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    Re: Poor's "Line of Position Computer"
    From: Hewitt Schlereth
    Date: 2008 Sep 19, 22:45 -0400

    I once had a circular slide rule that was an aluminum disk about 8,5
    inches in diameter. It wasn't specifically for celestial; just C, D,
    Sin/Cos and Tan scales. As I recall the scales were on in a spiral
    which made them many feet long. I used to use it now and then to work
    the fundamental sin-cos formulas for Hc and Z. I believe it was called
    a Pickett 'Atlas'.
    On 9/19/08, d walden  wrote:
    > 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
    > http://books.google.com/books?id=KPg3AAAAMAAJ&printsec=titlepage
    > 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
    >  >
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