# NavList:

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

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Re: Calculator v. Slide Rule for Ex-Meridian Correction
From: Bob Goethe
Date: 2016 Jan 4, 14:59 -0800

My dad is a 93 year old mechanical engineer who worked all his life with petroleum pipelines.  He decided to retire in 1988-ish, "before they make me put a computer in my office."

He used to say, "A young engineer can come up with an answer to a particular engineering problem, and do so to 16 significant digits...but it takes him a whole afternoon to generate the program to *give* him that number.  I can usually come up with an answer to 3 significant digits - which is often all you need - in two minutes."

Doubtless his preferences have influenced me in my liking for being able to do blue water navigation "with no batteries required."

I have used a slide rule to work the Ageton sine/cosine equations during a trans-pacific sail, and the slide rule served me well.  In particular, I used my slide rule to work out the daily bearing and distance to our destination via a great circle route.

I had figured out, before my departure, that I could work those equations with a slide rule in pretty much the same time - within a few seconds - that I could do so with a calculator.

I used Pub. 249 for reducing sextant sights where the celestial object had a declination plus/minus 29°.

Using a slide rule meant I didn't have to pack along Pub 229 just to do a once-a-day great circle calculation...or to reduce the occasional star sight where the declination was too far north.

As an "I don't own a sailboat" sailor, I cannot simply stash navigational references on my boat and then have them always available.  Whatever celestial aids I need must travel to my starting port in my suitcase.  The slide rule (at several ounces) is more compact than Pub. 229 (at several pounds).

Once 3-D printing has advanced a bit further, my ambition is to build the "Mark 1 Navigator's Slide Rule" and then make 5 more to sell to the entire group of people in the world who, like me, would be interested in such a thing.

Bob

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