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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: sextant precision.
From: Frank Reed CT
Date: 2005 Jun 21, 01:30 EDT
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 that... 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. -FER 42.0N 87.7W, or 41.4N 72.1W. www.HistoricalAtlas.com/lunars