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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Slocum's lunars
From: Frank Reed CT
Date: 2003 Dec 23, 18:56 EST
From: Frank Reed CT
Date: 2003 Dec 23, 18:56 EST
H Prinz wrote:
"I have read this article, but I don't think the author makes his case."
Do you have a copy by any chance?
You noted:
"1. Slocum does not say that he found an error in the Almanac, he says he
found it in the "tables".
2. Slocum says that a "logarithm" was wrong. RA and Dec are not tabulated
as logarithms.
3. Slocum says a "column" was in error, not the whole almanac."
As Jan Kalivoda (I think) correctly noted in an earlier message, it was common for navigators to refer to all of their numbers as logarithms no matter what they were mathematically. But I agree that this is a little less likely than a problem with an actual table of logarithms or a problem with using those tables.
"Furthermore, RA and Dec don't enter the distance computation, unless the
altitudes are computed (and if so, as with some precise methods, only as a
small correction)."
That sounds right to me. If you're near the Marquesas in June, the error in calculated altitude for the Sun if taken from a time that's off by 12 hours would amount to only about a quarter of a degree. This could be a significant error, but not enough to put him in the wrong place by "hundreds of miles" as Slocum describes.
And:
"So the error could only have affected the reduction of the time sight that was taken in connection with it. May we really assume that Slocum did not know how to do a time sight?"
Of course he KNEW how, but that doesn't mean he would have been infallible in their execution.
And you wrote:
"And finally, a twelve hour shift of the time argument can affect
declination of the sun by up to 12' around equinox. If we grant Slocum
that he performed at least noon sights regularly, this puts his various
claims about the accuracy of his navigation to within a few miles in an
interesting light."
Slocum frequently claims that his latitudes were consistent day ater day, but if your latitude was off by 12 miles for a few months, 5 on average, you would not necessarily notice it. It's one disadvantage of "sailing alone".
You wrote:
"In short, the problem that I have with van der Werf's explanation is that
we can accept it only if we assume that Slocum did not know what the hell
he was doing."
It's not quite that bad. Everyone, no matter how smart or competent, makes mistakes. One of my favorite quotations on quality control in recent years followed the investigation into the loss of the Mars Climate Orbiter. Here it is:
"People sometimes make errors. The problem here was not the error, it was the failure of NASA's systems engineering, and the checks and balances in our processes to detect the error. That's why we lost the spacecraft." --Dr. Edward Weiler, NASA's Associate Administrator for Space Science.
And that's absolutely right. The idea that a bunch of NASA engineers messed up a "unit conversion" became a late night tv joke, but mistakes like this are inevitable. The key is to create systems of quality control that catch those errors.
Error detection is one of the most difficult and important parts of practical celestial navigation. Slocum found his error one way or another. It sure would be wonderful if we had access to his notes.
One of my error detection experiences: last summer I did a series of lunars and they were all out by about 23.5 minutes of arc (in the corrected LD). I was very puzzled to say the least. Eventually I found my error but the only way to do it was to labor through the whole thing thinking carefully about each and every input. I had neglected Daylight Savings Time.
Frank E. Reed
[X] Mystic, Connecticut
[ ] Chicago, Illinois
"I have read this article, but I don't think the author makes his case."
Do you have a copy by any chance?
You noted:
"1. Slocum does not say that he found an error in the Almanac, he says he
found it in the "tables".
2. Slocum says that a "logarithm" was wrong. RA and Dec are not tabulated
as logarithms.
3. Slocum says a "column" was in error, not the whole almanac."
As Jan Kalivoda (I think) correctly noted in an earlier message, it was common for navigators to refer to all of their numbers as logarithms no matter what they were mathematically. But I agree that this is a little less likely than a problem with an actual table of logarithms or a problem with using those tables.
"Furthermore, RA and Dec don't enter the distance computation, unless the
altitudes are computed (and if so, as with some precise methods, only as a
small correction)."
That sounds right to me. If you're near the Marquesas in June, the error in calculated altitude for the Sun if taken from a time that's off by 12 hours would amount to only about a quarter of a degree. This could be a significant error, but not enough to put him in the wrong place by "hundreds of miles" as Slocum describes.
And:
"So the error could only have affected the reduction of the time sight that was taken in connection with it. May we really assume that Slocum did not know how to do a time sight?"
Of course he KNEW how, but that doesn't mean he would have been infallible in their execution.
And you wrote:
"And finally, a twelve hour shift of the time argument can affect
declination of the sun by up to 12' around equinox. If we grant Slocum
that he performed at least noon sights regularly, this puts his various
claims about the accuracy of his navigation to within a few miles in an
interesting light."
Slocum frequently claims that his latitudes were consistent day ater day, but if your latitude was off by 12 miles for a few months, 5 on average, you would not necessarily notice it. It's one disadvantage of "sailing alone".
You wrote:
"In short, the problem that I have with van der Werf's explanation is that
we can accept it only if we assume that Slocum did not know what the hell
he was doing."
It's not quite that bad. Everyone, no matter how smart or competent, makes mistakes. One of my favorite quotations on quality control in recent years followed the investigation into the loss of the Mars Climate Orbiter. Here it is:
"People sometimes make errors. The problem here was not the error, it was the failure of NASA's systems engineering, and the checks and balances in our processes to detect the error. That's why we lost the spacecraft." --Dr. Edward Weiler, NASA's Associate Administrator for Space Science.
And that's absolutely right. The idea that a bunch of NASA engineers messed up a "unit conversion" became a late night tv joke, but mistakes like this are inevitable. The key is to create systems of quality control that catch those errors.
Error detection is one of the most difficult and important parts of practical celestial navigation. Slocum found his error one way or another. It sure would be wonderful if we had access to his notes.
One of my error detection experiences: last summer I did a series of lunars and they were all out by about 23.5 minutes of arc (in the corrected LD). I was very puzzled to say the least. Eventually I found my error but the only way to do it was to labor through the whole thing thinking carefully about each and every input. I had neglected Daylight Savings Time.
Frank E. Reed
[X] Mystic, Connecticut
[ ] Chicago, Illinois
