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Dear Fred,
Here are some of my recent observations.
The purpose was to estimate my sextant accuracy,
to determine its index correction,
and simultaneously to learn how to make observations
of maximal precision under the ideal conditions.
All observations were made from my second floor balcony,
under the perfect weather conditions.
The sextant was SNO-T, made in 1990,
with two scopes.
I used the standard refraction table (did not measure
air temperature and pressure).

1. Index error tests by the Sun
Oct. 3, LT 13:20
           off'        on'
           32.4        32.6
           31.6        33.2
           31.7        32.6
           32.4        33.0
           32.3        32.9
           32.2        32.4

Aver:      32.1        32.78
SD          0.3'        0.1'
Average Observed sun radius: 16.22. Value from Almanac: 16.0
Correction: -0.2'.

2. Index error tests by stars: 0.0'
I mean that with stars of 2-nd magnitude I did repeated tests
of the star-to-itself distance and the stable result is always
less than 0.1'. So I read it from the drum as 0.
I made 20-30 such measurements.

3. Star-to star distances.

Oct. 2, LT 0:30
Altair-Deneb (Galileo scope)
Sextant distance: 38d00.0', 38d00.4', 37d59.9'.
Average: 38d00.15', SD 0.3'
Calculated distance: 38d00.1'
Correction: 0.0'

Oct. 2 LT 0:45
Vega-Deneb (Galileo scope)
Sextant distance: 23d50.3', 23d50.8', 23d50.5'.
Average: 23d50.5' SD 0.2'
Calculated distance: 38d00.1'
Correction: -0.4

Sept. 29 LT
Arctur-Vega (Inverting scope)
Sextant distance: 59d05.8', 59d06', 59d06', 59d05.8', 59d05.8
Average: 59d05.9' SD 0.1'
Calculated distance 59d05.9
Correction: 0.

4. Sun altitudes with artificial horizon (Davis, vegetable oil).
Assumed position: N 40d27.2', W 86d55.8'

Oct 3. LT 15:10
GMT            SEXTANT            DEVIATION          SCOPE
20:28:27       60d19.1'           +0.0               Galileo
20:30:34       59d42.1'           -0.6               Galileo
20:32:36       59d04.5'           -0.2               Galileo
20:34:07       58d35.8'           +0.2               Galileo
20:35:35       58d09.8'           -0.2               Galileo
20:38:51       57d09.2'           -0.1               Galileo
20:44:30       55d23.5'           +0.1              Inverting

5. Some conclusions.
It seems from these data that the sextant "has no instrumental
error for
practical purposes", and that the index correction
is 0. (The "correction" is always less than SD.)
The star test for the index error seems to be more accurate
than the Sun test.
The inverting scope seems to be more accurate for stars.

Further conclusions.
All reductions in the tables presented above were made
with the Nautical Almanac and computations with
exact formulas (using my computer).
The "Complete on board Celestial Navigator" which I also
tried gave errors about
0.4' in its almanac part, and up to 3' because of rounding
in its sight reduction tables.

Alex

   

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