# 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, 18:23 -0500

Hi Greg

The Bygrave is generally good 1 minute, occasionally larger in extreme cases.

The 0'.1 available from HO 229 (and others) is true.  The calculation error is no greater than that.  However, with your expected 'good case' fix to be within 3 nautical miles, that extra (30X) precision is just noise, no?

Don't get me wrong.  I have all volumes of HO 229.  Its the last tabular method before electronic computerization.  I also have HO 204, which is a tabular method rarely described here, as the tabulated values include RA and Dec!

In terms of speed of result, its just absolutely impossible to beat the Bygrave, except by electronic means.   The rational provided to avoid it seemed to beg a response.  Yes, there is a decrease in the resolution of the result, but the increased resolution affords you little benefit!

Much has been written before about a linear slide rule.  The general consensus is that a standard slide rule simply does not have enough resolution over its length to provide a reasonable result.  The Bygrave increases the length tremendously, by wrapping the scales like a helix around the cylinders, providing a reasonable 1' fix.

On Jan 20, 2014 12:32 PM, "Greg Licfi" <cfi@licfi.com> wrote:

Hi Gary,
Perhaps I'm just misinformed; and in this case I would love to be - so please take no offense if I am.
I think I read somewhere that a Bygrave sliderule can only resolve 1 arc minute(?).
This might be better suited to a aircraft with a bubble sextant (+/- 2 arc minutes after averaging ?) then a ship.
I currently do my calculations with: H.O.229, a smart phone app, ICE, and a scientific calculator.
all of which seem to agree within +/- 0.1 arc minute with the USNO. If it (the Bygrave sliderule) can resolve
better than that I would be a convert for sure. Also; how much resolution do you need on the sliderule?
Back in my college days 3 or 4 decimal places was considered good with a 'K&E Decalon(?)5'
BTW: I'm a big fan of ICE - I run it on my laptop & smart phone with something called 'dosbox' My smartphone
app uses USNO software and data, ICE was a USNO product, as is H.O.229. so you would expect them to all
yield the same results (no?)
~Greg

This new ship here is fitted according to the reported increase of knowledge among mankind. Namely,
she is cumbered end to end, with bells and trumpets and clock and wires, it has been told to me, can call
voices out of the air of the waters to con the ship while her crew sleep. But sleep Thou lightly. It has not
yet been told to me that the Sea has ceased to be the Sea.
—Rudyard Kipling

On 01/20/2014 04:28 AM, Gary LaPook wrote:

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{at}rwlw.com>
To: garylapook---.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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