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Geoffrey Kolbe you wrote: Unless you are trying to pass an exam, the 'proper' definition does not really matter does it? So long as you fully understand what YOU mean by the symbols you use, then "how to use them properly" should not be a problem. The important thing is to understand the principles.

I’m not sure that I’d agree with that Geoffrey.  In celestial as with many things, e.g. the law, the ologies, mathematics, we are often required to learn from books, research the work of others, and to leave records of our own.  Sometimes, those records are even classed as legal documents, e.g. a navigators log and charts and the calculations that go with them.  Therefore, it’s desirable to have conventions for the naming of variables and their definitions, and ideally as few different ones as possible.  E.g did you learn F=ma or p=mf at school, and were you ever required to work with that wonderful unit of mass  'the slug'?  One can only wonder where the n in Zn came from.  Perhaps the origin of Zn was ‘the angle a star makes with respect to north at the observer’s zenith’.  DaveP