# 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
From: Gary LaPook
Date: 2014 Jan 20, 01:12 -0800
But if you are thinking of doing celestial computations on a normal sliderule you should consider using the Bygrave formulas instead of the normal cosine formula because they give greater accuracy. See:

http://fer3.com/arc/m2.aspx/Bygrave-formula-accuracy-10-inch-slide-rule-Hirose-jul-2009-g8985

http://fer3.com/arc/m2.aspx/Bygrave-formula-accuracy-10-inch-slide-rule-LaPook-jul-2009-g9019

Or, you can make your very own Bygrave sliderule which provides even greater accuracy, complete plans are available here:

gl

From: David Cortes <dcortes@rwlw.com>
To: garylapook@pacbell.net
Sent: Sunday, January 19, 2014 10:48 PM
Subject: [NavList] Using a slide rule for celnav

```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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