NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Lunar trouble, need help
From: Jeremy C
Date: 2008 Jun 15, 15:52 EDT
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From: Jeremy C
Date: 2008 Jun 15, 15:52 EDT
Jim wrote:
I took the time to interpolate between your first and second Sun and
Moon altitudes. I then used the online calculator using interpolated
values for the altitudes and the time of each LD observation to
determine the error in the observed lunar distances which I found to
be:
3) 1.4
4) 1.0
5) 1.0
6) 1.2
7) 0.9
There has been a great deal written on this forum regarding issues in
observing lunars. I think it is fair to say that getting initial
results similar to what you have here is not entirely uncommon.
Some have found that they have had to "train their eye" so to speak by
predetermining an angle and seeing what the perfect alingment should
look like.
Since the results suggest about a 1' systematic error, IE is first and
foremost in question.
Did you have hazy conditions?
I think the list would agree that to get really accurate results,
conditions have to be perfect.
Otherwise, difficult to add much more. You may want to have a look at
some recent discussion on lunar distance accuracy as there is an
excellent posting from Frank summarizing sources of error.
Jim
I took the time to interpolate between your first and second Sun and
Moon altitudes. I then used the online calculator using interpolated
values for the altitudes and the time of each LD observation to
determine the error in the observed lunar distances which I found to
be:
3) 1.4
4) 1.0
5) 1.0
6) 1.2
7) 0.9
There has been a great deal written on this forum regarding issues in
observing lunars. I think it is fair to say that getting initial
results similar to what you have here is not entirely uncommon.
Some have found that they have had to "train their eye" so to speak by
predetermining an angle and seeing what the perfect alingment should
look like.
Since the results suggest about a 1' systematic error, IE is first and
foremost in question.
Did you have hazy conditions?
I think the list would agree that to get really accurate results,
conditions have to be perfect.
Otherwise, difficult to add much more. You may want to have a look at
some recent discussion on lunar distance accuracy as there is an
excellent posting from Frank summarizing sources of error.
Jim
--------------------------
Jim,
The 1' of arc error is more what I expected, the trouble is that I had is
the 35' arc error when I punched in the near limb observations into Frank's
program, when I switched to "far" limb, I got about 1' of LD error which is
more of what I expected. I am wondering where I went wrong.
I perfectly understand the issues to get those last few arc minutes
down to zero, but 35' is way outside my standard shooting errors so I was
wondering if anyone could spot where I went a bit askew.
I ran into a new LD difficulty that could explain some of
the error: stack gas. The relative wind over the ship combined
with the azimuth of the moon so that the moon lay in the middle of the
engine exhaust cloud and its visible heat waves which I am sure skewed the
LD observation a bit. Still it shouldn't have been 35'.
I intend to take this summer to make some LD observations from land (I
have a plan on how to mount a sextant to a sturdy photographic tripod to assist
in these), and learn a bit more about the nuances of lunars as most of it is
outside of my ken.
I will also say that getting lunars at sea will be inherently more
inaccurate then land. Roll alone can change my height of eye several feet
and ship's vibration also make it more difficult to get a steady platform (it
pretty much forces you to shoot while standing.) I am actually quite happy
with about 1' of error for sea lunars. Sitting on a beach, I'd expect a
bit better however. I don't have logs, but I was wondering how accurate
old time sailors got with their lunars at sea?
Thanks to everyone on their input.
Jeremy
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--~--~---------~--~----~------------~-------~--~----~
Navigation List archive: www.fer3.com/arc
To post, email NavList@fer3.com
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