NavList:

   

Reply

Alex, you wrote:
"And I continue to maintain that taking one shot per week with the purpose
of investigation of lunars accuracy is ridiculous."

"Ridiculous" seems like a rather strong word here. How many lunars do you
shoot per week, on average? In any case, the impression that I get from the
article is that White was simply entertaining himself by shooting lunars now
and then from his home (on a postman's holiday, he goes for a long walk...
on an astronomer's holiday, he shoots lunar distances?). White was like you
or me shooting backyard lunars. He did not set out to investigate the
accuracy of lunars, probably understanding very well that they were DECADES
obsolete for sea use. But at some point, when he pondered the accuracy that
he was able to achieve, White, like a number of other land-based lunarians
in the second half of the 19th century, decided to pen a "letter to the
editor" praising the virtues of lunars and bemoaning the fact that nobody
uses such onble observations at sea anymore.

And you wrote:
"There is a logic in not applying excentricity error. But this makes his
results highly suspicious. Especially if we interpret them as single shots."

Sorry, Alex, but I can't understand how you could begin to say this. What
makes you say his results are "highly suspicious"?  Can you back this up??

I wrote:
"I propose that his arc error table would have read something like this:
30d: 0", 60d: +10", 90d: 0", 120d: -20". "

And you replied:
"Highly improbable. Especially on a good sextant."

Really? The arc error on a sextant could not have that pattern?? How "highly
improbable" would that be? Are you saying that if we looked through a few
dozen modern sextant certificates, we could never find one with a similar
pattern?

And you wrote:
"See Simms theoretical formula for the arc error."

The formula in Simms was one of the first attempts to deal with the issue of
arc error. But that particular theoretical formula (assuming we're talking
about the same one) applies to eccentricity only, literally out-of-centering
of the arc relative to the axis of the arm. But if the arc itself has error
in its milling, then that equation is not really relevant since two (or
more) sources of error are combined in a way that cannot be distinguished in
observational practice. In the end, you really have to just sit down and
measure the arc error at a number of well-spaced angles and make a little
table --just as we see with 20th century sextant certificates.

And you wrote:
"Averaging observations taken a week or more apart has nothing to do with
sea practice. Or even with land travel. It only makes sense when you
determine longitude of a fixed observatory."

Yes. That was the case at hand, wasn't it? Surely you agree with what I
wrote previously, that we can average sets of four in White's list and get
reduced error?

And you wrote:
"You misunderstood what I said. I did not say anything of the language. I
was talking about the whole approach, "philosophy". There is one approach:
to find what the truth is. For example, to evaluate the lunar method
accuracy. And another approach: to convince people in something which you
already know or believe. To promote some agenda."

Thanks, I can see better what you meant now. But people always have agendas.
No one studies at random. And more specifically, no one studies the accuracy
of a practical art like celestial navigation UNLESS they have some practical
reason, some agenda. Incidentally, my German is too poor to read the details
in the paper by Bolte. Why was he conducting such a study at a point in time
when lunars were obsolete (at sea)? Was he tasked to do this by some higher
authority? Was it his own idea? What was his agenda?

I had written earlier
"There is ample evidence for them [lunar observations] in the logbooks."

And you asked:
"Evidence of WHAT? That lunars were widely practiced? Nobody seems to deny
this. Or that "lunars can give you your position reliably to 6 or to 15
miles" ?"

Evidence of actual practice --that's the key to it all: if you want to
understand the history of navigation, then go to primary source documents.
And maybe evidence of accuracy, too, if you can puzzle it out. Go to the
logbooks, the newspapers, the letters written at the time (though I am
writing this directly to you, I do not mean it as instruction to you, just
so we're clear, I am just speaking of general philosophy here). The
experiments that you or I do today have some relevance to lunar observations
practiced historically, but only "some". There were so many variables in
play that the observational limit of a modern observer with a modern sextant
really only tells us about modern observers with modern sextants.

Of course, modern observers using modern sextants to shoot lunars are
relevant to other issues. Lunars are a good way to test our skills and test
our sextants and even have a bit of fun. And you might even use them in a
sailing race where celestial navigation gets you a little handicap on time.
Take lunars, and win the Bermuda Race! How's that for an agenda?? :-)

 -FER
http://www.HistoricalAtlas.com/lunars



--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---

   

Reply