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 21, 08:56 -0500
George wrote:
> =================
>
> And there, I suspect, lies the problem, that doesn't occur when a
> real
> horizon is used, offset by a precise semidiameter. Nor does that
> problem occur with a bubble horizon, with any other body than the
> Moon, because everything except the Moon is symmetrical about its
> centre, and symmetry is easy to estimate precisely.
You may be on to something George. It makes perfect sense, but that only accounts for observations with a bubble horizon. I will have to dig out my logs, but I have had similar bad luck with sea horizon observations. I admit to not having taken a sea horizon observation of the moon for quite a few years though.
George wrote:
> Robert's phrase, about a gibbous Moon, "there was plenty to work
> with", is particularly revealing. No, there wasn't plenty to work
> with, there was no more to work with than for the flimsiest
> crescent
> Moon.
Robert responds:
Actually George, there was. But I am trying find an adequate description.....there was enough of the moon's visible outline that I could line this up with the inner circumference of the bubble and enough of the top and bottom of the moon that I could determine when it was approximately in the centre of the bubble.
I agree that with a crescent moon, there is a lot of guesstimation.
Tell you what: next full moon, I will take a round of observations and report my findings.
George wrote:
> Then, Robert's technique implies that he must somehow estimate the
> centre point of that half-circle, and put it in the centre of the
> bubble. Although the cusps show up precisely when the Moon's disc
> is
> about half-lit (at first and last quarter) their position is much
> harder to estimate when the Moon is nearly gibbous, or when it's a
> fine crescent. So taking the mid-point between the cusps would be
> an
> unreliable method, when those cusps are very blunt or very fine..
Robert responds:
A valid argument, however, when observing a gibbous moon, my technique is a combination of judging the centre of the moon and alignging its visible outline with the outline of the bubble.
George wrote:
> I suspect that Robert would probably admit that he assesses the
> Moon's
> centre in a semi-intuitive manner, saying to himself "that looks
> about
> right" when he thinks it is so, without applying any particular
> rule
> to the job. And in that case, it seems plausible that there lies
> his
> difficulty, that such a procedure may turn out to be roughly
> correct,
> but may well be liable to errors of a few arc-minutes, in a Moon
> with
> a diameter of about 30'.
Robert responds:
Absolutely! Moon shots for anything less than a full moon are a combination of any and all of the above. But refer to my earlier comments.
George, you ought to get yourself a bubble sextant and try some observations with it. There are still a lot of RAE MK IX-A's kicking around your side of the pond.
George wrote:
> Paul Hirose, always dependable on such matters, with his quote
> from a
> bubble-sextant manual, hits the nail on the head. It suggests, for
> Moon observations, that the appropriate limb, upper or lower,
> rather
> than the Moon's centre, is centred in the bubble, and then
> corrected
> accordingly.
Robert responds:
Without having tried it, the method strikes me as somewhat unreliable. I won't discount it just yet, without trying the technique. Something to look forward to. I will try this out and report back to the list.
But we are getting somewhat off track. That bubble sextant/horizon observations a fickle and fraught with errors is a given. I pointed this out right from the start. The thrust of my question had to do with the once popular notion that moon observations are unreliable. This has been the case for me, both with a sea horizon and a bubble horizon (which in itself is unreliable) and has obviously been the case for other mariners, both past and present, otherwise the notion would not have gained a foot hold in the first place.
Therefore I get back to my original question about whether or not there is some factual basis to idea that moon shots are unreliable.
Thus far, most of the list members do not subscribe to this notion. A few others have literally "peeped" in with the laconic sentiment that "I haven't had much luck with moonshots either".
But George may be on to something when he discusses the vagaries of the moon's symatry. I should do some digging on this matter.
Robert
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From: Robert Eno
Date: 2006 Aug 21, 08:56 -0500
George wrote:
> =================
>
> And there, I suspect, lies the problem, that doesn't occur when a
> real
> horizon is used, offset by a precise semidiameter. Nor does that
> problem occur with a bubble horizon, with any other body than the
> Moon, because everything except the Moon is symmetrical about its
> centre, and symmetry is easy to estimate precisely.
You may be on to something George. It makes perfect sense, but that only accounts for observations with a bubble horizon. I will have to dig out my logs, but I have had similar bad luck with sea horizon observations. I admit to not having taken a sea horizon observation of the moon for quite a few years though.
George wrote:
> Robert's phrase, about a gibbous Moon, "there was plenty to work
> with", is particularly revealing. No, there wasn't plenty to work
> with, there was no more to work with than for the flimsiest
> crescent
> Moon.
Robert responds:
Actually George, there was. But I am trying find an adequate description.....there was enough of the moon's visible outline that I could line this up with the inner circumference of the bubble and enough of the top and bottom of the moon that I could determine when it was approximately in the centre of the bubble.
I agree that with a crescent moon, there is a lot of guesstimation.
Tell you what: next full moon, I will take a round of observations and report my findings.
George wrote:
> Then, Robert's technique implies that he must somehow estimate the
> centre point of that half-circle, and put it in the centre of the
> bubble. Although the cusps show up precisely when the Moon's disc
> is
> about half-lit (at first and last quarter) their position is much
> harder to estimate when the Moon is nearly gibbous, or when it's a
> fine crescent. So taking the mid-point between the cusps would be
> an
> unreliable method, when those cusps are very blunt or very fine..
Robert responds:
A valid argument, however, when observing a gibbous moon, my technique is a combination of judging the centre of the moon and alignging its visible outline with the outline of the bubble.
George wrote:
> I suspect that Robert would probably admit that he assesses the
> Moon's
> centre in a semi-intuitive manner, saying to himself "that looks
> about
> right" when he thinks it is so, without applying any particular
> rule
> to the job. And in that case, it seems plausible that there lies
> his
> difficulty, that such a procedure may turn out to be roughly
> correct,
> but may well be liable to errors of a few arc-minutes, in a Moon
> with
> a diameter of about 30'.
Robert responds:
Absolutely! Moon shots for anything less than a full moon are a combination of any and all of the above. But refer to my earlier comments.
George, you ought to get yourself a bubble sextant and try some observations with it. There are still a lot of RAE MK IX-A's kicking around your side of the pond.
George wrote:
> Paul Hirose, always dependable on such matters, with his quote
> from a
> bubble-sextant manual, hits the nail on the head. It suggests, for
> Moon observations, that the appropriate limb, upper or lower,
> rather
> than the Moon's centre, is centred in the bubble, and then
> corrected
> accordingly.
Robert responds:
Without having tried it, the method strikes me as somewhat unreliable. I won't discount it just yet, without trying the technique. Something to look forward to. I will try this out and report back to the list.
But we are getting somewhat off track. That bubble sextant/horizon observations a fickle and fraught with errors is a given. I pointed this out right from the start. The thrust of my question had to do with the once popular notion that moon observations are unreliable. This has been the case for me, both with a sea horizon and a bubble horizon (which in itself is unreliable) and has obviously been the case for other mariners, both past and present, otherwise the notion would not have gained a foot hold in the first place.
Therefore I get back to my original question about whether or not there is some factual basis to idea that moon shots are unreliable.
Thus far, most of the list members do not subscribe to this notion. A few others have literally "peeped" in with the laconic sentiment that "I haven't had much luck with moonshots either".
But George may be on to something when he discusses the vagaries of the moon's symatry. I should do some digging on this matter.
Robert
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
