🚀 go-pugleaf

It looks like UT (or maybe UTC) = GMT = GCT = Z. 

Here is a reply I received from David J. Bryant via Mark Horton
<cbosgd!mark@Berkeley.ARPA>:
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Date: Sun, 29 Jan 84 18:56:43 est; From: mark@cbosgd.UUCP (Mark Horton)
To: wcwells@ucbopal; Subject: star gazers time

Here is an answer to your star gazers questions

Date: Sun, 29 Jan 84 18:21:08 est; From: djb (David J. Bryant)
To: mark; Subject: Re: can you please answer the "star gazers" question in this?

Astronomers time stamp observations and events according to Universal
Time (abbreviated UT).  This is based on atomic clocks and mathematical
standards according to definition of the International Astromical Union
(I.A.U.).  Further, Greenwich Mean Time (GMT) is defined to be the same 
as UT, and applies to the standard meridian (longitude 0 degrees).  Greenwich
Civil Time (GCT) is synonymous with GMT.

WWV and WBS (in England) broadcast UT (which is therefore also GMT, and
for the military, Z ("Zulu") time.  This is a worldwide standard reference,
and probably the best for your purposes.  (Elaine says that CompuServe
used UT for ALL their record keeping, login accounting, etc.)

There are other time schemes as follows:

	Apparent Time - The time indicated by a sundial.  Because the
		     Sun's speed across the sky is not uniform, this
		     varies throughout the year, and is useless for
		     time reckoning.
	Mean Time  - To avoid the non-uniformities of Apparent Time, a
		     fictitious "mean sun" is invoked; this moves across
		     the sky at a constant rate throughout the year.  Mean
		     Time, like Apparent Time, is purely a function of your
		     longitude, and so is too variable for much civil use.
	Civil Time - This is what the clock on the wall is set to.  It
		     is based on the concept of Time Zones, Daylight
		     Saving Time, etc.  Each time zone is homed on a
		     central meridian, and the mean time for that meridian
		     is used as the civil time throughout the time zone.
		     Other adjustments (Daylight Saving Time) apply as
		     required.
	Sideral Time - Unlike all the above, which is based on the 24 hour
		     solar day, Sideral Time is based on the 23h 56min
		     "star" day.  There are numerous flavors of Sideral
		     Time that compensate for variations, but I don't
		     see any real reason to go into much detail.  This is
		     not a time scheme you want to use.

Interestingly, all time schemes can be converted to any other time scheme,
provided you specify latitude, date, etc.

If you would like a more thorough, informative description of these (and
other) time systems, let me know.  I think this should be enough to answer
your question about "star gazing" time.  Personally, I encourage you to
use UT, and to follow other standard astronomical (I.A.U.) conventions to
deal with issues associated with time/date stamping, since astronomers have
already worked out these problems.

	David



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Now why UT and UTC?  I suspect that UT is the time based on the atomic
clock from a specific point in time and that UTC is UT which has been
leap second adjusted.

Bill Wells
wcwells@Berkeley.ARPA