On Mon, Oct 22, 2018 at 2:12 AM Mark H Weaver <mhw@netris.org> wrote:

Universal Time (UT) is not a measure of physical time, but rather is a
> measure of the rotation angle of the Earth with respect to distant
> quasars.  A UT second is identified with a fixed amount of rotation of
> the Earth, which equals 1/86400 of a mean solar day.  That's why every
> day has 86400 UT seconds.


Quite right.  Buit the whole point of UTC is that its seconds are not
angles,
but SI = TAI seconds.  There are a variable number of these in a day, and
a UTC clock will indeed report 23:59:60 at the end of a day with a leap
second in it (and other civil-time clocks will similarly report :60 in
whatever
hour, according to their timezone offsets).  See
<https://en.wikipedia.org/wiki/Leap_second#/media/File:Leapsecond2016.png>,
which is a screenshot of https://time.gov displaying the last leap second.

Now there are indeed exactly 86400 _Posix_ seconds in a day, which is
achieved by giving two seconds the same label if it is a leap day.  But
that has nothing to do with either TAI or UTC.


> UTC is kept within 0.9 seconds of UT1 (a
> version of UT with certain corrections applied), so over long time
> periods, with the leap seconds taken into account, UTC seconds are equal
> to UT seconds.
>

No, in the long run UTC time is equal to UT1 time.  That's not the same
thing at all.

-- 
John Cowan          http://vrici.lojban.org/~cowan        cowan@ccil.org
I am expressing my opinion.  When my honorable and gallant friend is
called, he will express his opinion.  This is the process which we
call Debate.                   --Winston Churchill