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Jeremy, you wrote:
"I am wondering what people are typically getting for errors?  I am
thinking that this is a bit much and I think with a bit more practice
I can do better."

Yes. But in may not be so much about practice...

These are the critical factors:
1) Sun not stars. Most people seem to get much better results with the Sun
as the other body. I also find that I get great results with Jupiter and
Mars. When the other body is a star my results are typically about 50%
worse. In addition, when you look through historical logbooks, it's clear
that the Sun was generally the object used in the majority of lunars (on the
order of 80% or so).
2) telescope magnification. The unaided human eye (corrected with eyeglasses
or contacts) has a resolution of about 1 minute of arc. If you want to see
differences as small as, say, one-tenth of a minute of arc, then you need a
telescope with a magnification of 10x.
3) index error. Obvious, yes, but it's much more important with lunars. Us
the Sun limb method (or the same with a gibbous Moon in twilight, which is
my favorite) and do a bunch of trials and average. You need the best
possible index correction you can get.
4) general sextant adjustment. Go through and double-check all the little
alignment errors. The mirrors need to be perpendicular to the frame. The
telescope axis needs to be parallel to the frame. Etc. One nice thing about
doing lunars is that it forces you to test all these things carefully.
5) overall instrument quality. If the instrument has any "slip" or looseness
in the components, that will show up here. Not much you can do if this is
the problem.
6) arc error. After you take care of everything else, if you still get
errors larger than a fraction of a minute of arc, then you probably have arc
error. This is supposed to be listed on the table in the case, but
frequently those tables are old or filled with zeros that may indicate
cheating on the testing. The nice thing about arc error is that it is
correctable. This is yet another good reason to shoot lunars. If you get
reliable errors for certain angles, you are essentially compiling a fresh
error table. When you have accumulated enough data, your sextant is good as
new --when the new errors are applied-- and every sight you take (lunar or
otherwise) will benefit.
7) sight geometry and astronomical conditions. It helps when both objects
are higher than five or ten degrees. The sky should be clear (one can do
normal altitude sights through high cirrus but not lunars). Some
uncomfortable orientations can make the process much harder.


Myself, on individual lunars, I get errors with a standard deviation of
about 0.25 minutes of arc when all the above are working in my favor. When I
take four lunars in a row and average, which was a common technique
historically, I get a mean error about half as large: around 0.1 minutes of
arc standard deviation.

Please note: your mileage may vary! Also note that a standard deviation is
not a maximum error. When we meet in Mystic in July, let's plan a time when
the Moon is "in distance" (as they used to say).

 -FER



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