# NavList:

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

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Re: Using a slide rule for celnav
Date: 2014 Jan 20, 19:36 -0500

Hi David Cortes

I would not advise you to attempt celestial navigation reductions with a "typical" linear slide rule, as your question seems to imply.  The resolution simply won't be enough to adequately determine a fix (your location).

There are many ways to reduce the celestial triangle.  A slide rule isn't the best choice until the transition is made to cylindrical rules and in particular, the Bygrave.  (Gary LaPook has developed what he calls the 'flat bygrave.  Details here on the list.)

On Jan 20, 2014 1:47 AM, "David Cortes" <dcortes@rwlw.com> wrote:

```To Navlist:

I learned how to use a slide rule back in high school, and it's been 45-plus years.  Can some of you old-timers tell whether it's possible to multiply sin by sin or cos by cos, etc.  n one continuous operation, without putting the rule down to write down the number of the first calculated sin or cosin, etc.?

David

-----Original Message-----
From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of Frank Reed
Sent: Monday, January 20, 2014 12:13 AM
To: dcortes{at}rwlw.com
Subject: [NavList] Re: What is a "Class A" sextant?

My understanding of the Kew "Class A" rating was that it was an overall rating. It was the certification required for sextants given to Royal Navy cadets. It combined several factors, and the instrument had to meet various standards on several tests.

You may remember a NavList discussion a few years back about tables of "star distances" published in about 1905 for use with Lord Ellenborough's method of testing sextant arc error at sea (*). In the introduction, the authors say that a "Class A" certification implies among "other things" that the centering error (or "arc error" as we would call it today) amounted to less than 1' of arc maximum. Classes B and C would presumably permit progressively greater arc error, and this same source says that the sextant would be "rejected" (in other words, worse than class C) if the arc error was greater than 3'.

*that discussion was in March 2010, and here's my first message on thee subject, specifically addressed to you personally, in fact. :)

-FER

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