NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Antoine Couëtte
Date: 2010 Mar 30, 00:06 -0700
Hello George,
You just wrote :
QUOTE
"What's needed is altitude corrected for index error and dip, but for
nothing else.
(although the correction process itself isn't at all fussy about the exact
details of those altitudes, so in truth, it would matter little how, or if,
such corrections had been applied)."
UNQUOTE
This is exactly the kind of information I was looking for. So this is great, Thank you.
We should then consider that the published Altitudes are (most probably) corrected for only index error and dip.
From my just recent "playing with numbers" a few minutes ago on some examples from this page, I am now (almost) sure that the published Lunar altitudes are NOT corrected for Augmented Semi-Diameter and are NOT corrected for Parallax either. Whether they are actually corrected or not for refraction and/or dip still remains to be further fully confirmed. Since the combined result of both effects (Dip + Refraction) is much smaller than the combined effect of both Augmented SD + Parallax, I have to work on a much bigger number of examples from this page to be able to fully clarify this issue and I will let you know then.
*******
YOu also wrote :
QUOTE
I suggest that Kermit is concentrating on aspects of a lunar distance that
do not matter. But I'm aware that he is a fellow who is keen on exactitude.
George.
UNQUOTE
It all depends on the way we tackle Lunars. We probably might have different viewpoints on Lunars.
Like our most Prestigious Forebears, you ,George, might seem interested in ONLY one result which can be obtained from LUNARS, i.e. the Greenwich Time determination, whether Apparent or Mean, in order to subsequently determine Longitude (only), through the then traditional computing processes which - for lack of the subsequent improvement given by Capitaine de Frégate Marcq Saint Hilaire - always separated the computation of Longitude from the computation of Latitude. Both such computations were totally distinct by nature and they did not have to be carried out at the same time in the day.
And I certainly fully respect your approach which I would qualify as almost 100% traditional. And I will publicly claim here that RN Captain James Cook is one of the most fascinating Men I keep admiring. (An other one is Johann Sebastian Bach ... hello Wolfgang ..., but for some other OT reasons)
On the other hand, we should not forget either that LUNARS can also yield additionnal very valuable information on the Observers' positions. This was not very common knowledge then to our Great Navigation Ancestors. In a recent post, Frank indicated that some Astronomers of these times had been aware of such potential additionnal benefits (i.e. the knowledge of the Observers' Positions as a cherry on the cake), but they failed to devise any practicable computation method acceptable for use onboard a vessel.
Stating, and even demonstrating and repeating for centuries now that the knowledge of accurate heights is NOT A MUST for Lunars is (somewhat) acceptable, but ONLY IF we keep the "traditional viewpoint" (i.e. access to ONLY the Greenwich Apparent/Mean Time in order to determine the Observer's only Longitude).
In other words, we all know - it has been "proved" many times - and understand that "just average" heights determinations (i.e. less accurate than what the best available sextants could then achieve) were (more than) sufficient to accurately correct for the well understood effects of Parallax and Semi-Diameters which kept drawing so many Mathematicians' efforts. On top of that, "just average" heights determinations were also sufficient to fine-tune the Clearing processes through paying due attention to both Temperature and Pressure which DO both have a significant and quite appreciable effect on clearing results.
Nonetheless, and by some current standards, "just average" heights data is nothing short of a MOST REGRETTABLE lack of accuracy, should we happen to wish determining the Observers's Lat/Long positions, with both coordinates solved simultaneously to-day through our well known Marcq Saint Hilaire's method.
This is why I have personnally felt that an accurate and thorough understanding of such published historical data is paramount, and not just simply " aspects of a lunar distance that do not matter ".
Equally important is a good feeling about their implied limitations given the openly admitted lack of accuracy of some of them (i.e. while Distances between Limb(s) kept being recorded with the utmost care - yes ... VERY good reasons for that, and we ALL agree here - , UNFORTUNATELY - although fully understandably by their standards then - it (definitely ?) was "in the air" then that it was not worth bothering trying to achieve the same accuracy level on both heights adjacent to the Distance observation).
This is simply why I am (overly?) cautious about exactitude.
EXACTITUDE could just be the name of "trying to get the MOST of available historical data", no more, but certainly NO LESS.
With this philosophy in my mind, I have been looking for comments on my early inaccurate assumptions, and I do highly thank you again for the very valuable light you shed on these most exciting topics.
And more generally my grateful comments do not concern you only, but also many other contributors, and if we are to acknowledge that a number of them are able to give clear views and explanations on matters which are not always cristal-clear per se, I would certainly also include at least Frank to my Thanks List.
Thank you again to all of you and to you George in particular, and
Best Regards, till next time ...
Antoine
Antoine M. "Kermit" Couëtte
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