A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David Pike
Date: 2019 Dec 19, 13:04 -0800
Mike Freeman you asked:
In Mary Blewitt Celestial Navigation book it describes how to make a plotting sheet...
On a piece of paper mark your Longitude scale across the bottom, 1 inch to represent 1 degree for example. Line AB
Draw a line from the bottom LH corner at exactly the angle of your assumed Latitude. Line CD
Draw vertical lines from the Longitude scale to meet the line drawn at an angle.
The intersection of the vertical lines on line CD is your Latitude scale.
Is this method OK or just for practice plotting?
Almost OK, but not quite. You forgot the last bit. Suppose you started from 53N 3W. Draw the 53-degree angle line towards the 2W line. Then stick one point of your dividers at 53N 3W and scribe an arc from where the angled line crosses 2W back to the 3W line. Where the arc crosses the 3W line, that’s 54N. You’ve effectively made the latitude interval = sec lat x the longitude interval. I know Frank hates this method, but for years and years one of the questions on the RAF Basic Navigator Training Final Exam was “You are carrying out a maritime patrol and you receive a message rerouting you to an area off the top of your chart”. All you have is your dividers, a pencil, and your Douglas protractor. What are you going to do?”. You got more paper by folding the bottom of your chart under so that the back of it appeared at the top of your chart.
If you think about it you PLs might be above or below your 53N line, so you ought to draw a 53.5-degree line upwards and a 52.5 degree line downwards from 53N 3W to constuct a 54N line and a 52N line. Ideally, choose a longitude interval divisible by 60 to make it easy to plot assumed positions. Don’t forget you’d also have to construct a nm scale to plot your intercepts by dividing your latitude interval by 60. Unless you yearn for your school days doing tech drawing, it’s much easier to use one of Franks universal plotting sheets available on this site. Attached is a chart segment I put together that way as part of a ‘I shot the Sun with ….’ Pamphlet when I was demonstrating with the Astrovan a couple of years ago and was too mean to give everyone a proper chart to work with. DaveP