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Poor's "Line of Position Computer"

 

Poor's "Line of Position Computer" is mentioned in Bowditch.  It had a diameter of about 15 inches.  A question was raised once in the group, but I couldn't find any follow up.  It would seem, few were made.  I haven't seen one available.  Finding no other real circular slide rule type methods (that's circular not cylindrical, which has been discussed on list), I decided to build one myself.  Details of the computer can be found in:

Simplified Navigation for Ships and Aircraft    By Charles Lane Poor

Unfortunately, in the Google books version, the fold outs don't.  I found a hard copy quite cheap online.

 

Several possibilities for making one presented themselves:

 

1 Print out two halves from google pdf and paste together.
pros: cheap, anyone can do it, easy, little investment of time or money
cons: pdf scans are low resolution, hard to read, and enlarging makes things worse

 

2 Copy from book and blow up.
pros: have entire fold out, high resolution scans possible, enlarging is better
cons: need the book

 

3 Create from scratch using postscript.
pros: can customize, can print high res at any size, can learn how scales are made
cons: have to create postscript file

Well, I tried all three.  Number one is quick and easy, but the result in not very satisfactory.  Two is better,  but Hour Angles are in hours using Roman Numerals and style is very much early twentieth century (not that that's necessarily all bad!)  So, I went for three.  I wrote a FORTRAN program to generate the postscript file making it easy to make changes and quickly iterate and see the results.

 

Attached is a 8 1/2 x 11 inch pdf of the result.  At this size, index marks are no less than one degree.  It's still a work in progress, so any comments or questions are welcome.  The postscript file and FORTRAN source are available.  I made the circle and arm using transparency film and used a paper clip as a clamp.

 

Some hints: it's easier in practice than it seems from reading the book, some scales increase CW, some CCW; be careful,  a quick "cheat sheet":

 

Poor's LOP Computer      

To find alt      
set circle at dec   
clamp arm at lat   
move circle to t   
read arm is number on 3a is 2a/1, 2b/10 ,2c/100 & NOTE  
set circle at 0   
clamp arm at zd scale=L~D=abs(signed difference)   
move circle to number on 3b   
read arm is alt   
      
To find z      
set circle at t   
clamp arm at alt   
move arm to dec   
read circle is z   
      
      
NOTE:      
if  dec to lat less than 60 deg     
and circle left of 0.0     
and arm right of lat 60 deg     
 and t is on 2b    
 then number is 2b/1    
 or     
 and t is on 2c    
 then number is 2c/10    
      
if  dec to lat greater than 60 deg     
and circle right of 0.0     
and arm left of lat 60 deg     
then number is number/10