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Tom, your mention of GCT got me thinking, my memory said same as GMT but I decided to look it up and found the following:

 

 

Below is a partial definition  of GTC. from this page.

 

."Greenwich Civil Time -- GCT

In 1928 the IAU approved the use of this term as a synonym for Universal Time. The US almanac used this term from 1925 until 1952 to indicate that days were being reckoned from midnight. The British almanac could not be convinced to use the term GCT, and continued to use the term GMT with the new astronomical meaning."

 

 There are many other definitions of times defined and this page would be a good place to look and review this subject. The article explains the various ways time was defined over the years and includes much data that is use full to the navigator. For example, "that days were being reckoned from midnight" wasn't it always ?  Take a look to find out.

 

On another site is the following article that shows the derivation of "Z" time,  its also very interesting. interesting.

Hmmm, that's curious. I sent 10 and only three are in the note.

Im resending some and include the back.

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