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On Sep 11, 2005, at 5:37 PM, george huxtable wrote:

> Ken Gebhart wrote-
>
>
>> Re Mr. Piccer?s 5 min error.  Such an error with a plastic sextant is
>> not
>> unusual no matter how careful one is. As has been mentioned before,
>> the
>> critical areas of a sextant are the pivot of the arm, and the worm
>> screw of
>> the micrometer.  Granted that plastic injected molding should yield
>> identical parts, but the assembly is still done by hand.  Any
>> variation in
>> inserting the pivot, and glueing the holding cap on it could easily
>> contribute to such an error.  Same goes for the worm screw.  Add in
>> the many
>> other sources of error that have been discussed earlier, and I am not
>> surprised at all.
>
> Because it's a measurement by reflection, to get a discrepancy of 5'
> requires an error in sextant reading of 10'. Ken knows a lot more about
> plastic sextants than I do, and I am not familiar with the Davis
> sextant
> that was used. But I would be VERY surprised to detect an error of 10'
> in
> my plastic Ebbco.
>
> George.

Bowditch cautions that the beginner will often be disappointed with the
results of his or her first sights.  Further practice may reduce the
error to under a minute.  I would say to practice quite a bit before
blaming the sextant.  That said, a metal sextant is much to be
preferred for precise, accurate measurements.

Fred

   

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