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    Re: Pub. 249 question
    From: Brad Morris
    Date: 2017 Nov 27, 14:47 -0500
    Hello Frank

    You wrote
    There's no problem inventing reasonable algorithms for selecting stars appropriate for sights today. Decide your rules and apply them. Reverse-engineering tables from decades ago and then applying those exact algorithms uncritically today does not make us better navigators... 

    -------

    Point well taken!  The potential list of candidates in that 70 year old tome was necessarily short.

    But consider the Astronomical Almanac's "Bright Star List.  That list is quite extensive (approx 1500 stars) and given a hard set of rules, could provide a robust set of stars for a fix anywhere on the planet.  With the tiniest bit of computational power, an app could easily recommend list given your EP (or GPS position, if that floats your boat).  Add in star identification (like a Sky Scout).  Add in a way to mask stars that are behind the clouds, with a subsequent recommendation and there might really be something.

    At least, that is the way I interpreted this discussion.  The search for a set of rules that could be applied, albeit with a modern, up to date, extensive list of stars.

    Since you promised that there is "no problem inventing a set of rules" I will defer to your expertise.  Perhaps you would like to provide a first cut at those rules.  I certainly would appreciate that 

    Brad





    On Nov 27, 2017 2:14 PM, "Frank Reed" <NoReply_FrankReed@fer3.com> wrote:

    David Pike, you wondered:
    "Has anyone any idea what “continuity” means in the explanation in AP3270/HO249 where it says “Continuity was sought in regard to both latitude and hour angle, particularly for latitude where changes are not immediately evident by inspection”?"

    The only person who could know for sure is long gone. I don't mean that he (or she, but probably he) has necessarily died, but certainly the memory of the exact meaning of that phrasing in the mind of whoever wrote it has long passed its expiration date. But I agree with Dave Walden on the likely meaning. Really, I think what you can take away from this is that the author is saying that there is no fixed algorithm -- no binding set of rules. Instead, where possible, the same set of stars was continued across as many bands of LHA and Latitude as possible even in circumstances where another choice might have technically better satisfied a narrowly-defined set of rules. In the context of your comment about aviation, imagine a navigator who plans a set of sights at latitude 59° N and then is unable to take actual sights for ten minutes and by then the aircraft is already well north of 60° N. It would be annoying to discover that the recommended stars on the two pages differ. The lists must change eventually, but where possible it would be better to have some "continuity" than to select stars which are ideally positioned by azimuth separation. Of course, it's very likely that the lists would have been compiled for broad bands of latitude in the first place, so the problem wouldn't necessarily arise. 

    While it's always interesting to try to understand how things were done historically, especially if it leads to insights in the nature of the navigation methods decades ago, some portion of this discussion has verged on a type of "ancestor worship"... as if it is a valuable goal to do things exactly as the old wise men did them seventy years ago, merely to emulate their holy acts! There's no problem inventing reasonable algorithms for selecting stars appropriate for sights today. Decide your rules and apply them. Reverse-engineering tables from decades ago and then applying those exact algorithms uncritically today does not make us better navigators... 

    Frank Reed
    Conanicut Island USA

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