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
Trevor K wrote:
"I think Frank's evidence from whaling log books has already given you one answer: In at least some cases, it seems to have been the practice to take two lunars a couple of days apart. That way, they would have been worked up separately and a careless error in one would not have been automatically repeated in the second. If the first one confirmed
the navigator's DR, then the second lunar might not be needed. "
Although that's a worthy speculation, I'm not sure it works. If you want an independent test, you just use two teams. Labor was cheap back then. Pairs of lunars a day or two apart (as seen in the logs) would seem to require some other explanation. I've already mentioned that I think lunar distance training was a possibility... but there could easily be other explanations.
And wrote:
"If the DR was highly suspect, then two lunars that roughly agreed would have given grounds to call it a celestial fix and start a new DR reckoning. If the DR and the first two lunars were all very different, then do a third lunar on another day and see what that indicated."
That's also an interesting hypothesis, but is there evidence for it? From what I have seen, they didn't do what you're describing.
And wrote:
"In the context of lunars: It is entirely possible to navigate your way around the ocean with no means of estimating longitude at all, save for DR. After all, the Norse operated a regular trade route from Norway to Greenland and back (without touching land in between) on that basis starting nearly a thousand years ago. "
And you could do that today! And Slocum could have 105 years ago... :-)
Trevor K mentioned:
"I doubt that it was any accident that steamers started the practice of carrying multiple chronometers (if, in fact, they did)."
Apparently chronometers were sufficiently inexpensive before steamers dominated the oceans. In most of the whaleship logbooks I've seen from the 1840s and 1850s, there is mention of a second chronometer. Whaleships did not live or die by tight schedules and they were not operated by large rich corporate entities, yet even they could justify the cost of more than one chronometer at that (fairly early) date.
Trevor K added:
"Greater navigational certainty paid off in hard cash and so justified the cost of the instruments."
Yes, and as I mentioned at some point, this choice is ALL about money. Navigators in the 19th century didn't do lunars or trust chronometers because of theory or romanticism. They followed the money (whether in profit made or in risk avoided). Chronometers won because they became cheap.
Frank E. Reed
[X] Mystic, Connecticut
[ ] Chicago, Illinois
"I think Frank's evidence from whaling log books has already given you one answer: In at least some cases, it seems to have been the practice to take two lunars a couple of days apart. That way, they would have been worked up separately and a careless error in one would not have been automatically repeated in the second. If the first one confirmed
the navigator's DR, then the second lunar might not be needed. "
Although that's a worthy speculation, I'm not sure it works. If you want an independent test, you just use two teams. Labor was cheap back then. Pairs of lunars a day or two apart (as seen in the logs) would seem to require some other explanation. I've already mentioned that I think lunar distance training was a possibility... but there could easily be other explanations.
And wrote:
"If the DR was highly suspect, then two lunars that roughly agreed would have given grounds to call it a celestial fix and start a new DR reckoning. If the DR and the first two lunars were all very different, then do a third lunar on another day and see what that indicated."
That's also an interesting hypothesis, but is there evidence for it? From what I have seen, they didn't do what you're describing.
And wrote:
"In the context of lunars: It is entirely possible to navigate your way around the ocean with no means of estimating longitude at all, save for DR. After all, the Norse operated a regular trade route from Norway to Greenland and back (without touching land in between) on that basis starting nearly a thousand years ago. "
And you could do that today! And Slocum could have 105 years ago... :-)
Trevor K mentioned:
"I doubt that it was any accident that steamers started the practice of carrying multiple chronometers (if, in fact, they did)."
Apparently chronometers were sufficiently inexpensive before steamers dominated the oceans. In most of the whaleship logbooks I've seen from the 1840s and 1850s, there is mention of a second chronometer. Whaleships did not live or die by tight schedules and they were not operated by large rich corporate entities, yet even they could justify the cost of more than one chronometer at that (fairly early) date.
Trevor K added:
"Greater navigational certainty paid off in hard cash and so justified the cost of the instruments."
Yes, and as I mentioned at some point, this choice is ALL about money. Navigators in the 19th century didn't do lunars or trust chronometers because of theory or romanticism. They followed the money (whether in profit made or in risk avoided). Chronometers won because they became cheap.
Frank E. Reed
[X] Mystic, Connecticut
[ ] Chicago, Illinois
