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This is a resend in a different format, as the previous offering gave some
readers some trouble. My apologies for that.

======================

Oh, dear... We're on to Sobel again.

One of the many problems with Sobel is that, as a journalist rather than a
navigator, she didn't understand what was going on, but
thought she did.

Which was how she came to write such stuff as (on page 5) "Precise
knowledge of the hour in two different places at once - a
longitude prerequisite so easily accessible today from any pair of cheap
wristwatches - was utterly unattainable up to and including
the era of pendulum clocks".

It is of course true that the most difficult part of that problem, the
knowledge of Greenwich Time, was resolved by the introduction
of the chronometer, and later that job could be done by a cheap wristwatch.
But she has the notion that to get the longitude you
only have to compare it with local time, to be read off the other cheap
wristwatch. There's no mention of any need for the important
step of going out and taking an observation for local time, if the sky is
clear enough to make that possible. The only words about
that topic are to the effect that "the navigator resets his ship's clock to
local noon when the sun reaches the highest point in the
sky...", a statement that's defective on so many levels, that it shows
nothing but her lack of understanding.

And a similar problem besets Frank's recent posting, when he writes-
"Lunars were very common in navigational practice before
chronometers became widely available, but not in the way one might at first
imagine. The system was not lunars in place of
chronometers but rather dead reckoning, or what they called "longitude by
account," supplemented by lunars. And after chronometers
became common enough in the USA, the rule was longitude by chronometer
supplemented by lunars, at least for a while."

Even with a chronometer on board, longitude by chronometer was available
only after taking an observation for local time, for an
object well away from the meridian. That might be made from the Sun, at
morning or afternoon, or from a star at twilight, if the the
sky was clear at the auspicious moment. In between such moments, and
whenever the sky was obscured by cloud (and it was
unpredictable how long that might last) the ship's position was determined
by dead reckoning, from the last fix. The introduction of
a chronometer did not dispose of the need for dead reckoning. It was the
only prime means of short-term navigation, available in all
weathers, until satellites appeared.

The introduction of the chronometer made big changes to the way navigation
could be done, in providing a reasonably dependable
source of Greenwich Time which remained good for a month or two, the
duration of a voyage between sighted headlands. It wasn't a
difference in principle, though, if a navigator carried some sort of
pocket-watch that he could trust to a minute a day, or
thereabouts. For a short time, after a lunar observation had provided a
check on the time showed by the watch, that watch could be
used, just like a real chronometer, to bridge over the interval to the next
lunar. Lunars were unavailable over a few days around
new moon, and such a technique could be used to bridge over any such gap.

Frank wrote: "Navigators didn't shoot lunars every day or even close to
every day." No they didn't shoot lunars every day, for one
thing because lunars weren't available every day. If a navigator was
competent to measure star-lunars, they were available over many
more days than Sun-lunars. Nor was it necessary, even when lunars were
being relied on for longitude, to take them every day, if a
ship was in mid-ocean, when her longitude mattered little. But in the lunar
era, a conscientious navigator would take a lunar when
the opportunity arose, when the Moon was visible in a clear sky. It was
part of the "professional pride" that Henty has referred to.
But more rarely possible, if the navigator didn't have star-lunars in his
toolbag. Frank's account seems to allow for the use of Sun
lunars only. Was there really no mention at all of any star lunars? If so,
it may be that the navigational standards on the vessels
to whose logs Frank has had access may have been somewhat more casual.

George.

contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
----- Original Message -----
From: "Frank Reed" 
To: 
Sent: Wednesday, March 17, 2010 7:13 PM
Subject: [NavList] Re: David Thomson and his lunar tables


John,

Aboard American vessels, lunars were used for longitude right through the
1840s though almost never after that. One of my favorite
examples is the gold rush ship "Sabina" in 1849. I wrote up its navigation
and use of lunars for NavList back in 2005. You can read
about it here: http://fer3.com/arc/m2.aspx?i=024812.

Lunars were very common in navigational practice before chronometers became
widely available, but not in the way one might at first
imagine. The system was not lunars in place of chronometers but rather dead
reckoning, or what they called "longitude by account,"
supplemented by lunars. And after chronometers became common enough in the
USA, the rule was longitude by chronometer supplemented
by lunars, at least for a while. Lunars were never used alone, which was
probably the vision that many of the nautical astronomers
had for them back in the 18th century (and sometimes the impression that
people get today when they hear about them). Navigators
didn't shoot lunars every day or even close to every day. The rough pattern
was to shoot lunars for a few days around First Quarter
and a few days around Last Quarter. That check on the longitude every
fortnight was a nice way of locking out the slowly
accumulating uncertainty in a longitude by dead reckoning or a longitude by
a potentially cranky chronometer.

As for Sobel's book, it definitely oversells the immediate victory of the
chronometer and converts Maskelyne into more of a villain
than he probably was (though there were plenty at the time who thought him
a bit of a villain, too). It took decades, in fact nearly
three-quarters of a century, to build up the little industry of artisans
that could manufacture chronometers reliably and in
sufficient quantities to bring the price down and satisfy the market. The
version of history told in Sobel's "Longitude" was the
standard version of the story among most people who told the history of
navigation in the latter half of the twentieth century. She
didn't invent it. Certainly the image of a "lone genius" beating the system
resonates... even if it didn't quite happen that way.
Apart from this fairly significant flaw, I think there's an enormous amount
of value and some very fine prose in Sobel's
"Longitude". For an alternate opinion, on the NavList main page, search for
the phrase "despicable little bookling". :-)

And the idea that the astronomers and mathematicians were irrationally
opposed and in some cases dogmatically opposed to "mechanics"
and their devices still has plenty going for it. Maskelyne and a few others
just couldn't see the simplicity of a solution based on
complicated machines. They didn't get it.

I'm attaching a page from a logbook from 1809 with an interesting example
of a lunar worked out. The text towards the lower left
reads "Running for Madagascar from Cape Agulhas. this obs. agrees very
nearly with acct." (in other words, the lunar longitude
agrees very nearly with the dead reckoning longitude --so he's happy). At
the very bottom of the page, with respect to another lunar
observation, he writes "Cape Orfui bearing NW 6 Leagues which would make
its long. about 51-10 --by Bowditch its long. is 51-38."
The actual longitude of Cape Guardafui, the eastern tip of the Horn of
Africa, is about 51-15 so he's doing pretty well with lunars!
Also it's interesting to note that he is carrying a copy of Bowditch and
using it as a reference for longitudes of places, but he
has NOT worked his lunar observations using the procedures recommended by
Bowditch back then.

And I'm also attaching a photo of a set of Australian postage stamps
commemorating Cook. It shows the geometry of shooting a lunar
in the middle pane. I could nitpick: the details aren't quite right, but
it's impressive to see something navigational on a stamp
anyway.

-FER


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