NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Moon altitude problems.
From: Robert Eno
Date: 2006 Aug 20, 23:42 -0500
George Huxtable wrote:
> Robert Eno questioned Frank Reed's statement-
>
> | If the line is tilted so that the
> | > upper cusp is to the right of the lower cusp (and the Sun is to
> the
> | > right), then the Lower Limb should be used.
> |
> | Robert asks:
> |
> | Frank, perhaps I am misinterpreting your description and/or
> visualizing this
> | incorrectly, but it seems to me that under such conditions, would it
> not be
> | the upper limb that should be used?
>
> Frank has it right.
Robert responds: I "got it" in the interim. A question of misinterpreted
terminology on my part.
George wrote:
> Maybe he had inherited similar prejudices, from another old salt, in
> his own early days. And now, Robert is doing his best to pass those
> prejudices on to another group of navigators, us. All very well, if
> there is reasoning and hard evidence to back it up, but those so far
> have been conspicuously lacking.
Robert responds:
And here George completely misunderstands me. I would never presume to pass
on any of my own views to anyone on this list. My contention is that while
the textbooks and most navigators it seems, assure us that moon observations
are perfectly accurate, there are a number of seasoned navigators who
believe, based on their own observations, that moon shots are unreliable. I
cannot speak for my friend except to say that he has decades of actual
(pre-GPS) sea-going experience as a master mariner and that he later taught
navigation and mathematics to budding seafarers. This indicates to me that
his views on the accuacy of moon observations cannot be simplistically
attributed to mere prejudices inhereted from another old salt.
Your comment about "hard evidence" being "conspicously absent" is
misleading. At no time, did I suggest that I had hard evidence to support a
case for moon observations being unreliable. Throughout this discussion I
have repeatedly stated that my comments are based on my own personal
experience; that there are other mariners that are of the same opinion; and
that there may be some factual basis to this contention. You will also
remember that, in not so many words, I asked for opinions from other list
members. If I had "hard evidence" to support the idea that moon shots are
unreliable, I would not have posted the message in the first place because
it would have been a non-issue for me.
George wrote:
>In Robert Eno's case, when using a bubble sextant
> attachment, errors
> are far more likely to stem from its own inherent inaccuracies, rather
> than any problems with the Moon itself. I have no experience at all
> with bubble devices, but one possible difficulty comes to mind, with
> the Moon and a bubble sextant.
Robert responds:
And I did clearly acknowledge the inherent inaccuracies of bubble
sextants/attachments in my original post. Nevertheless, I countered that my
observations of Vega, which were far more difficult to take than the moon's
and therefore should have been less accurate, turned out to be better than
the moon observations anyway.
George wrote:
When observing the Sun, as I understand
> it, the aim is to match the disc of the Sun with the disc image of the
> bubble, and try to keep it there.
Robert responds:
That is pretty much the case, although it is better to have a bubble which
is slightly larger than the disc of the sun. Not too much larger but just a
couple of "hairs".
George wrote:
How does that work out with the
> non-disc of the Moon, when only a part of that non-disc is the crisp
> edge of the appropriate limb? What do you centre, against what?
Robert responds:
This is where it gets tricky. Sometimes, you can see the faint outline of
the rest of the moon, while other times you have to almost "imagine" the
shape of the rest of the moon and try to align this, and the portion which
you can see, in the centre of the bubble. My observations of a few nights
ago, were of a gibbous moon, so there was plenty to work with.
George wrote:
> the problems that Robert finds with a bubble sextant relevant to an
> observer who can see a horizon? Have a significant fraction of the
> many Moon observations he relates been measured against a real
> horizon, and do the same discrepancies occur then?
Robert responds:
Both. I would actually have to dig out my old logbooks to see what my
natural horizon observations were like but I remember them being none too
satisfactory. Other mariners (not all) I have spoken to about this have
related similar "bad luck" experiences with the moon.
George wrote:
> If Robert is prepared to show us his own observation log, so that we
> can check over the problems he tells us about, we may be able to
> detect whether he sees observational scatter or some systematic error.
> And perhaps, some possible defect in the way that those all-important
> corrections have been made.
Robert responds:
I will send these to you off list as not everyone may interested in looking
at them. In any case, I hasten to reiterate what I already stated in my
first posting: 8 observations (4 moon & 4 Vega) do not constitute a
statistically defensible position; especially with a bubble horizon. I
mentioned these recent data to illustrate a pattern that I have experienced
for years: moon observations, for me anyway, have been more often than not,
unreliable.
George wrote:
> Until now, all we have seen is hearsay evidence. And prejudice.
Robert responds:
Again, I believe you misrepresent me. I never indicated that my contention
was correct and in fact questioned my own personal observations as possibly
being the result of continuous re-inforcement of a bad habit (see my first
posting). Prejudice? Harsh words George. I would more describe my
sentiments as "gun shy" (where moon observations are concerned). This is
predicated on experience. I simply do not get good results with the moon.
George wrote:
>
> By the way, the Moon altitude correction tables, in the back pages of
> the nautical almanac, were devised with a horizon sextant, rather than
> a bubble sextant, in mind, so they include a semidiameter correction.
> Special
> rules are given for applying them to a bubble sextant, and presumably
> these have been rigorously followed. I hope so.
Robert responds:
Indeed, I did follow the special instructions in the NA pertaining to bubble
sextant observations.
Data to follow offlist.
