NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Gary LaPook
Date: 2017 Oct 21, 15:24 -0700
Greg, thanks for the comment and the kamal implementation.
My question was why Mr Gatty tabulated the -31' Hc rather than the apparently more correct value of -51'?
Regards,
Tony
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Because you use the standard 50' for computing Hc for which you need to allow for the minus 16' of SD so you add the 16' SD to the minus 34' of refraction for a sight taken lined up with the visible horizon to find Hc for navigational purposes. To make it clear, if you were planning to plot an LOP from your observation of the sun's upper llimb lined up with the horizon then use minus 50' for your correction to compute Hc (and any dip, very small from a life raft, just the square root of your height of eye, less than 2 minutes sitting in a life raft.) If you are just trying to find latitude using the method in the raft book then you do not need to apply SD.
gl
