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Henry, you wrote:
"Thanks for your interesting reply. I am of the opinion that Lunars
generally received a bad name as far as accuracy is concerned primarily for
two reasons."

I should say right at the top here that lunars were, in fact, widely used by
those navigators who needed them (navigators wo could not afford
chronometers) in the early part of the 19th century. There are references to
them all through the old logbooks [many of these logbooks are available
online at mysticseaport.org]. They did not have a "bad name" regarding
accuracy until late in the 19th century.

You suggested:
"1. The relative inaccuracy of early instruments. Let's remember that the
octant, as originally produced was not fitted with a tangent screw and gear
for easy and positve measurement of small arc increments. Although the
vernier appeared rather early on, it was the incorporation of the tangent
screw that allowed more positive fine adjustment. I also certainly cannot
believe that some of the earlier instruments made of wood with ivory arcs,
although works of art, could be relied on as respects accuracy over a wide
range of condition to be met at sea."

Certainly, instrument accuracy was always a big issue and still is for
lunars. I agree with that. Regarding your specific comments on octants, I
know of no one who shot lunars with a wooden octant in the era. A metal
sextant --with tangent screw, telescope, and assorted shades-- were all
recommended right from the very beginning of the lunars era. Even in 1763,
Maskelyne spells out these requirements in detail. There's clear evidence
that navigators carried two types of instruments: octants, like you've
described above, for daily latitude observations (almost always Noon Sun)
and sextants for lunars. Of course, if there WERE any navigators who
declined the advice to get a proper sextant, and this advice was in
practically every navigation manual and guide to shooting lunars, they would
definitely have been disappointed with the results. Wooden octants are
comparable in accuracy to modern plastic sextants (e.g. Davis).

You also wrote:
"2. The mathematical methods of clearing the distance, especially the so
called approximate methods were a wonderment of complication, in some
instances requiring the application of numerous corrections which in
themselves sometimes defied reason. Most of the epitomes I have read did not
apply to reason, but simply listed rules to follow. In my view,at least, a
relatively simple explanation of the problem related to a trigonometrical
format would have better served the mariner. The entire subject of spherical
trigonometry does not appear as complicated as some of the approximate
methods."

Here, I DO disagree. The great majority of the lunars that you will find
worked out in 19th century logbooks are series methods (erroneously known
sometimes as "approximate" methods). The steps involved in working them were
no more complicated than working an ordinary time sight --something which
nearly every navigator could do with ease. The amount of work was about
three times greater, but the steps and mathematical skills were not greater.
I've seen many examples of lunar observations worked out on spare pages in
old logbooks, and not once have I seen any of the straight triangle methods,
like Borda's, in use. They used the series methods, like Bowditch's
Principal Method, Thompson's method (adopted into Bowditch in 1837 just
before his death), Witchell's method, variants of Lyons' method, Bowditch's
Original Method (known in Europe as the method of Mendoza Rios, although
never claimed by MR himself) and so on. There's no doubt that other direct
methods, including Dunthorne's and Borda's and Krafft's were in fact used,
but they were less popular. The choice of method was not considered a "big
deal" in the day. Many navigators seem to have learned a couple of different
methods if for no other reason than for intellectual amusement.

You also wrote:
"With an accurate instrument and a little practice, I see no problem in,
under favorable conditions at least, attaining an accuracy of less than 0.5
arc minutes in measurement of the distance."

Yes, I agree. As I've said, I find that modern navigators with good sextants
(equipped with a 7x or better telescope, which I consider relatively
important) can do even better than this on a regular basis.

And you wrote:
"I am not of the opinion that sea state has a significant effect on the
observer, unless a vibratory effect is present, such as when a vessel be
laboring or pounding, or when vibration is enduced by operating machinery.
It does seem usual for a vessel to have been eased off or hove-to so as to
provide more favorable conditions for observation. "

There appears to have arisen lately the idea that lunars were only shot from
large, stable vessels. In fact, many rather small vessels in the 19th
century used them regularly.

And you wrote:
"You referred me to your Lunar Tables, with which I am well familiar."

Just so there's no misunderstanding here, did you try the tool on my web
site which lets you CLEAR a lunar online?? You put in your observations, and
it comes back and tells you how much error you made. What do you get when
you plug in your raw numbers? (I'm not talking about the predicted lunar
distance tables here).

 -FER
www.HistoricalAtlas.com/lunars



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