A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2017 Nov 27, 10:37 -0800
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...
Conanicut Island USA