NavList:

He wrote-
"I shot an altitude of the sun, then the moon, then 5 lunar distances,
followed by another sun and finally another moon altitude.  (in retrospect,
I should have shot the 2nd moon first then finally the second sun)."

George replies: Yes, his "retrospect" is correct; that would have put the mean timing of
Moon altitude and the mean timing of  Sun altitude both very close to the
same moment as the mean time of the lunar distances.

 

Jeremy adds:

 

I thought of this well after the sight, and made a mental note to do it that way in the future.

and added-
"My first trouble was with the moon altitudes.  The Hs of the sun was nearly
2x and on opposite bearings as the moon so I was getting massive flashes of
the sun where it hit my horizon mirror and bounced back through the scope.
I have a feeling that my altitudes of the moon are none too accurate."

Well, they don't need to be VERY accurate, because they are needed only to
calculate a correction. But I am interested in his problem of sunlight
getting into the scope, and the way he has explained it.

Yes, sunlight can be a real problem when observing Moon altitudes,
especially when the two bodies are nearly opposite in azimuth. But Jeremy
also refers to "The Hs of the sun was nearly 2x and on opposite bearings as
the moon". But it wasn't; the Moon was roughly twice the altitude of the
Sun! And is the height of the body being observed relevant anyway? I think
not. The problem is at its worst when the Sun can just peep around the edges
of the index mirror, and illuminate the horizon mirror with scattered light
(but direct light is usually masked out by the frames), and that always
occurs at a Sun altitude of around 60º, depending a bit on the details of
the sextant design. In this case the Sun was only at 33 deg altitude, behind
the observer, so the light was coming in around the side of his head, or
over it, but just how was it getting into the scope so badly? Just what was
the sunlight bouncing off? Is Jeremy clear about that?

 

Jeremy Replies:

 

George is right here.  The sun was 1/2x the height of the moon which is what I meant.  The sun was behind me and was reflecting off of the horizon mirror into the scope.  Shooting with the Tamaya, with 4x40 scope, I had to put my finger over the lower part of the index mirror to block the sun from reflecting into the scope, this made adjustment of the micrometer drum to get a more exact moon altitude difficult at best.  I will attach a photo of the sextant.  You can see that the horizon glass is angled so that the sun, at about 30 degrees as I discovered, can be reflected into the scope if directly behind you.  The reflections are not unusual, I have seen my mast head light (which was behind me) during the twilight shots as well. 
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George wrote:

By stating chronometer times of GMT, Jeremy has pre-empted the answer, of
course. If you know your GMT, there's no need for a lunar;  you can simply
cross position lines from the measured altitudes of Sun and Moon, and get a
much more precise answer, in both lat and long, that a lunar could ever
provide. The only point of taking a lunar is to determine GMT (and rather
imprecisely, at that) and correct your on-board clock from it. Then, armed
with that time, you can work out the true position of one or more bodies,
measure altitudes, and cross the resulting position lines. Of course, in
this case, he wasn't setting us an exercise, he was asking for our help (
and got it, I'm pleased to see).

 

Jeremy replies:

 

In this case, shooting sun and moon lines are not very helpful for getting a position.  Since the azimuths are nearly exactly opposite, and small error in the sight or plotting will lead to a large error in position as the LOP's will be nearly parallel.  This is the main reason I did not attempt to use these lines in my "day's run" exercise.

 

My purpose at this point in shooting lunars is to see how accurate I can get.  I am trying to refine my shooting techniques more then anything (still needs work obviously).  I want to see what my error is.  Eventually I will work these down on times not so precise to get the historical flavor.

 

It is a similar vent of why I shoot sights here at sea (besides giving the list exercises).  I want to make sure that I am figuring a position close to GPS fixes so that I can be fairly confident in my work if and when I lose the ability to determine electronic fixes.

 

Jeremy