NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Gary LaPook
Date: 2015 Jul 14, 00:10 -0700
I still don't know where the formula comes from. The Bygrave method starts with determining the co-latitude by subtracting the latitude from 90 degrees. After doing that you calculate Greg's "X," my "W" or Bygrave's "y." (I will use my nomenclature from this point on.) The formula for this is W = arc tan (tan declination/cos H) ( or cos "t" or cos LHA) they all work with a calculator) but to get them in the ranges of values available on my flat Bygrave or on to a standard 10 inch slide rule you have to convert LHA into the range of 0 to 90 degrees. See the attached form from my flat Bygrave website which shows this conversion and the computation of "W."
Then you either add or subtract W from co-latitude depending on the LHA and on whether the latitude and declination are of the same name or contrary name.
From my website:
"We also use the top of the form to determine if we will be adding or subtracting the intermediate value of "W" to co-latitude. This is determined by the column of the LHA and by the names of the latitude and declination. In our example we can see that we will be adding "W" since LHA is in the last column and the names of the latitude and the declination are the same. Just circle the "+" mark at the bottom of this column and also place a "+" mark on the "W" line under the co-latitude line. (Illustrations 5 and 6) Shown on the form, but not happening in practice, are the two cells for LHA between 90º and 180º and 270º and contrary declination because such combinations can never happen with the celestial body above the horizon. .So the rule simplifies to:
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Greg,
This if/then statement is a little confusing to me.
For contrary name: 90° - X - L = Y Y<90° Y = Y
It seems like I don't need to be concerned for when Y = Y. That case rather takes care of itself. I think I might be more concerned about what to do when d and L have contrary names, and Y needs to be corrected/changed in some fashion.
Have I missed something here, or does the if/then statement need to be tweaked a little bit?
Thanks,
Bob
