# NavList:

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

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Re: Traditional navigation by slide rule
From: Francis Upchurch
Date: 2016 Jan 22, 04:53 -0000

Great minds think alike! ( I know, fools seldom ..!)

I’ve just done similar to Hewitt.

Copied the S scale from my 1960s school Thornton and stuck it to the blank reverse side of the slide. (took about 5 minutes and copier gives exact correct size).Put this side in and bingo, 2 sine scales moving against each other.

See photos for sin 30x sin 20 direct gives 0.1710 and 9.84 Degrees direct with one move of the slide and 2 moves of the curser.

So the old school 12” brought honourably out of retirement to do test drive, proof of principle for any more AhHa moments!

So following on from Brad’s Ah Ha, I shall now try to design the perfect cylindrical cel nav rule along the following lines.

·         Similar size to Bygrave/MiniFuller. Still have some left over drain pipes but may ask Wolfgang for some of his. With magnifiers, this should give 4-7 place accuracy. My scales are in degree and minutes, so mostly to 1’ depending on position on the scales.

·         Dedicated celnav rule for cosine formulae and derivatives, eg for Hc, Az, Karl type lunar clearance, great circle etc.

·         Innermost tube has cos and sine scales. Outermost tube likewise. (i.e no C/D (or is it A/b) slide for ordinary slide rule functions) this really therefore a sort of “Bygrave” for cosine formulae. Not a general purpose rule.

·         Could maybe add 3rd scale to say outermost curser tube (? Tan or cotans) to do Bygrave formula / But I like KISS and probably will use Bygrave for Bygrave and this one for just cosine formulae.

On another, but related subject, my Thornton manual (yes available still free on line!) describes Sd and Td differential trig scales (on the original slider) which it claims are more accurate throughout the angle range than standard sines and cosines.

I had never heard of or used these as boy. Does anyone know about these and maybe we should be using these, Bob, especially on 12’ linear rules as you intend?

Thanks Bob, Brad, Gary, Hewitt et al for setting me off on yet another harebrained cel nav slide rule adventure!

I am very busy at moment so this may take a while!

Best wishes

Francis

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