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George H, you wrote:
"I think an important point is being lost, in this discussion of determining
lunar distance from a thin crescent Moon, by both Frank and Jim. It boils
down to Frank's sweeping-statement - "Celestial navigation back then was a
daytime activity." "

I had the strangest feeling of deja vu reading this. Then I went to the
archive and searched on "sweeping statement(s)". It turns out I wasn't just
imagining things! 

The reason I think it's important to state rather bluntly that navigation in
the nineteenth century was a daytime activity is because so many modern
navigators have been trained on twilight sights. Although there seems to be
a recent trend, among those still using celestial, towards running fixes
using daytime Sun sights primarily, there are many navigators who
immediately think of night-time shots when they hear about lunar distances.
It's a hard image to get out of one's head. Of course, the stars CAN be used
for lunars, and they certainly weren't avoided, but it seems to have been
much, much more common to use the Sun. That's the evidence that I have seen
in historical logbooks.

Celestial navigation in the nineteenth century was primarily a daytime
activity. Star sights were counted as fancy or exotic sights. Sun-Moon
lunars were very much preferred. Lunars using the stars were unusual, but
not "rare". Last Spring, you may recall, I wrote up a rather detailed
account of a navigator who had no choice but to use the stars for his lunars
(the navigator of the Erin, taken as a prize by the British in 1807, sailing
to Bermuda: http://fer3.com/arc/m2.aspx?y=200705&i=102871). I have yet to
encounter any documented cases of lunars using one of the planets, so I
would say that those sights do qualify as rare (not never used, just rarely
used).

And you added:
"If a navigator chooses to restrict himself to daytime lunars (with the Sun)
then yes, the dimness of the Moon crescent against an illuminated sky
presents a real challenge in perceiving exactly where its edge should be,
because of low contrast. However, this is just the situation for which
night-time star-lunars were intended."

Yes, that's right. That's exactly why the almanac publishers included the
stars. They, of course, very reasonably, designed the system to be used each
and every day, except for roughly four days around New Moon, and that's
essential for emergency use. The more mundane, regular use was a different
matter. The navigator could afford to wait for the convenience, and accuracy
of Sun-Moon lunars around First and Last Quarter. At least, that's how it
appears from evidence I've seen in logbooks.

And:
"Frank writes, in another sweeping-statement- "That's really all that was
necessary since dead reckoning for longitude was "good enough" for ten days
or more ...", regarding ten days without an observed longitude with
equanimity. I doubt whether any real navigator, approaching an unseen coast
after a week or more against contrary winds, under square rig, would share
Frank's confidence in his DR. He would, by then, be desperate to get a
longitude, and that's what a star-lunar could often provide."

Navigators in the era appear to have trusted their dead reckoning longitudes
much more than you imagine. And indeed, lunars helped confirm that
confidence. Now there were always cases where bad weather, bad instruments,
or other factors spoiled a DR longitude much more quickly. Those navigators
might have benefited from taking lunars more often and, yes, just as you
say, that's the time when they would have been forced to try the lunars
stars.

And let me re-iterate a point that I've made before: if you want to
understand how those navigators 200 years ago worked, you have to go back
and look at their logbooks, their calculations scribbled on the endpapers of
their navigation manuals, their own stories about what they did. The
practice of navigation is not the same as the textbook cases.

Also, for those of you who've connected to broadband Internet recently,
don't forget to explore the collection of logbooks and other original
documents available on the web site of the library of Mystic Seaport:
http://www.mysticseaport.org/library/initiative/MsList.cfm. You can explore
dozens of actual 19th century logbooks, quite a few with evidence of lunar
observations.

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


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