Re: Date and time on Cindu: yearly update

On Sun, May 18, 2008 at 8:38 PM, Tristan McLeay <conlang@...> wrote:
> For whatever it's worth, the fact that there exists a new standard UTC
> doesn't mean that people stopped using the old standard GMT, so GMT
> would've been correct for a long time after the introduction of UTC.
As a time zone designation, GMT is still in use, even officially.  As
a time standard, it is not "incorrect", merely obsolete.  But the
question was whether or not UTC and GMT refer to the same time zone,
and the answer is that yes, they are intended to.

> For instance, Australian time zones were based on GMT until a few years ago
> (so we would've skipped a few seconds at the end of one year, rather
> than just one every now and again).
OK, first, "GMT" has multiple meanings: it can just mean the time zone
against which all others are measured, a.k.a. the time in the UK when
it's not Summer Time.  It can also refer to a time standard, that is,
a reference by which to set clocks, which is no longer in use.  In the
latter capacity it was replaced by UTC.

However, your talk about skipping seconds makes no sense.  GMT and UTC
have never differed by more than a fraction of a second.  Both are
approximations to the same time specification, which is one where
12:00 means the average time of solar noon at Greenwich.  In other
words, both are tied to the Earth's rotation.  They differ only in the
mechanism of the approximation:  UTC uses atomic time, with occasional
leap seconds to make up for the fact that the Earth isn't as good a
timekeeper as an atomic clock; GMT doesn't use atomic time at all, but
merely goes directly by the local mean solar time in Greenwich.  There
is no offset between them.

Originally, GMT was defined such that zero hours (0:00 GMT) was NOON.
It was defined by astronomers, see, and it's more convenient for
astronomers if the date doesn't change during the night. Starting in
the 1920's it was changed such that zero hours is midnight, and that
source of potential 12-hour confusion was really the only reason the
term was officially deprecated as part of the switch to a civil time
standard based on atomic time.








--
Mark J. Reed <markjreed@...>

Reply