NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: No Lunars Era
From: Frank Reed CT
Date: 2004 Dec 6, 18:45 EST
And:
And:
From: Frank Reed CT
Date: 2004 Dec 6, 18:45 EST
Alex E wrote:
"Did lunars really qualify for the Longitude Prize"? The condition of the
prize, if I remember correctly was to find a reliable method of determining
longitude with 30' precision."
There was a sliding scale of prizes. 60' accuracy would win 5000 pounds,
40' would win 7500, and 30' wins 10,000 pounds.
And:
"Can one really guarantee this with lunars?"
Guarantee? No. There are no guarantees in navigation. I can get lunars
accurate to 0.2 to 0.3 minutes on a regular basis (and hence 6 to 9 minutes of
longitude), and this seems to be the typical expectation for lunars at sea in
the 19th century.
And:
"You have to be able to measure distances (sometimes large) with better
than 1' accuracy."
Yes. Now this business of "sometimes large" angles is perhaps asking too
much of the method. People today tend to view lunars as pure theory. But the way
they were used in practice is what's relevant to their usefulness historically.
When you look through old logbooks and check when lunars were taken, patterns of
use emerge that are not apparent in the navigation manuals like Norie and
Bowditch. First, the Sun was the second object in the vast majority of lunars
shot at sea --something like 80 to 90% of lunars were Sun-Moon lunars. Second,
the elongation was typically between 50 and 100 degrees with an average roughly
around 80. Lunars were taken typically for two or three days every two weeks.
It's a pattern that confused me when I first noticed it in the logbooks; why two
days in a row and then nothing for two weeks? But it's rather obvious when you
look at the Moon's phase at the time of the observations. First Quarter and Last
Quarter were the prefered times for shooting lunars. I should add that these
two patterns in the observations are not entirely independent. If you
limit yourself to Sun-Moon lunars, a two-week pattern is bound to emerge. The
part that is striking though is that lunars were done during a relatively
specific period of the lunar month.
And:
"(With all this irradiation, collimation and other sort of corrections)."
"(With all this irradiation, collimation and other sort of corrections)."
Ahem. Just because things are discussed on this list ad nauseam
does not mean they were serious problems for navigators in practical situations.
"I measured quite a lot of lunar distances from my balcony, under best possible conditions with a modern sextant, which most rate as very good, using computer reduction."
"I measured quite a lot of lunar distances from my balcony, under best possible conditions with a modern sextant, which most rate as very good, using computer reduction."
That's all well and good, but you're still a rank beginner, right? A
navigator might spend ten years at sea.
And wrote:
"My conclusion is that I am reasonably confident in 1' accuracy (of the distance) but not much better. Some of my lunars are 0.6 and even 0.7 off."
"My conclusion is that I am reasonably confident in 1' accuracy (of the distance) but not much better. Some of my lunars are 0.6 and even 0.7 off."
But those occur under specific circumstances, right? Large angles, and
always positive error, right? This should be telling you something. Either,
find the source of that error and fix it. Or, accept it as an error of unknown
origin and subtract that amount as an instrument error (it may very well be just
that). Or, avoid those angles! As noted above, navigators limited themselves to
certain angular ranges in the great majority of cases.
Also:
"Second, as we know from the Lewis and Clark story, even professional astronomers/surveyors had difficulties with reducing the sights."
"Second, as we know from the Lewis and Clark story, even professional astronomers/surveyors had difficulties with reducing the sights."
They didn't reduce them, and I agree completely with Ken Muldrew that this
was the source of their difficulties. They had no "feedback", as he puts it
somewhere, from their other navigational information.
And concluded:
"Only once I tried to reduce a lunar observation "by hands", but still using a primitive electronic calculator. My experience shows that this is not easy:-)"
"Only once I tried to reduce a lunar observation "by hands", but still using a primitive electronic calculator. My experience shows that this is not easy:-)"
I'm sure you agree that the evidence of "once" is no evidence at all.
Lunars were NOT, repeat NOT, difficult from a calculational standpoint. Indeed,
the "time sight" which was a daily calculation in the chronometer era involved
almost exactly the same mathematical steps and procedures. A lunar calculation
was roughly three times the length of a time sight calculation hence more
tedious but not more difficult than a time sight. Did you take a look at the
lunar calculation that I posted under the thread "A Lunar from the 1840s"?? That
is a COMPLETE lunar calculation from observations to GMT (this is in the second
period where lunars were being used to check the chronometer rather to find the
longitude directly).
Frank R
[ ] Mystic, Connecticut
[X] Chicago, Illinois
[ ] Mystic, Connecticut
[X] Chicago, Illinois