NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Moon altitude problems.
From: George Huxtable
Date: 2006 Aug 20, 19:04 -0500
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.
| Keeping along the same lines as my original posting, minus the coded
| explicatives in deference to George Huxtable's sensibilities...
Robert has completely misunderstood me. I am not at all bothered
whether he uses "coded explicatives" , or even real explicatives, in
his postings. It's nothing to do with any such prudishness. It's just
a plea that posters choose an appropriate threadname that is relevant
and searchable, for the benefit of those that will wish to unearth a
topic in the future. Indeed, it seems likely that the curious
characters in that threadname have also caused more than one such
message to be rejected in transit as spam; that was an additional
defect I had not foreseen.
| ... I posed this
| question to a good friend of mine, a Master Mariner with three
decades of
| seagoing experience (prior to GPS) and who taught celestial
navigation at
| the Nautical Institute in Nova Scotia for many years after he left
his
| seagoing service.
|
| My friend's opinion of moon shots contains even more colourful and
| descriptive explicatives than those which I offered in my original
post; as
| can be expected from a hard-boiled merchant mariner. Essentially
though, he
| told me that moon shots tend to be inaccurate and unreliable and
that in his
| career, he avoided them if he could. The moon, my friend says, is
too fast
| and too close. There are too many variables with the moon.
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.
Yes, some extra care has to be taken with corrections for the Moon,
but these are well known and well tabulated in the almanac, and
provided that has been done correctly, there's absolutely no reason
why a Moon observation should be less accurate than any other (in my
own view). 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. 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. 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? Are
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?
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.
Until now, all we have seen is hearsay evidence. And prejudice.
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.
George.
contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
From: George Huxtable
Date: 2006 Aug 20, 19:04 -0500
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.
| Keeping along the same lines as my original posting, minus the coded
| explicatives in deference to George Huxtable's sensibilities...
Robert has completely misunderstood me. I am not at all bothered
whether he uses "coded explicatives" , or even real explicatives, in
his postings. It's nothing to do with any such prudishness. It's just
a plea that posters choose an appropriate threadname that is relevant
and searchable, for the benefit of those that will wish to unearth a
topic in the future. Indeed, it seems likely that the curious
characters in that threadname have also caused more than one such
message to be rejected in transit as spam; that was an additional
defect I had not foreseen.
| ... I posed this
| question to a good friend of mine, a Master Mariner with three
decades of
| seagoing experience (prior to GPS) and who taught celestial
navigation at
| the Nautical Institute in Nova Scotia for many years after he left
his
| seagoing service.
|
| My friend's opinion of moon shots contains even more colourful and
| descriptive explicatives than those which I offered in my original
post; as
| can be expected from a hard-boiled merchant mariner. Essentially
though, he
| told me that moon shots tend to be inaccurate and unreliable and
that in his
| career, he avoided them if he could. The moon, my friend says, is
too fast
| and too close. There are too many variables with the moon.
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.
Yes, some extra care has to be taken with corrections for the Moon,
but these are well known and well tabulated in the almanac, and
provided that has been done correctly, there's absolutely no reason
why a Moon observation should be less accurate than any other (in my
own view). 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. 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. 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? Are
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?
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.
Until now, all we have seen is hearsay evidence. And prejudice.
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.
George.
contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
