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From: Frank Reed (no email)
Date: Sun Dec 07 2003 - 21:40:10 EST
George Huxtable wrote:
"Frank, where were you a few months ago when the list was wrangling over the
question of the dips and bumps in the Earth's gravitational field (and whether
you could shorten a passage by navigating around them)?"
Can you point me to the right spot in the message archives? I would love to
see how the discussion turned out.
Re my "day job", the Centennia Historical Atlas, you wrote:
"It's a story that I, for one would like to learn more about, please."
Since it's a commercial product, I don't think I should spend time on it on
this list, but I'll tell one fun story since I'm at an anniversary. In November
1990, I ran my first advertisement about "Centennia" (way back then it was
called the "Millennium" historical atlas --back before everything was
millennially monikered). Two weeks later (13 years ago this week), I received a letter
from the History Department at the US Naval Academy informing me that they had
been hoping for something like this for years and they were ready to buy 1200
copies a year. I thought to myself, 'wow, this business is easy... if I get an
order for 1200 every month!' And the rest is history.
"It's time that something was done to redress the great injustices that were
done to Maskelyne in that Sobel book."
A few months ago, I found your review of Sobel's "Longitude" in the archives
for this list and printed a few copies for the other navigators at Mystic
Seaport. They were amused to say the least. And we agree with the general
sentiment though maybe not every word <g>.
"Yes, it's a weakness that applies to other plastic sextants, including some
Ebbco models. It was got round in some early sextants, in the days when
plane-parallel glass was hard to find, by mounting the shades so they could be
rotated through 180°, then averaging."
Before I forget, the errors in the shades of my Davis sextant are smaller
than about 1.5 minutes or arc, so they're not so bad for ordinary celestial
navigation. I intend to buy another despite the flaws.
And you wrote:
"And those telescopes were so long that the sextant would have to be held
right out at arm's length, as some old illustrations show. Presumably a modern
Plath is superior in both respects. Does Frank have access to any "historical"
sextant to make a comparison, I wonder?"
I might be able to experiment. I'll look into it.
Regarding backyard lunars, you wrote:
"Using that system, is Frank able to deduce the longitude from the observed
lunar distance, with altitudes calculated and not measured, and no prior
knowledge of chronometer error or longitude? Can this be done without any measured
altitudes at all?"
No. Naturally you need a time sight at some point to get local time. But this
is easy and can be done when convenient under the assumption that you have
access to an ordinary "watch" (as opposed to a chronometer) to keep local time
for a few hours. Since this is a separate process both historically and
theoretically, I don't think it interferes with the legitimacy of the lunar
observation. I have done one good Sun altitude (from the nearby shore) along with a set
of lunars so that the time sight was nearly coincident with the lunar
observation, but it doesn't really add anything to the process.
And:
"I suspect that part of it is the removal of that great trigonometrical
bugbear, the "clearing" of the distance, by the use of calculators/computers."
Well, in many ways, that was over and done with 25 years ago. Letcher laid it
all out nicely in his book. Modern interest ironically probably derives in
part from Sobel's book. And there's also the effect of the Internet. Twenty-five
years ago, I knew only one person who knew what a "lunar" was.
And:
"Another is, oddly enough, the advent of GPS, which has taken away interest
from classical navigation as a means of finding where you are, and diverted it
into understanding the immense importance that scientific navigation played in
the opening-up of the World. That's my own view, anyway."
I agree. There's a funny quote in a navigation textbook from c.1905 (by the
Earl of Dunworthy??). His chapter on lunars begins with a long explanation of
why lunars are "a foolish thing" but a "fascinating mathematical problem". The
same could be said of all of celestial navigation in the year 2003. A hundred
years ago, professional navigators were finally being relieved of the annoying
task of learning how to reduce lunars. They had long since ceased to be
practical, and so the last word on them for decades was from a generation of
navigators who saw them only as pointless torture required to pass a licensing exam.
There are a few students today who are being "tortured" by celestial
navigation when they know they will never use it in any practical setting (odds of 999
to 1 against). They're a lot like those last students of lunars a hundred
years ago. But I think that they are few enough in numbers that their influence
on navigation will be relatively insignificant. Today most students learn
celestial navigation of any flavor for reasons of personal pride and personal
challenge. Anyone can push a button on a GPS, but only a handful can use the Sun
and Moon and the stars and planets to cross the sea. They can claim the legacy
of thousands of years of tradition. They are mariners. Everybody else...?
They're just "boaters". <g> That, I think, is the main "selling point" of celestial
navigation today including less practical components like lunar distances.
Frank E. Reed
75% Mystic, Connecticut
25% Chicago, Illinois
(I'm flying out of the snow tomorrow so I may be offline for a couple of days)
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