# NavList:

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

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Re: Ogura IJN
From: Greg Rudzinski
Date: 2017 Aug 9, 11:03 -0700

David,

You might want to give a slide rule a try for solving azimuth. I like slide rules that have the S scale just above the C scale on the slide (see Post Versalog 1460). First solve for prime vertical if the quadrant of the meridian angle is in doubt. The arc sin of  (sin declination divided by sin latitude) = PV.  To solve for Z = arc sin (cos dec sin t / cos Hc) . Use Zn rules to get azimuth. I prefer the slide rule to the Rust diagram seen in Weem's Line of Position Book. The Ix graph is the best compact azimuth method and fits on one side of a standard sheet of paper. All useful formulae and rules can be jotted down on the blank back side for inclusion into a short table, sextant box, or wallet.

Greg Rudzinski

From: David C
Date: 2017 Aug 8, 21:22 -0700

Peter wrote

Some further discussion of Ogura's method and its relatives by Charles H. Cotter is on-line at http://www.cambridge.org/core/services/aop-cambridge-core/content/view/S037346330003393

Thanks for the link. This raises a few points.

I must take a global view of navigation. I admit that in the past I have approached the subject as an Anglophile. It is only  recently that Weem's Line of Position book has joined Lecky, Burdwood and  Inmans on my list of "must add to my collection". Ogura will also be added.

I note that copies of Ogura's English edition were destroyed by fire and earthquake. Cotter does not mention the plates - presumably they survived and this is why the tables were reprinted so quickly.

Cotter stated that the complexity of short tables increased if they provided azimuth as well as altitude.  He also said that this complexity was why short methods did not become popular. Something I do not understand is why the ABC tables did not become the default for azimuth. They are (in my opinion) quick and easy to use. Altitude by a simplified Dreisonstok and azimuth by ABC  tables seems to me (sitting in my armchair) to be a quick and easy way of working a sight. When I work a sight by cosine formula and calculator I usually pull out Norie to determine the azimuth.

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