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From: Bill (no email)
Date: Sun Apr 03 2005 - 16:01:53 EDT
> That's good! By the way, did you preset the sextant by checking the predicted
> distance first? This is 100% acceptable --it was the recommended procedure
> historically right from the beginning.
Did not preset the first. Had been playing with an artificial horizon and
Celestaire/Johnson bubble scope when I noticed the Sun and Moon, so just had
at it with the scope off to get preliminary alignment. Did roughly
precalculate the second, and did use the scope on it too.
Have since learned how to precalculate star distances, and have been using
that for stars. Helps when I'm going for Procyon/Aldebaran and wind up with
Procyon/Cappella instead.
> With the angle preset, you aim at the
> Moon and then rotate the instrument around the line of sight until the other
> object pops into view. This should happen when the "horns" of the Moon are
> nearly pependicular to the frame of the instrument. See why??
A line perpendicular to a line through the horns points at the Sun? Had not
thought about using that to align the sextant. Thanks.
> And you wrote:
> "Encouraged, I went back out and tried to obtain an Hs using the tips on your
> lunars page. Hold sextant in both hands, tweak, rest, tweak, rest,
> tweak.... By now (20:14:06 UT) the moon was higher and did not have much
> contrast against the sky and wispy clouds, but managed to get an Hs I had a
> high level of confidence in. Plugged the Hs in and results were 2.6' error,
> 1 d 19.2' error longitude."
>
> Seems unlikely, doesn't it? An error that big is much too big to believe,
> assuming you had a scope attached to your sextant. In my early experience, I
> found that these large errors usually resulted from the sort of silly mistakes
> that people only make when they're trying something new. For example, you
> might right down the time incorrectly by five minutes. Focusing so intensely
> on getting the angle right, it's much more likely that we'll screw up
> something else that should be really simple.
Looked for the classic ID-10T errors. Sextant came in with the Hs still on
it, so double checked that. Pretty confident the time was correct--no signs
of transposed minute (14) or second digits (03), and doubt transposed
seconds would matter that much. So pretty much a DSO error as close as I
can figure it. For some reason I manage to be out in left field shagging
pop flies about every 10th to 20th observation. Generally attribute that to
doing too many observations in a row.
Moon was approx. 53d, and dim against the sky. Given the Sun was lower, it
might have made sense to us it in the horizon glass instead of the Moon, but
it seemed easier to have the Moon in the glass as it was difficult to see.
> Today should be nice for Sun-Moon lunars if you're weather is decent.
Too early for this aging party-hearty chinchilla. Hoping for a lunch-hour
shot Monday or Tuesday.
> And:
> "This is going to be like a termite doing calculations for landing the rover
> on Mars, but I think on it. ;-)"
>
> Cool. Just remember, this ain't rocket science. Lunars aren't anywhere near as
> difficult as they're often made out to be. By the way, have you read Arthur
> Pearson's 'Backyard Lunars' article on his web site at www.lunardistance.com
> <http://www.lunardistance.com> ?
I have not, but will give it a look. Thanks. At this point studying and
distilling your "Easy Lunars' page. Looking for clues for the strange
number in the calculations readout after running through the usual suspects
and coming up empty. I still maintain it is not a use of the word "easy" I
am familiar with ;-)
Bill
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