|
|
|
|
|||||
|
||||||
From: Frank Reed (no email)
Date: Sat Dec 06 2003 - 22:43:54 EST
Last week, George Huxtable wrote:
"Are you a new name to our list? If so, you are indeed welcome, especially
with the message you bring. Lunar observers, as opposed to lunar pontificators,
are few and far between. "
Hey I can pontificate with the best of 'em <g>, but the proof is in the
practice. I decided last year when I got back into lunars that I couldn't talk
about them unless I could do them well myself.
And thank you for your welcoming words. I think I should describe my
background and interests. First, I'm a physicist by training specializing in
gravitation. But my real interest has always been astronomical calculation and the
practical arts connected to such calculations. I grew up in the Mystic,
Connecticut area, and a year ago I moved back here to Mystic to work on a project for
the local planetarium and celestial navigation center (just to be clear, I'm
done with that project now so I don't presently work there and do not represent
them in these messages). 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.
Mystic is the home of "Mystic Seaport", a museum dedicated to America's
maritime heritage with a focus on the 19th century. For those who don't know of it,
Mystic Seaport has a large collection of small boats, vessels, and several
large ships including three National Historic Landmarks. The centerpiece of the
collection is the Charles W. Morgan --the last of the wooden whaling ships and
the oldest commercial vessel in the United States, built in 1841.
The Planetarium at Mystic Seaport naturally emphasizes the history of
celestial navigation in its daily programming though somewhat indifferently before I
started working there last year. Many visitors to Mystic Seaport are at least
aware of the story of Harrison and his chronometers from Dava Sobel's little
book and the various tv documentaries and other multimedia that it spawned. 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".
Step One: shoot some. Twenty-five years ago (when I was 17) I bought a Davis
plastic sextant (the same model which is currently priced at about $195). It's
a nice instrument and works well. I started there. To reduce my sights, I
began by using Arthur Pearson's spreadsheet. I discovered that I could get 1-2
minute accuracy in the lunars using my plastic sextant. This depended on careful
and frequent index correction measurements since plastic sextants change
their adjustment with temperature. But I couldn't do better than that and I often
found that the error would jump for no apparent reason to 3 or more minutes in
a series of sights. After some experimentation, 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"). Now I might have
been able to develop procedures to work around this problem, but at this
point I decided it was time to get serious and switch to a high quality sextant.
The Seaport Planetarium has a small collection of sextants for celestial
navigation classes (and of course also a collection of preserved instruments). I
borrowed the sextant from the class collection that I felt would best suit my
needs for shooting lunars. 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. With this instrument in hand, my lunars immediately improved in
quality and the process started to seem even "practical" (I place that word in
quotation because there is scarcely any part of celestial navigation that really
qualifies as practical now that GPS has solved the problem of navigation).
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. I have designed my web calculators with small screens in mind.
This means I can view them from a portable device --like my cell phone. Why a
cell phone? Mostly because kids like them. Celestial navigation is dusty and
moldy to most young people and lunars even more so. Pulling out a cell phone to
work a lunar has an amazing effect on younger students. To give it a try, visit
"fer1.com" (a short address since it's intended to be entered on a cell
phone). The lunar distance calculator is on the page named "Moon and Sun". [please
note: fer1.com is presently set up entirely for my entertainment so there are
many sections that are "not ready for prime-time"]. 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.
I also did a half-dozen lunars sets by shooting the altitudes along with the
lunar distance. I did these from shore points near here where I could see the
horizon. I also did one set from a boat but only a few miles offshore. Even in
the middle of the night, shooting the altitudes along with the lunar
presented no difficulties at all. My procedure was to shoot two altitudes of each
object first, then shoot four to six lunar distances, and follow that up with two
more altitudes of the objects taking note of "watch time" for each
observation. I would average the lunars graphically and take out the time of the "best
lunar". With that time I would interpolate the objects' altitudes. This
interpolation is more than adequate to get the objects' altitudes to +/-6 minutes of
arc which is all that is required to reduce the lunar distance sight. This
graphical interpolation method works great and the whole process takes no more
than ten to fifteen minutes. But for the most part, I no longer bother with
direct measurement of the altitudes; I've satisfied myself that I can make the
measurements when and if I need to do so.
I usually do the lunars observations themselves with both hands on the
sextant frame since the angle at which you have to hold the sextant is often awkward
--and even painful after a few minutes. Holding he instrument stably is
critical. To make this work, I would alternate between adjusting the sextant angle,
making small adjustments of the micrometer, with the actual process of
looking through the sextant. The lunar distance changes so slowly that you don't
really need to keep a hand on the micrometer while observing (as is essential
with altitude observations).
A very accurate index correction and careful adjustments to eliminate
perpendicularity error and the other sextant errors are essential for these delicate
observations. I find that the best index correction observations come from
"limb to limb" observations of the Sun or the horns of the Moon. Be careful if
you try this with the Sun: if you forget the horizon shades, you'll shoot your
eye out!
It's also critical to get accurate altitude corrections including the effect
of temperature and pressure. You can't ignore these if you want accurate
results from the reduction of your lunar observations. This is one minor problem
with some of the spreadsheet solutions and also Letcher's method which folds the
refraction calculation directly into the reduction process. The method I use
to reduce lunars when I want to be very accurate is based on the approach in
Chauvenet and is closely related to Letcher's technique (I derived it myself
but it's in the same family as those two methods).
Shooting "practical lunars" for many months taught me one important lesson
about chronometers: you need one. The only way to do navigation "purely" by
lunars is to stay in port for three or four days every month around New Moon. If
that's unacceptable (and I think it's clear that it is) then you need a
chronometer that can run accurately for three or four days. Of course once you have
that, it's not much of a jump to acquire a chronometer that will run well for
the entire duration of a typical 18th/19th century sea voyage. The chronometer
had to be developed to cover the period around New Moon. The lunar distance
method was not complete without the chronometer.
In over 250 planetarium lectures in 2003, I discussed the Moon as a "natural
clock in the sky" and the fact that it was used by early 19th century
navigators to cross oceans. I also invited a few dozen museum visitors over the summer
to shoot their very own lunar (when weather and timing and moon phase would
permit it). I've also shot lunars from the shores of Lake Michigan and from a
half-dozen spots along Interstate-80 as I drove from Illinois to Connecticut
--I can claim that I've crossed 800 miles of the United States using lunar
distances to determine my longitude (well, ok, the road signs helped a little... <g>
). So they're not dead yet, and maybe the 21st century is the time to breathe
some life into the long lost lunars.
Frank E. Reed
75% Mystic, Connecticut
25% Chicago, Illinois
(I'm suposed to be in Chicago right now but we're getting buried by a
blizzard in Connecticut this weekend so my travel plans have been pushed back a few
days).
|