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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Bowditch tables and sexant parallax
From: Bill B
Date: 2005 Apr 23, 18:33 -0500
From: Bill B
Date: 2005 Apr 23, 18:33 -0500
Regarding Bowditch tables (now 15, 16, and 17) for determining distance from an object of known height. Table 16 uses a plane right triangle. Smallest distance from object of on the tables ranges from 0.1 nm to 100 yards to .5 nm--0.1 nm for table 16. My Astra IIIB has an aprrox. vertical distance of 2.25" between the center of the horizon mirror and the index mirror. Using a plane right triangle, and knowing the distance to the object, I should be able to calculate a useable correction for parallax. For 100 yards approx. 0d 2' 9", for 0.1 nm approx. 0d 1' 4", for .5 nm approx. 13". Distance must be known to calculate the above, and distance is what we want to solve for using the tables. Is it reasonable to hold the sextant horizontally, align the sides of the object between the horizon glass and mirror, and use that angle (off the arc so, add to sextant measurment) plus/minus IC to correct to the actual angle? Bill
