Fantastic! Thank you for the summary, I will add it to the FAQ (with
appropriate attribution of course).
-ben
On Sun, Jun 22, 2003 at 10:36:02PM +0000, [EMAIL PROTECTED] wrote:
> Thanks Eugene.
>
> I'll try to rephrase this, because it
> would be good to have it in the FAQ.
> If somebody can explain it better, or
> more correctly, please help me!
>
> What's up with GMT, TAI, UTC, and UT1?
>
> Before 1972, the "international time" reference
> was GMT. In GMT, all days have the same number
> of seconds. A day starts at "midnight" and has
> 86400 seconds.
>
> TAI is another time measuring system, in
> which seconds depend on "atomic time" only,
> instead of the Sun-Earth position.
> TAI days have 86400 seconds, and it's
> origin is in 1958 January 1.
>
> Parallel with those, there exists UT1, which
> is the "astronomical time".
> UT1 depends only on Sun-Earth position.
> UT1 - TAI is some fractional seconds.
>
> In 1972 UTC was introduced, in order to
> approximate "international time" to
> "astronomical time".
> Now, whenever the difference between UTC and
> UT1 is big enough, a leap second is introduced.
> UTC is synchronized to TAI, which means that
> UTC - TAI is an integer number of seconds.
> UTC - UT1 is some fractional seconds.
>
> > [EMAIL PROTECTED] schreef:
> > >
> http://hpiers.obspm.fr/eop-pc/earthor/utc/leapsecond.
> > > html
> > >
> > > "... Since the system was introduced in 1972, "
> > >
> > > The table starts in 1972. Before that, GMT was
> > > in use - not UT1!
> >
> > Not true. UT1 existed at least since 1958. In that
> year, TAI (atomic
> > time) was synchronized with UT1.
> >
> > UT1 is based on the observed length of the day
> (corrected for some
> > large scale effects like the movement of the
> poles). As it is
> > observation based, it is not a good timescale to
> base civil time on.
> > UT was used for that. Periodically the
> correspondence of UT and UT1 was
> > checked, and UT was corrected by changing the
> length of the day, and by
> > adding a small time adjustment to account for the
> accumulated
> > difference.
> >
> > Civil time was based on UT: GMT was UT+0000, MET
> was UT+0100, EST was
> > UT-0500 IIRC (as long as you're not in Australia).
> >
> > So the role of UT/GMT was taken over by UTC, not by
> UT1.
> >
> > Eugene
> >
>
