[LEAPSECS] the big artillery

Steve Allen sla at ucolick.org
Sat Nov 1 19:31:37 EDT 2014

On Sat 2014-11-01T16:50:57 -0400, Athena Madeleina hath writ:
> So days may come and go, but UTC with or without leap seconds meets
> its definition just fine - for those who just think of it as a
> universally agreed-upon time reference that's coordinated by timing
> labs.   It is not amibuguous if this universal reference coincides
> with UT1 to .9 seconds until 2020 and then less closely thereafter -
> that's just the way it would work out.

I disagree, and so does the existing documentary record.

In the appropriate contexts there are days of Terrestrial Time,
International Atomic Time, Barycentric Coordinate Time, Geocentric
Coordinate time, GPS system time, BeiDou system time, etc.  Each of
those days is 86400 SI seconds in its own reference frame.

In other contexts there are days of Universal Time, Sidereal Time,
Ephemeris Time.  Each of those days is 86400 of its own kind of

UTC is unique by being the only time scale where 86400 of its seconds
is not the same as one of its days, and the complex web of reasons why
UTC is different is most obvious in the explanatory note from the 1964
IAU General Assembly
In that note they clarify that Atomic Time and Universal Time are
recognized as distinct concepts with technical definitions, both of
which are needed in some context.  Every other agency involved in the
definition of radio broadcast time signals has statements in the
documentary record in agreement with that.

The two demands placed on radio broadcast time signals were
1) they must provide a measure of elapsed uniform atomic seconds
2) they must provide a measure of earth rotation as Universal Time
The CCIR decided to eschew the entrenched notion of 86400 seconds = 1
day as a practical way to satisfy the two incompatible demands for the
purposes of radio broadcast time signals.  This is effectively the
same as the doomed never to succeed World Calendar and its intercalary
Worldsday.  The subsequent proliferation of other time scales shows
that UTC was not favored by users with precise technical needs of
either flavor.

In the past 50 years the needs of users of radio broadcast time
signals have changed.  It is no longer imperative that radio broadcast
time signals directly provide Universal Time, and it has become
imperative that the radio broadcasts provide uniform measure of
elapsed time.  This is saying that requirement 2) has lost importance,
not that the definition of Universal Time has become deficient.  UT is
just not what is needed by the principal users of the broadcasts.

The ITU-R cannot erase a century of textbooks, laws, statements, and
algorithms which all recognize Universal Time as a subdivision of
calendar days of earth rotation.  Whereas it is true that Universal
Time is not time, it is also true that Atomic Time is not date in any
way that calendars have meant since antiquity.

At WRC-15 next year the ITU-R has no choice about the nature of the
radio broadcast time signals; they must become a purely atomic time
scale.  The ITU-R does have a choice about the name of the time scale
in the radio broadcasts.

If WRC-15 decides to change the nature and retain the name then they
will be repeating the mistake their predecessors made in 1970: trying
to pretend that one technical time scale can serve two incompatible
technical purposes.  I expect that will result in yet another
generation of confusion and flame wars, until all of the people and
systems who know the current definition of Universal Time are gone.

Steve Allen                 <sla at ucolick.org>                WGS-84 (GPS)
UCO/Lick Observatory--ISB   Natural Sciences II, Room 165    Lat  +36.99855
1156 High Street            Voice: +1 831 459 3046           Lng -122.06015
Santa Cruz, CA 95064        http://www.ucolick.org/~sla/     Hgt +250 m

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