NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Real accuracy of the method of lunar distances
From: Frank Reed CT
Date: 2004 Jan 7, 04:50 EST
From: Frank Reed CT
Date: 2004 Jan 7, 04:50 EST
George H wrote:
"It's been noticeable that even in these days of highly precise sextants and timekeepers, many of the lunar measurements reported to this list have been out by a minute-of-arc or two from their expected value"
From my experience, I would bet on sextant adjustment problems. There are few observations that test a sextant and a navigator as strenuously as lunars. But there are some others besides lunars! More on this later...
And George H wrote:
"Perhaps part of the lost lunar art was in knowing just how much to allow for irradiation when taking a Sun-lunar"
Possibly, but I doubt it. With decent shades, "irradiation" should never be a problem.
And:
"Perhaps some lunar observers on this list have managed to achieve the same accuracies that their predecessors were doing 200 years ago. If so, I hope they will tell us about it."
As I've written, I have no problem getting lunars within 0.3 minutes of error (on average, 9 minutes of longitude) with a good sextant.
And:
"Of course, what we next have to consider is the effect on the accuracy of a lunar of a pitching, rolling, deck, finding a patch to lie down where the square sails overhead didn't blot out the two objects you needed to see at the same time."
The rigging can also *help* with sights. I frequently do lunars by lining up the more difficult object (a fainter star at night, or a slim crescent moon in daylight) with a tree limb or an overhead wire. This is a fair "cheat" since you can do the same thing with rigging on a sailing ship.
Frank E. Reed
[X] Mystic, Connecticut
[ ] Chicago, Illinois
"It's been noticeable that even in these days of highly precise sextants and timekeepers, many of the lunar measurements reported to this list have been out by a minute-of-arc or two from their expected value"
From my experience, I would bet on sextant adjustment problems. There are few observations that test a sextant and a navigator as strenuously as lunars. But there are some others besides lunars! More on this later...
And George H wrote:
"Perhaps part of the lost lunar art was in knowing just how much to allow for irradiation when taking a Sun-lunar"
Possibly, but I doubt it. With decent shades, "irradiation" should never be a problem.
And:
"Perhaps some lunar observers on this list have managed to achieve the same accuracies that their predecessors were doing 200 years ago. If so, I hope they will tell us about it."
As I've written, I have no problem getting lunars within 0.3 minutes of error (on average, 9 minutes of longitude) with a good sextant.
And:
"Of course, what we next have to consider is the effect on the accuracy of a lunar of a pitching, rolling, deck, finding a patch to lie down where the square sails overhead didn't blot out the two objects you needed to see at the same time."
The rigging can also *help* with sights. I frequently do lunars by lining up the more difficult object (a fainter star at night, or a slim crescent moon in daylight) with a tree limb or an overhead wire. This is a fair "cheat" since you can do the same thing with rigging on a sailing ship.
Frank E. Reed
[X] Mystic, Connecticut
[ ] Chicago, Illinois
