UTC vs GMT

[Markus G. Kuhn]

In message <199610032320.QAA04986 at shade.twinsun.com>, Paul Eggert wrote:

> That's a good suggestion for Etc/UTC and Etc/Universal, since anybody
> who chooses those zones probably wants `UTC'.  (Also, Etc/UCT should
> probably generate `UCT' instead of the current `GMT'.)

What is UCT supposed to mean and why is it defined there? I have never
heard this term before. Is it supposed to be "Universal Coordinated
Time"???

Two alternative original abbreviation proposals were CUT (English:
Coordinated Universal Time) and TUC (francais: temps universel
coordine), but UTC was selected both as a compromise between the
French and English proposals and because the C at the end looks more
like an index in UT0, UT1, UT2 and a mathematical style notation is
always the most international approach.

See also <URL:http://www.boulder.nist.gov/timefreq/faq/faq.htm>:

  Why is the abbreviation for Coordinated Universal Time "UTC" instead
  of "CUT"?

  In 1970 the Coordinated Universal Time system was devised by an
  international advisory group of technical experts within the
  International Telecommunication Union (ITU). The ITU felt it was best
  to designate a single abbreviation for use in all languages in order
  to minimize confusion.  Since unanimous agreement could not be
  achieved on using either the English word order, CUT, or the French
  word order, TUC, a compromise of using neither, UTC, was adopted.

  Is Coordinated Universal Time (UTC) the same thing as Greenwich Mean
  Time (GMT)?

  Greenwich Mean Time (GMT) is a 24 hour astronomical time system based
  on the local time at Greenwich, England. GMT can be considered
  equivalent to Coordinated Universal Time (UTC) when fractions of a
  second are not important. However, by international agreement, the
  term UTC is recommended for all general timekeeping applications, and
  use of the term GMT is discouraged.

> As far as I know, GMT hasn't been an official reference time since the 1920s.

Yes, I have seen various references to some IAU conference in 1928
where an agreement was reached that the term Univeral Time (UT) shall
be used, and this term has since then indeed be used quite
consistently in the astronomical literature.  I have never read GMT in
any recent paper in "Astronomy and Astrophysics", only UT (especially
when second precision is not relevant), or UTC/UT1/etc. if subsecond
precision is relevant.