# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: A-5 Course and Distance Computer
From: David Pike
Date: 2021 Jun 30, 09:59 -0700

David Masters you wrote:  There is a note at the bottom of the instructions that says, "If arrow points to warning space, solve by using equivalent DL." I'm not sure what that means, how to apply the value, or if it is referring to the scale on the outer ring.

David
DL is the difference in latitude between start and destination.  The very bottom instruction on the Distance side tells you how to get the equivalent DL.  You just set the computer up differently and read the equiv DL from the third scale in from the outer edge.  It's the same scale that you use for setting DL.  Look at the outer Course scale on the Course side.  Like any slide rule the divisions starts wide apart from 89.5 degrees and get closer and closer until beyond 45 degrees moving towards 0 they’re too close to be readable, so they’ve not tried to mark them.  They’ve left the sector blank.  If you end up in this sector, you must go back to Distance side and read an ‘equivalent DL’.  Then you return to the Course side and use that.  It’s some kind of alternative scale which works best between 0 and 45 degrees.  The divisions are wide apart around 0 and useless above 45 degrees.  It’s a mirror image about 45 degrees of the first Course scale.  I’m still trying  to work out the ‘magic’ behind this.  I blame Lewis Carroll myself.  The graph of Secant 3/2pi to 2pi (from All Silly Tabby Cats) pushed pi/2 radians to the right, or maybe the log of this, might be worth toying with.  My model A-5 is almost complete.  DaveP

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