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## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Checking a sextant calibration.
From: Jared Sherman
Date: 2003 Oct 6, 23:26 -0400

```Paul,it has been pinted out to me that when I said "position" I confused all
the navigators on this list, who use that word in the jargon meaning of
"exactly where am I".

As I am not a navigator I meant it only in the common sense and I should have
said "line of position" since one reading and reduction will only place you
along one line of position. (Which to me, is still "a position" in the larger
sense. If nothing else, I know I am on the rim a a circle drawn on Planet
Earth, and sometimes that is all the precision you will get.)

By taking one sight and reducing it, you will get one "position" in that
general sense. By all means, complicate the exercise by taking multiple
sights or otherwise getting a real position. I did not refer to this and
should have made clear I was dismissing it, because it is unnecessary to the
exercise.

Get one line of position, or position, as you prefer. Now recompute that
position by any means you please, using a bogus observed height (i.e. a
different sextant scale reading) that is off by 20".

If you have simply reduced one sight, your new LOP will be off by small
distance. If you have used you sights or other means to fully calculate a
real position, you will have done a lot more work--but you will still find
the final result is going to be off by a small amount. In any case, that
amount will demonstrate the effect of a sextant which is in error by 20". In
the case of a single reduction, the line of position will move as the
distance along the intercept moves. How far does that distance along the
intercept move? About 1/10th of a mile per tenth of a minute (6 seconds). My
reduction program said 0.1 mile per 0.1 minute until I changed by .3 minute
and it changed by .4 miles, someone else can tell me why that is right or
wrong, probably program rounding.

The point was just to say that one can take one reading--without even doing a
series for a proper position--and then diddle the numbers for that one
reading, to imitate a badly calibrated/manufactured sextant. If the
calibration card is missing and the sextant is "guessed" to be +-20" from the
manufacturing of the arm and scale, the fast single reduction then repeated
with a bogus number, will show that even if there is a +-20" error on the
scale, the final position (or position line) will be off by only a small
amount.

Which comes back to the simple question that is all I was addressing. If one
does not know the calibration error for the arm, and one does not know if the
sextant is built +-10" or 9" or even 30", one can still be confident that
errors of that magnitude are insignificant and readily ignored. Or, one can
consider them extremely significant, depending on one's own opinion.

But with two simple calculations, one can see with one's own eyes exactly how
large/small a difference those seconds make, and draw one's own conclusions.

That's all I was attempting to demonstrate, I apologize if I did not make that
clear enough and my illustrations were misleading.

```
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