# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Re: Bowditch tables and sexant parallax**

**From:**Frank Reed CT

**Date:**2005 Apr 27, 02:44 EDT

Bill you wrote: "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?" If I've understood you right, yes. I've been thinking along similar lines for a "laboratory" test of arc errors. The parallax within the instrument (because of that 2.25" distance between the line of sight through the horizon glass and the index mirror that you're describing) is something that we can calculate exactly and correct for. For example, I could do an index correction by looking at a an index card across the room. If the top and bottom of the card are aligned (assuming that's the same as the distance between the line of sight through the horizon glass and the index mirror), then my sextant should reading should be identical to the I.C. based on objects observed at great distance (the standard method for checking index correction). For other angles, I am guessing that the parallax goes as d*cos(h) where h is the angle read off the sextant and d is that distance (2.25" in your case). -FER 42.0N 87.7W, or 41.4N 72.1W. www.HistoricalAtlas.com/lunars