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Subject: Re: Sextant Errors
From: Bruce Stark (Stark4677@XXX.XXX)
Date: Sun Jul 07 2002 - 16:13:49 EDT
Robert! Glad to see you're on the LIst!
Janice and I have been gone for a several weeks, and I have a lot of catching
up to do.
Regarding side error, I'd say it's important to have the index mirror as near
perpendicular as you can get it, using the usual procedure given in all the
manuals. Then, if you want to see a star and its reflection side-by-side you
can get that when you adjust the horizon mirror. Good idea if you want to
check index error before or after a round of star altitudes—and have perfect
eyesight. I'm no optician, but don't believe such a small side-error could
have any practical effect.
Those of us who see stars as squirming blobs should stick to the method
Maskelyne, Moore, Bowditch, etc. preferred for finding index error. That is,
taking a series of measurement of the sun's diameter on and off the arc.
Those who have good eyesight should be getting better results from their
lunars than I am, and if I miss GMT by more than a minute of time with a
sun-lunar it's reason to look for a blunder. A while back I worked the six
sun-lunars Lewis took at the Three Forks of the Missouri and at Camp
Fortunate in 1804. Lewis did better than I generally do, and he was sick and
had a lot on his mind.
I'm thinking maybe some of the lunarians on the list haven't given enough
thought to getting both mirrors perpendicular to the frame of the sextant.
You hardly ever have to make this adjustment, but should check it now and
then to make sure it's perfect. Index error, on the other hand, is best left
as it is. But measure and record it just about every time you take a lunar.
Another possibility is confusion about how the instrument error shown on the
maker's certificate inside the sextant box should be applied. Some believe
this error should be corrected for by applying it with the OPPOSITE sign to
that shown on the certificate. But, with sextants I've seen, the certificate
shows CORRECTIONS, to be added or subtracted according to the + or - sign
given.
Finally, I seem to get much better results, especially with the longer
distances, if the telescope is centered on the part of the horizon mirror
where the clear and silvered parts meet, and contact is made there, next to
the silvered part. I THINK this helps keep the observation parallel to the
frame of the instrument, but perhaps am kidding myself. Feedback would be
appreciated.
Bruce
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