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One point that I believe to have not been brought up is the vibrational
problem encountered in making sextant observations at sea - generally
engine and/or propeller induced. Frequently, it becomes impossible to
steady the sextant to the degree desired. This consideration is sometimes
magnified when using higher powered telescopes. Wind conditions, as well
as vessel movement, also significantly affect sight accuracy
independently of vibration, unless the observer can arrange a sheltered
location with adequate body support. In my previous postings regarding
Lunar Distances actually taken at sea, I indicated with little emphasis
these considerations to have been a problem with respect to attainable
accuracy.

Actually, I believe we are talking apples and oranges when comparing
terra firma sights with shipboard sights as respects attainable accuracy.

Incidentally, sailors don't get po over a little criticisim now and then
- ships were too small and too crowded not to get along with all - a
little controversy was and is good for the sole, and keeps one on one's
toes.
                                        Henry

On Sat, 12 Feb 2005 02:35:33 -0500 Alexandre Eremenko
 writes:
> Pierre,
>
> I can confirm what Frank says:
> I have two scopes, a usual 3.5x and an inverting 7x.
> After long trials I found that it is hardly possible
> to achieve good accuracy in star-to-star distances
> with my 3.5x. In all my observations (from the land,
> I have not tried my sextant in sea yet) I find the 7x
> far superior to the usual scope.
> I cannot say it is easy with the 7x, sometimes I am 0.3-0.6 off,
> but after long
> practice I can achieve 0.2 to 0.3 accuracy most of the time,
> with my 7x scope, both for
> lunar and star-to star distances.
> Averaging a series of 5-6 such
> observations usually gives 0.1 accuracy.
>
> Let me share some training experience:
>
> a) my common mistake in the beginning was to observe
> very bright Moon. It obscures the star. One has to use
> a filter, sometimes two, on the Moon, so that the star is
> well visible when it touches.
> Same applies to a pair of stars where one is substantially brighter.
> In general, better results with star-to-star distances can be
> achieved with stars that are not very bright. Same applies to index
> check with stars: use a weak star.
>
> b) With lunar distances, it is hard to understand in the beginning
> how should it look when the star really "touches" the Moon.
> I used the following method with Frank's online lunar calculator:
> I measure a distance and reduce it immediately to see the error,
> then I measure it again, trying to correct my mistake,
> and reduce, and repeat
> this until I get
> a good result consistently.
> The procedure works even better with star-to-star distances:
> they change so slowly that you can preset your sextant on
> the exact distance, and then see how it looks. And then
> slightly turn the drum and try to achieve the same view again.
>
> Alex.
>
>
> > > Frank Reed a �crit:
> > >
> > > Pierre Brial, you wrote:
> > > "Not yet. My sextant has a mechanical accuracy of 0.2', but
> despite
> > > the
> > > fact I have adjusted it, repeated tests on star distances show
> me that
> > > it is difficult to have a practical accuracy of under 0.8'. But
> may be
> > > this is also because of my lack of experience or inaccuracies in
> my
> > > eye."
> > >
> > > Do you have a good telescope on it? I have found that I can only
> get
> > > down to 0.2' accuracy with a 7x35 monocular on my sextant.
> >
> > I think you got the point. I've got a 3.5x38 telescope. Recently I
> tried
> > a lunar distance with Saturn. The ephemeris give for Saturn a semi
> > diameter of 0.2', that is a whole diameter of 0.4'. But through the
> > scope, I see only a spot. A bright one, but nevertheless a spot.
> So the
> > magnification is too small to get a 0.4' accuracy.
> >
> > > "Are these [Bowditch] notes available somewhere ? Foreign
> accounts on Reunion for
> > > 18th century are not well known here..."
> > >
> > > It's in the Boston Public Library in Massachusetts. I have only
> read
> > > summary accounts, but next time I'm out east, it's on my list of
> > > things to read. I'll let you know if there's more of interest
> > > regarding Reunion.
> >
> > I will be very grateful if you can find informations about this.
> Also
> > let me know in case these note will be published.
> >
> > Best regards
> >
> > Pierre Brial
> >
>

   

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