NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Lunars: Three simultaneous observations
From: Frank Reed CT
Date: 2004 Sep 21, 00:51 EDT
From: Frank Reed CT
Date: 2004 Sep 21, 00:51 EDT
Last week Gordon T wrote:
"BTW, I know a lot of talk has been given to "Lunar Distance" methods,
and it is fun to see how they did them, but it does not seem to me
to be a very practical method."
I guess that depends on what you mean by "very practical". Lunars were certainly used actively in the late 18th and most of the 19th centuries (with their heyday from about 1775 to 1845 and declining rapidly thereafter). Of course, they were never used in the same fashion as chronometers. It was neither practical nor necessary to shoot a lunar every day. In the logbooks I've explored, even when actively used, a typical rate was about once a week. Lunars provided an excellent "check" on the dead reckoning which was the primary method of longitude navigation in this era.
And wrote:
"There are at least three simultaneous observations that have to be done and tons of calculations"
Many myths developed about lunars after they faded away. Their difficulty has been greatly exaggerated. It is often repeated that lunars required three simultaneous observations. This just isn't so. In the very earliest editions of his "Navigator", Bowditch describes the technique for shooting lunars when you're alone. I've done it myself on many occasions. Labor was cheap on large vessels in the 19th century. They had large crews, and it was simply convenient to put a couple of junior officers to work observing the altitudes simultaneously with the lunar distance, but it wasn't required. As for "tons" of calculations, that's just too "weighty" <g>. A lunar can be worked by hand on half a page of paper in less than twenty minutes. You should really try it for yourself if you haven't yet. If you do, I'll say again here that the single most important prerequisite for successful lunars is an excellent sextant properly adjusted.
And:
"I can see why they "dumped" these methods as soon as they got decent chronometers."
Sure. Navigation is a practical art. Although there were academics and lunarian eccentrics trumpeting the lasting importance of lunars as a check on the chronometers in the late 19th century, practical navigators saw the situation differently. If you had asked one of them back then 'what's your best backup when your two main chronometers fail?', he probably would have answered 'a third chronometer'. The situation is similar today with respect to celestial navigation in general. It has been rapidly abandoned in favor of GPS, and the best backup against the possibility that your GPS receiver gets fried is probably another GPS receiver.
And:
"I remember reading about some ship finding an island and noting the position. A few years latter another ship spending days trying to find the island without success. Then another ship finding the same island again without any problems"
This would not be due to the weaknesses of lunars though. You should take a look at some of the positions generated during Cook's first Pacific voyage during which lunars were used extensively (they brought an astronomer). The longitudes are exceptionally good. Navigators using lunars and later chronometers had no problem finding islands charted by Cook.
Frank R
[ ] Mystic, Connecticut
[X] Chicago, Illinois
"BTW, I know a lot of talk has been given to "Lunar Distance" methods,
and it is fun to see how they did them, but it does not seem to me
to be a very practical method."
I guess that depends on what you mean by "very practical". Lunars were certainly used actively in the late 18th and most of the 19th centuries (with their heyday from about 1775 to 1845 and declining rapidly thereafter). Of course, they were never used in the same fashion as chronometers. It was neither practical nor necessary to shoot a lunar every day. In the logbooks I've explored, even when actively used, a typical rate was about once a week. Lunars provided an excellent "check" on the dead reckoning which was the primary method of longitude navigation in this era.
And wrote:
"There are at least three simultaneous observations that have to be done and tons of calculations"
Many myths developed about lunars after they faded away. Their difficulty has been greatly exaggerated. It is often repeated that lunars required three simultaneous observations. This just isn't so. In the very earliest editions of his "Navigator", Bowditch describes the technique for shooting lunars when you're alone. I've done it myself on many occasions. Labor was cheap on large vessels in the 19th century. They had large crews, and it was simply convenient to put a couple of junior officers to work observing the altitudes simultaneously with the lunar distance, but it wasn't required. As for "tons" of calculations, that's just too "weighty" <g>. A lunar can be worked by hand on half a page of paper in less than twenty minutes. You should really try it for yourself if you haven't yet. If you do, I'll say again here that the single most important prerequisite for successful lunars is an excellent sextant properly adjusted.
And:
"I can see why they "dumped" these methods as soon as they got decent chronometers."
Sure. Navigation is a practical art. Although there were academics and lunarian eccentrics trumpeting the lasting importance of lunars as a check on the chronometers in the late 19th century, practical navigators saw the situation differently. If you had asked one of them back then 'what's your best backup when your two main chronometers fail?', he probably would have answered 'a third chronometer'. The situation is similar today with respect to celestial navigation in general. It has been rapidly abandoned in favor of GPS, and the best backup against the possibility that your GPS receiver gets fried is probably another GPS receiver.
And:
"I remember reading about some ship finding an island and noting the position. A few years latter another ship spending days trying to find the island without success. Then another ship finding the same island again without any problems"
This would not be due to the weaknesses of lunars though. You should take a look at some of the positions generated during Cook's first Pacific voyage during which lunars were used extensively (they brought an astronomer). The longitudes are exceptionally good. Navigators using lunars and later chronometers had no problem finding islands charted by Cook.
Frank R
[ ] Mystic, Connecticut
[X] Chicago, Illinois
