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    Re: Sight reduction methods
    From: Frank Reed
    Date: 2015 Nov 3, 13:58 -0800

    Hi Dale,

    Welcome aboard (if you haven't been welcomed enough yet). There are dozens and dozens of sight reduction methods. Some deserve the name, some don't. Some methods of sight reduction don't have names at all. Others deserve to be split in two.

    You mentioned "law of cosines" as a method of sight reduction. May I ask, where did you pick up that terminology? I've heard it from quite a few people recently, and I'm curious to learn why people began referring to a basic mathematical equation as a "method of sight reduction". It's a bit like saying "addition" is a method of sight reduction. :) There are many procedural techniques founded on the law of cosines. Each of them could be counted as a distinct method of sight reduction.

    You also asked about HO 249 (also known as "Pub. 249" since the Hydrographic Office no longer exists while the "Publication" has continued). This is an excellent example of a method that deserves to be known by two names since it's really two very different ways of analyzing sights. Volume 1 of Pub.249 involves essentially no calculation apart from the LHA of Aries. It's a lookup table where we read out the altitude and azimuth for integer values of latitude and LHA Aries. Six bright stars are solved and tabulated at that latitude and LHA Aries. By contrast, the other two volumes (really one set of tables split in two) are a different sort of sight reduction intended for general objects including the Sun and planets.

    Another set of "HO" tables worth experimenting with would be HO 208. The best reason for learning these is that they are at the center of a really excellent book on celestial navigation that every navigation enthusiast should acquire: John Letcher's Self-Contained Celestial Navigation with H.O. 208. This is a very fine textbook of celestial navigation, well-written and loaded with good advice. The inclusion of the tables HO 208 is just a bonus.

    And you should learn about the hav-Doniol method which was a subject of much discussion among NavList members a few months ago. It's a short method that requires one longhand multiplication but in trade it gets by with only a short mathematical table. It's efficient and convenient if you want a paper and pencil approach. You could also experiment with various slide-rule style sight reduction methods like the Bygrave cylindrical slide rule and Gary LaPook's "flat Bygrave". The options go on and on...

    Collecting sight reduction methods can turn into a form of "stamp collecting". You only need one, but the process of collecting them can get addictive. Beware the curse of sight reductio ad absurdam!! Ha ha. :)

    Frank Reed
    Conanicut Island USA

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