NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2026 Aug 3, 17:36 -0700
Hello Jacquelin Hardy.
You asked:
"Does that mean using the ABC Tables in Nories Tables or using the British Admiralty Chart No. 5000 (Weir Diagram)"?
That's the trouble with ABC. It stands for "three easy steps". Easy as ABC! :)
Can you remember anything else about the context in question? Or even just the general run of the conversation? Of course, there are the ABC cases that you mentioned. Also the general calculation that I provide in my "Modern Celestial Navigation" and other workshops uses three simple quantities that I call A, B, C. Just for the record, and not because this necessarily what you saw previously:
A = cos ZD / cos Dec / cos Lat
B = tan Dec × tan Lat
C = A - B
And then,
dLon = acos C
Lon = GHA +/- dLon
This is nothing but the standard spherical law of cosines written in a calculator-friendly clean format.
Frank Reed
