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The first test results are puzzling:
it seems that this pocket sextant described in my previous message
is more
accurate than I expected, but on the other hand
there is a systematic error I cannot explain.

June 11, Sun from a beach, very good weather,
Lat 54d27'N, Long 10d11'E, (taken from from Multimap.com) near the mouth
of Kielerforde.
Dip was taken from the table, after careful measurement of the height.

7:53 a.m., average of 5, measured alt 23d28' true: 23d31'.5 error -3'.5
8:15 a.m., average of 6, measured alt 26d43'.6 true: 26d47'.0 error -3'.4
8:30 a.m., average of 6, measured alt 28d36'.9 true: 28d40'.2 error -3'.3
9:26 a.m., average of 5, measured alt 36d57'.0 true: 37d00'.5 error -3'.5
9:57 a.m., average of 8, measured alt 41d31'.1 true: 41d34'.7 error -3'.6

Is not this strange?! It looks like if I use IC=-3.5 the
results would be nearly perfect, as good as with a best full size
sextant. These are the averages of course, but I actually took care
to reduce ALL observations, and the error of EACH individual
observation deviates from -3'.5 by no more than 1'.
(This the very best one can hope for with the vernier reading to 1')

Observations are takes from three different points near the shore,
both with upper and lower limb.
Of course I double chesked and triple checked the index correction
(using the flat roofs about 0.5 mile away). It is zero!

So where could these -3.5 possibly come from?
This cannot be any anomalous dip (the whole dip correction
was between -2.2 (when I sat on a stone just next to the water edge)
and -3.5). It is hard to believe to such large arc irregularity at
such moderate angles.
The only thing I did not check is the index mirror perpendicularity,
because I have no idea how to do it on a pocket sextant.

Still need tests at larger angles (This is impossible to do
before I sail next week. Kiel is located at the end of a deep
and narrow fjord opening North.
So to see the Sun over the horizon I have to
travel 10 miles to the mouth of the fjord, and even from there
I can do observations only until 10 a.m. Stars are hardly visible at all
in summer because of the high latitude).

Alex

   

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