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    Re: sextant precision.
    From: Frank Reed CT
    Date: 2005 Jun 21, 01:30 EDT

    Henry H you wrote:
    "I have never been exposed  to a quantification of the error potential due
    to improperly ground shade  glasses in a quality instrument, however, such
    references as are available to  me, specifically Wharton & Field, tend to
    indicate any such error  existent to be inconsequential to observations
    made on the sea horizon, which  appears to be George's concern."
    In modern metal sextants (with glass  shades) I would hope that shade error
    would almost always be less than 0.1  minutes of arc. But I will add that I
    have seen an error of 1.0 minutes in  one shade in an otherwise excellent Plath
    sextant from the 1950s. An error of a  whole minute of arc, like that, is
    something that would be "nice" to  correct for in real observations though not
    entirely "necessary".
    In  plastic sextants, shade error seems to be quite common, and typically I
    have found it to be 1 or 2 minutes of arc. In certain combinations of shades,
    this can add up to 4 or 5 minutes easily. But we don't have to live with
    Shade error is a correctable error. If you can measure it  successfully --and
    we can-- it is very easy to correct. It is just a component  of I.C. when
    that shade is in use.
    42.0N 87.7W, or 41.4N  72.1W.

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