|
|
From: George Huxtable (no email)
Date: Sun Dec 07 2003 - 09:32:40 EST
Thanks to Frank Reed for an interesting mailing. I hope he gets out of his
snowdrifts soon.
A few comments from George, who admits to being, these days, a pontificator
rather than a practitioner of the lunar art.
It's a pleasure to be able to agree with just about every word that Frank
writes.
He said "I'm a physicist by training specializing in gravitation."
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)? I think a
consensus was reached in the end, but we could have done with some
authoritative words from a specialist.
"I am also the author/developer of the "Centennia Historical Atlas" CD-ROM
which is required reading for all students at the US Naval Academy but
that's another story."
It's a story that I, for one would like to learn more about, please.
"It occurred to me that Mystic Seaport could capitalize on that familiarity
by telling the other side of the story --the one in which Maskelyne is no
villain. And so I began an effort to bring back "the long lost lunars".
Good on you, as the Aussies would say. It's time that something was done to
redress the great injustices that were done to Maskelyne in that Sobel
book.
"I discovered that the most serious flaw in the Davis plastic sextant is in
the shades. In the middle of the night, I often needed a horizon shade to
reduce the Moon's brilliance when using one of the fainter "lunars stars"
(especially Hamal, Alpha Arietis). But the shades produced bad errors
(presumably "prismatic error").
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.
"It's a brass Plath sextant estimated to be from the late 1940s. Its best
feature for lunars is a 6x30 monocular which makes the Moon nice and big in
the field of view. This high-power monocular makes a BIG difference."
In the heyday of lunars, observers would use high-gain telescopes when they
could, but the disadvantage was in the restricted angle-of-view, so that in
rough conditions it would be difficult to get and keep the two bodies in
the field of view. 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?
"Mostly I shot my lunars from my backyard and calculated the altitudes. I
also wrote my own version of Arthur Pearson's spreadsheet calculator but
set up as a web page."
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?
"I would also be happy to talk about calculations at some point if anyone
is interested, but it's a big topic so I'll save that for another message."
Several of us are interested in such matters and would be keen to know
Frank's views.
"The chronometer had to be developed to cover the period around New Moon.
The lunar distance method was not complete without the chronometer."
Yes, and I would add that for long voyages, until chronometers got more
accurate and reliable, the chronometer method was not complete without the
lunar distance. Cook, for example, away for several years, relied on
correcting his chronometer readings using lunar distances. Mainly his
chronometer was used to interpolate for time over each passage of a voyage.
At the beginning and end of the leg, and sometimes in-between, its errors
had been determined by lunar distances (and sometimes moons of Jupiter).
"maybe the 21st century is the time to breathe some life into the long lost
lunars."
I agree, and suspect that more interest is being taken in lunars just now
than at any time in the last hundred years. This list is doing what it can.
As is the "Navigators' Newsletter." 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. 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.
George.
================================================================
contact George Huxtable by email at , by phone at
01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
================================================================
|