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Subject: A-12 bubble sextant results
From: Paul Hirose (paulhirose@XXX.XXX)
Date: Fri Dec 20 2002 - 23:11:28 EST
Near dusk today I pulled out my 1942 Link A-12 bubble sextant and took
some shots through a window. I made the sights from a standing
position and tried to make the coincidences rapidly but not sloppily.
However I did use a magnifying glass to read the vernier carefully to
the nearest minute.
Then I used the USNO's online calculator and a GPS-measured position
to obtain Hc. Right away it was evident the instrument had a +10
minute index error. Allowing for that, as well as refraction, I got:
UTC Ho Hc diff.
00:12:13 4 31.6 4 30.6 +1.0'
13:42 4 16.1 4 15.3 +0.8'
15:06 4 01.7 4 00.8 +0.9'
16:10 3 50.3 3 49.7 +0.6'
The results are amazingly good. In fact, I don't believe they are
typical. It doesn't seem likely someone badly out of practice as I am
(not that I ever was well practiced) should do so well with a zero
magnification sextant. This was probably just a lucky streak.
My Ho is still consistently high; a -11 minute index correction would
have made the numbers look even better. With the large index error and
rapid change of refraction at these low altitudes, I made the index
and refraction corrections separately rather than lumping them
together in the normal manner. There may be nonstandard refraction at
work too; we just had a storm pass through. Then there's the matter of
shooting through the window because I was too lazy to go out!
I plan to repeat the experiment to get a better idea of my accuracy.
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