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I had written-

| "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." "

and Frank replied-

| 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! 

That may have been similar, then, to the feeling I got of having seen it
before, when reading Frank's pronouncements, about celestial navigation
being a daytime activity, and about how many days DR would be trusted for.
Perhaps that just shows that at least Frank and I remain self-consistent,
even if neither of us seems to have profited, much, from earlier exchanges:
and we may be destined to replay them again...

| 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.

I suggest that the modern emphasis on twilight sights appeared because, once
precise and trustworthy times became available (from chronometers and then
radio), a round of star sights became the (only) way to obtain an
instantaneous, accurate position, that didn't rely on any assumptions of DR.
Before then, the uncertainty of time, from lunars and early timepieces, was
such that the DR assumptions, in a series of daytime running-fixes,
detracted little from the rough longitudes that were all that could be
expected..

I agree with Frank that in the era we are considering (which I take to be a
decade or two either side of , say, 1825) mariners would prefer a daytime
Sun lunar to a night-time star lunar, IF that option was available. But
often, it wasn't. There are more than twice as many nights in a month that
are available for a star-lunar, than there are days available for a
Sun-lunar.

Remember, these were not today's hobbyist lunarians, who can choose to take
their observations when conditions are "just right". They were men for whom
their living, and indeed their lives, depended on their navigational
competence. They required to know their longitude when it was necessary, and
when it was possible, not when it was easy. Lunar distances observations
were restricted enough, by phase of Moon, cloud cover, and rough seas.

Then, Frank concedes-

|  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...

Frank has now qualified the sweepingness by adding the word "primarily",  If
he had done so, first time round, there might have been no disagreement.

He continues-

|  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.

Well, it depends what is classed as "emergency use". Certainly, in the
mid-ocean phase of a long passage, a mariner could choose to check his
longitudes when it was convenient and easy to do so, which might explain to
some extent the emphasis Frank sees for daytime lunars. Is making a
landfall, after an ocean passage, "emergency use"?

| 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.

How does Frank know the true extent of that "trust"? Yes, in the absence of
observations, they had to calculate their DR; there was no alternative.
Before lunars and chronometers, that's what ocean voyaging relied on, with
all the well-documented perils we are so aware of. And indeed, long after
lunars and chronometers appeared, benighted mariners from many nations would
continue to voyage the oceans for many years, using latitude sailing only,
and risking their lives, and those of their crew.

| And indeed, lunars helped confirm that confidence.

Or undermine it, just as likely.

| Now there were always cases where bad weather, bad instruments,
| or other factors spoiled a DR longitude much more quickly.

More quickly than what? The ten-day figure he quoted earlier? That may have
been appropriate in mid-ocean, when position didn't matter, but absurd
elsewhere.

Frank calls in aid the logbooks available on the Mystic website. I have
looked into that interesting collection, but not extensively. I ask Frank to
tell us to what extent the logs he has examined are of whaling voyages, or
of coastal voyages along the East coast of North America, neither of which
has any great need for longitude observation. If he is offering statistics,
we need to know about the population he is sampling from.

I have looked into logs of whaling voyages from Britain in this period, into
Arctic waters, West of Spitzbergen. These always took place in the Summer
season. The whaling grounds were a broad target, so called for little
navigation to reach them, and the greatest navigational dangers occurred on
the homeward passage. But the whaling itself called for no real navigation:
it involved seeking whales, and avoiding ice floes, neither of which were
charted. From the time the Arctic circle is approached in Summer, stars
vanish, and the Moon is only occasionally seen above the horizon. When it
is, only Sun-lunars are ever feasible. And whaling-captains were not, in
general, sophisticated navigators; one who could work a lunar was a rare
exception.

So if Frank is offering an analysis of such matters, we need to know what
fraction of the log-books he has studied relate to port-to-port cross-ocean
trading voyages, or naval voyages, the sort of work for which lunar distance
navigation was intended. And what fraction of those log-pages relate to the
critical stage of such passages, approaching a landfall, rather than to the
mid-ocean period when position really doesn't matter.

George.

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

Where lu


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