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Bill Noyce said-

>By the way, George (& Arthur), Henning Umland's site also has
>a nice explanation of lunar distances in Chapter 7, which
>concludes with this statement (where D[Lapp] is the apparent
>distance):
>
>  The standard deviation of a time value obtained by lunar
>distance is inversely proportional to the rate of change of
>D[Lapp].... Finally, it should be mentioned that the rate of
>change of D[Lapp] (not D!) is also affected by atmospheric
>refraction and, even more, by the parallax of the moon [14].
>
>[14] The Lunar Distance Page, http://www.ld-DEADLINK-com

======================

Thanks for that useful information, Bill.

Henning's series of chapters  on "A short guide to celestial Navigation"
can be accessed directly at-
http://home.t-online.de/home/h.umland/page2.htm

The whole thing is really good reading, thouroughly done, and I recommend
it to any Nav-l member..

I have a version that was downloaded just a year ago, but Chapter 7, then,
didn't contain the passage that Bill quotes.

However, since then, Henning and I have had detailed and friendly
discussions about lunars (off-list, becauuse he isn't a Nav-l subscriber,
though was once).

In the version I downloaded a year ago, Chapter 7 , "Finding Time and
Longitude by Lunar Observations", was not about traditional lunar distance
observation at all, but about measuring lunar ALTITUDES, on-land. I had a
good go at persuading Henning about the deficiencies in that method, and
pointed him toward my series "about lunars". We also discussed parallactic
retardation.

I have since attempted to download the new Chapter 7, but my early version
of Acrobat can't accept the fancy modern typography, so I can't for now
print it out or even see it on the screen, except for patches. However,
from those patches, it's clear that Henning has completely revised Chapter
7, apparently along the lines that I suggested, I'm pleased to say.

But I have a nagging suspicion than perhaps he has also picked up my
warnings about the evil effects of parallactic retardation (about which I
have since recanted) and immortalised them in his own web pages.

I no longer think that- " The standard deviation of a time value obtained
by lunar distance is inversely proportional to the rate of change of
D[Lapp].... " (to use Henning's expression for apparent lunar distance),
but instead, that it is proportional to the rate of change of the true (or
cleared) lunar distance D, which is much more constant. It seems that I am
going to have to try to persuade him (as well as all those unconvinced
Nav-l members). Ah, well...

If Henning's statements are being quoted by Bill as evidence than my
original views were correct, and if those statements were indeed based, to
some extent, on those my views, then I have to regret that those arguments
of mine were so persuasive, now that I wish to back-track. They seem to be
coming back, in a circle, to haunt me!

George.

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