.  As Frank says, by the time you understand it, you realise you no longer need it.  It probably remains useful only to those people who complete celestial so rarely that they can never remember whether to add or subtract their own longitude during the calculation. 
To complete ‘real’ celestial calculations you need to know the difference in longitude on the celestial sphere between your own position on Earth and that of your chosen Sun, Moon, planet, or star on the celestial sphere at the time of your observation, except we don’t say ‘longitude difference’; we say ‘local hour angle’   You get this from the Nautical Almanac and use it in nautical tables.  Unfortunately, the body observed could be at any one of 360 longitudes depending upon the time of observation.  This would need you carrying around 360 different Nautical Almanacs.  We get round this by printing only one almanac for an observer sitting on the Greenwich Meridian.  For Sun, Moon and planet observations the hard worked navigator then must convert the Greenwich value to the value for his own longitude.  For stars it’s slightly more complicated but the roughly same principle.  Oops!  The music’s stopped.  Who’s going to supply Chapter 2 plus diagrams?  DaveP