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From: Alexandre E Eremenko (no email)
Date: Sat Jun 24 2006 - 09:34:30 EDT
I just returned from my short sailing trip,
from Kiel to Danish islands,
and report my observations with the new
pocket sextant (XIX century Troughton and Simms).
June 18, Sun, boat under motor, good conditions.
Local time 10 Average of 5 obs. error +0.6' (sigma=0.6')
Local time 12 Average of 5 obs. error 0.0' (sigma=1.5')
Local time 12:40 Average of 6, error 0.0' (sigma=1.0')
Local time 14:30 Average of 3, error -0.5'
June 19, hazy horizon, boat under motor, Sun.
12:30 Average of 6, error -1.8'
14:20, average of 3, error -1.1' (very hazy).
June 20 Sun at 11:50, under sails, waves 2 ft.,
average of 5 error -0.6'
June 20 Lunar at 10:00, boat under motor, conditions
very good. Average of 6, error
0.1' (sigma=1.4')
(Here is the whole sequence of errors of individual
observations: 1.4, -0.9, 0.8, -1.9, 1.6, -0.3
0.5, 1.2, -0.7, -0.3, -0.7, 0.1,
computed with Frank's calculator after coming home).
June 21, Sun was not visible (but our 31 feet boat was making
6 knots under sail all the day:-)
June 22, at 7 a.m. Sun from a beach,
Average of 6, no telescope, error 0.5',
Average of 6, with the telescope, error 0.0' (sigma=0.7).
Here is the whole sequence of errors:
0.5, 1.2, -0.7, -0.3, -0.7, 0.1).
June 23, sailed N of a shore, no horizon visible.
I also made some lighthouse observations,
taking their altitude to compute the distance.
The distances I measured were about 2-3 miles,
and the error was usually 0.1 to 0.2 of a mile.
Conclusions.
1. My pocket sextant is VERY good.
Much better than I expected.
(I never expected a good lunar with a pocket sextant:-)
Indeed, the scale can be read only to the whole minutes,
and I almost never had an error more than 2'
in an individual observation.
With good horizon it rarely exceeded 1',
even without a scope. The sextant has a pinhole
that serves instead of the zero-magnification tube.
Taking into account that the whole sextant fits
in a T-shirt pocket, I conclude that this was a really
outstanding device:-)
2. The vernier scale is not so hard to read, though it takes
somewhat more time than reading from a drum.
Index error was zero during the whole trip.
Small side error (probably 1-5') was always present. Adjustment had no
effect, and the sextant does not hold this adjustment well,
and I decided not to adjust it at all.
How to adjust the index mirror perpendiculatiry on
a posket sextant I still don't know.
2. Averaging
substantially improves
precision, as we see
from the two sample series above.
3. The only disadvantages of this sextant are
a) That the eyepiece is missing, as I said before.
This means that I cannot do a Sun observation on
altitudes less than 20 degrees, because of the strong glare
of the horizon.
b) there is absolutely no place to attach a strap.
So it is quite hard to handle in a small boat:
you have to hold it all the time in your hand, and only
one hand is free for recording the observations.
Alex
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