 Check out the bookstore at IRBS.com |
|
|
|
|||||
|
||||||
From: Frank Reed (no email)
Date: Mon Jun 06 2005 - 22:53:00 EDT
Fred you wrote:
"In response, I don't find the intercept method any more difficult to
understand geometrically than a noon shot. The basic flag pole
analogy, much derided here, seems adequate to me, only requiring that a
spherical surface and infinitely tall pole be substituted to achieve
reality. While building the infinitely tall pole, an instructor would
get to talk about parallax."
Yes, I totally agree that the essential principle of the circles of position
crossing is very easy to explain and that a substantial majority of people
who hear such an explanation do understand it. I've explained it over a
thousand times (I'm not kidding!) within the past few years to casual tourists and
groups of children as young as twelve years of age. And they get it, and
they're happy wit the explanation. But it's a long haul from that quick
geometrical explanation to the end goal of actually using the Nautical Almanac and
sight reduction tables. How long did it take you to learn so that you could do
it for real? How long do you think it would take someone a lot less
mathematically inclined than you are?
On a general note, let's remember here -- I have nowhere suggested that
everyone should throw their sight reduction tables and Nautical Almanacs
overboard. First and foremost, I felt it was important to counter the erroneous
claim, which I think you agreed with, that a navigator could not get a good
longitude at noon. Second, I am presenting a method of celestial navigation --and
it IS *real* celestial navigation-- that can be taught practically to students
in a very short period of time. They can have almost instant gratification.
And for a dying art, that's not such a bad thing.
And:
"Using pen and paper methods of sight reduction, LOP navigation is more
difficult, but not with a calculator or computer."
Very true about a computer especially. If you're reducing sights with a
computer, why not just use GPS? Clearly for practice sights, it's a nice
convenience to clear them with a computer. And there are some enthusiasts who like
going through the motions of using a sextant and doing all their clearing work
by computer, and the fact that they have no idea how to clear the sights by
hand doesn't matter to them at all. That's simply another market segment.
They're not "navigating" with celestial, but they like using sextants. To each
his own.
And:
"George's comments about taking a prolonged series of sights while
underway are well taken."
Nah. This is not time-consuming. Taking six to ten shots over 40 minutes to
an hour would require only six to ten minutes of work. You could make lunch
for a whole family at the same time. And besides, if you're not in a storm or
a race, there's an awful lot of free time on most ocean passages. Ask Joshua
Slocum...
And:
"a clear sky is needed. Why not take a quick morning, noon and evening
shot?"
Yes, a clear or partly cloudy sky is needed. Note that you don't need to see
the Sun continuously for this method --occasional "peeks" will suffice. Of
course, cloudy weather is the best argument against using celestial navigation
generally.
You also wrote:
"One point about a traditional noon shot is that the sun often peaks out
from behind the clouds at noon for a few moments even on days with
heavy overcast. There is a story famous to me about a passenger on a
ship asking why all the officers had their sextants out under a cloudy
sky at noon, only to be informed by the captain that the sun often
would peak out for a bit, and sure enough it did. That's also been my
experience at 36N in the mountains."
I think this is an 'urban legend'. I would bet that the rate of "Sun
peeking" on mostly cloudy days is constant throughout the middle hours of the day
and navigators have tended to remember those times when they got lucky right at
noon. I think a cloudless longitudinal stripe along the Sun's longitude
(which would exist if this tale were true) would show up pretty clearly in
satellite imagery! <g>
-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
|