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Bruce,

Thank you.  I have been researching my own question and came to the
same conclusion.  The relevant passage from the 1962 edition of
Bowditch was:

    Usually, it is preferable to make a single correction for all
    three errors, called INSTRUMENT ERROR.  Customarily, such a
    table is determined by the manufacturer and attached to the
    inside cover of the sextant case.  The sign of the error is
    reversed, so that the values given are for INSTRUMENT
    CORRECTION (I).

(The three errors are prismatic error, graduation errors and centering
error).  Elsewhere Bowditch makes the distinction between index error
and index correction, where you reverse the sign of the error to yield
the correction.  I did not find any passages in the 2002 edition of
Bowditch that contradicted the above, but also found little supporting
it.  My other two nav books, Turner and Toghill, also are relatively
silent on the subject.

The offending passage from Bauer was:

    So the instrument error taken from the certificate in the
    sextant box top is applied with reversed sign to sextant altitude.

This clearly is incorrect.  All the certificates illustrated by Bauer
indicate "correction" not "error," although only the old Husun
certificate indicates error existed in its instrument.

Making this change sure has straightened out my data!  I've been close
to pulling my hair out for several months trying to get my accuracy
close to my precision, even contemplating purchasing another sextant to
try to narrow things down.  Now I feel confident enough to try the
instrument out on the water (but still close to coast)!  It would help
if we lived closer than 350 miles to salt water.

By the way; John Luykx at Navtrak Nautical indicated that it might take
about 500 observations to calibrate my sextant; I probably still need
to get my standard deviation consistently under 0.2' before starting.

Fred

   

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