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Hi Alex + Frank,

In my previous postings I stated that I had taken a
number of Lunars at sea, in the South Atlantic, during
the late 1940s and early 1950s. Unfortunately, I have
been unable to find these observations amongst my
records, however, it is my recollection that results,
in terms of Longitude, were in the order of within
15-minutes of arc, as compared with my known positions
at the time of observation.

I used Borda's spherical method for clearing distances
but, as I previously pointed out, had and knew of no
then published tables of pre-calculated distances
against GMT - this necessitated that I calculate by
Great Circle methods the true distances to compare
with those cleared, all by hand calculation. This and
the simple fact that I had only 5-place logarithmic
tables left a greater window for error than I would
consider acceptable today.

There has recently been a considerable re-advancement,
as well as increased interest in the Lunar problem.
When I first became involved, most of my
contemporaries considered me a bit loony - no comments
please.

I appreciate that Alex's interest in Lunars seems to
be the accuracy of the angular measurement attainable
with the sextant, a matter of routine to me. I would
suggest to him that: 1) He equip his sextant with a
vertical handle, something which I long ago did to
ease the observation of horizontal angles in
coast-lining and position finding, and 2) For land
observations, consider mounting his sextant on a tilt
table or bracket atop a tripod, which allows
adjustment to a steady platform in the plane of the
observed distance.

Regards,

Henry


--- Alexandre E Eremenko 
wrote:

>
>
> Frank,
>
> > One of these days, clockwk.com is going to
> disappear,
>
> OK.
>
> > Considering how carefully he has spelled out other
>
> I disagree with this. He even did not care to
> mention
> his scope power:-)
> And I continue to maintain that taking one shot per
> week
> with the purpose of investigation of lunars accuracy
> is ridiculous. All books on the lunars strongly
> insist
> on averaging at least 5 shots. And I suppose White
> read the
> books he cites.
>
> > In a follow-up post, you decided that
> > this wasn't so ridiculous after all,
> > and I agree with your logic there.
>
> There is a logic in not applying excentricity error.
> But this makes his results highly suspicious.
> Especially if we interpret them as single shots.
>
> > I propose that his arc error table would have read
> > something like this:
> > 30d: 0", 60d: +10", 90d: 0", 120d: -20". It should
>
> Highly improbable. Especially on a good sextant.
> See Simms theoretical formula for the arc error.
> By the way, one may conclude from the article that
> this
> very formula was used to determine his arc
> excentricity
> correction:-)
>
> > "5. How Frank comes with the "mean error" of 0'.1,
> I don't know."
>
> > Take them in sets of four and average.
>
> 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.
>
> > I found your point "6" amusing.
> > You seem to be saying that you don't trust
> > him because he writes plainly and without
> > pseudo-academic pretense. Oddly
> > enough, I like him for that.
>
> 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.
> He seems to be using the second approach.
> I call the first approach "scientific".
> It has nothing to do with "language".
>
> > You know, they do have calm days on the oceans,
> Alex :-)
>
> Yes, they do. Have you seen
> www.math.purdue.edu/~eremenko/accuracy.html ?
> where the results of various observers are
> represented graphically?
>
> > That's Henry Halboth.
> > After getting a longitude accurate to within
> > 6 miles from a lunar distance
>
> I also have one or two lunars taken from sea with
> this accuracy. I even have a shot with a POCKET
> sextant
> from sea with this accuracy! I posted it 2 years
> ago.
> So what? I do not conclude from this that one can
> use a pocket sextant at sea to determine longitude
> within 6 miles:-)
>
> Henry, if by chance you are reading this,
> could you please tell us, how many times (perhaps in
> a certain definite period) did you make lunar
> observations at sea, and how many of those gave you
> longitude within 6 miles?
>
> > ...So that's ONE data point.
>
> Yes, exactly.
>
>
> > Finally, I would emphasize that lunars
> > were widely used in the early
> > nineteenth century --so they MUST
> > have been accurate "enough", whatever
> > "enough" means.
>
> For those British officials who made the longitude
> prize "enough" was 1/2 degree in longitude if I
> remember correctly.
>
> > There is ample evidence
> > for them in the logbooks.
>
> 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" ?
>
> Alex.
>
>
>
>
>


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