Robert
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
From: Robert Eno
Date: 2006 Aug 20, 23:42 -0500
George Huxtable wrote:
> Robert Eno questioned Frank Reed's statement-
>
> | If the line is tilted so that the
> | > upper cusp is to the right of the lower cusp (and the Sun is to
> the
> | > right), then the Lower Limb should be used.
> |
> | Robert asks:
> |
> | Frank, perhaps I am misinterpreting your description and/or
> visualizing this
> | incorrectly, but it seems to me that under such conditions, would it
> not be
> | the upper limb that should be used?
>
> Frank has it right.
Robert responds: I "got it" in the interim. A question of misinterpreted
terminology on my part.
George wrote:
> Maybe he had inherited similar prejudices, from another old salt, in
> his own early days. And now, Robert is doing his best to pass those
> prejudices on to another group of navigators, us. All very well, if
> there is reasoning and hard evidence to back it up, but those so far
> have been conspicuously lacking.
Robert responds:
And here George completely misunderstands me. I would never presume to pass
on any of my own views to anyone on this list. My contention is that while
the textbooks and most navigators it seems, assure us that moon observations
are perfectly accurate, there are a number of seasoned navigators who
believe, based on their own observations, that moon shots are unreliable. I
cannot speak for my friend except to say that he has decades of actual
(pre-GPS) sea-going experience as a master mariner and that he later taught
navigation and mathematics to budding seafarers. This indicates to me that
his views on the accuacy of moon observations cannot be simplistically
attributed to mere prejudices inhereted from another old salt.
Your comment about "hard evidence" being "conspicously absent" is
misleading. At no time, did I suggest that I had hard evidence to support a
case for moon observations being unreliable. Throughout this discussion I
have repeatedly stated that my comments are based on my own personal
experience; that there are other mariners that are of the same opinion; and
that there may be some factual basis to this contention. You will also
remember that, in not so many words, I asked for opinions from other list
members. If I had "hard evidence" to support the idea that moon shots are
unreliable, I would not have posted the message in the first place because
it would have been a non-issue for me.
George wrote:
>In Robert Eno's case, when using a bubble sextant
> attachment, errors
> are far more likely to stem from its own inherent inaccuracies, rather
> than any problems with the Moon itself. I have no experience at all
> with bubble devices, but one possible difficulty comes to mind, with
> the Moon and a bubble sextant.
Robert responds:
And I did clearly acknowledge the inherent inaccuracies of bubble
sextants/attachments in my original post. Nevertheless, I countered that my
observations of Vega, which were far more difficult to take than the moon's
and therefore should have been less accurate, turned out to be better than
the moon observations anyway.
George wrote:
When observing the Sun, as I understand
> it, the aim is to match the disc of the Sun with the disc image of the
> bubble, and try to keep it there.
Robert responds:
That is pretty much the case, although it is better to have a bubble which
is slightly larger than the disc of the sun. Not too much larger but just a
couple of "hairs".
George wrote:
How does that work out with the
> non-disc of the Moon, when only a part of that non-disc is the crisp
> edge of the appropriate limb? What do you centre, against what?
Robert responds:
This is where it gets tricky. Sometimes, you can see the faint outline of
the rest of the moon, while other times you have to almost "imagine" the
shape of the rest of the moon and try to align this, and the portion which
you can see, in the centre of the bubble. My observations of a few nights
ago, were of a gibbous moon, so there was plenty to work with.
George wrote:
> the problems that Robert finds with a bubble sextant relevant to an
> observer who can see a horizon? Have a significant fraction of the
> many Moon observations he relates been measured against a real
> horizon, and do the same discrepancies occur then?
Robert responds:
Both. I would actually have to dig out my old logbooks to see what my
natural horizon observations were like but I remember them being none too
satisfactory. Other mariners (not all) I have spoken to about this have
related similar "bad luck" experiences with the moon.
George wrote:
> If Robert is prepared to show us his own observation log, so that we
> can check over the problems he tells us about, we may be able to
> detect whether he sees observational scatter or some systematic error.
> And perhaps, some possible defect in the way that those all-important
> corrections have been made.
Robert responds:
I will send these to you off list as not everyone may interested in looking
at them. In any case, I hasten to reiterate what I already stated in my
first posting: 8 observations (4 moon & 4 Vega) do not constitute a
statistically defensible position; especially with a bubble horizon. I
mentioned these recent data to illustrate a pattern that I have experienced
for years: moon observations, for me anyway, have been more often than not,
unreliable.
George wrote:
> Until now, all we have seen is hearsay evidence. And prejudice.
Robert responds:
Again, I believe you misrepresent me. I never indicated that my contention
was correct and in fact questioned my own personal observations as possibly
being the result of continuous re-inforcement of a bad habit (see my first
posting). Prejudice? Harsh words George. I would more describe my
sentiments as "gun shy" (where moon observations are concerned). This is
predicated on experience. I simply do not get good results with the moon.
George wrote:
>
> By the way, the Moon altitude correction tables, in the back pages of
> the nautical almanac, were devised with a horizon sextant, rather than
> a bubble sextant, in mind, so they include a semidiameter correction.
> Special
> rules are given for applying them to a bubble sextant, and presumably
> these have been rigorously followed. I hope so.
Robert responds:
Indeed, I did follow the special instructions in the NA pertaining to bubble
sextant observations.
Data to follow offlist.
Robert
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
