A brief tour of time
UT0 is angle (but there is more than one angle involved in its values)
UT1 is angle (it is the rigorous quantity for earth rotation)
UT2 is angle (albeit hardly used anymore)
UT is angle (in some fuzzy pre-relativistic way)
Sidereal time is angle
The distinction between angle and time was not understood until this
past century, but by current understanding the above measures are
definitely not time.
GMT is not legally defined in a way that makes it easily classified as
either time or angle. (Its original use tends to classify it as angle
equal to UT1, and later use tends to classify it as UTC.)
TAI is a practical realization of proper time, as a statistical mean of the
participating clocks, reduced to the rotating geoid. (Note that the geoid
is not constant over geological time, therefore the rate of TAI may be
affected by lack of adequate geophysical modelling.)
ET is pre-relativistic time (presumably on the rotating geoid)
TT is proper time (TT = TDT). This is an ideal that should theoretically
tick in unison with TAI but may not because of defects in TAI.
When realized TT is a family of timescales derived from various
measures, but for practical purposes TDT = TAI + 32.184 s.
TDB is proper time in the frame of the barycenter that ticks in sync
with the mean ticks of TT. Technically TDB is ill-posed, obsolete and
should be replaced with TCB, but in practice it differs only in rate.
TCG is coordinate time in the frame of the geocenter that ticks faster
than mean TT because of the gravitational and rotational redshift of
clocks on the geoid.
TCB is coordinate time in the frame of the barycenter that ticks
faster than TCG because of the gravitational and velocity redshift of
clocks in the frame of the geocenter.
The realization of TDB, TCG, and TCB are families like those of TT that
depend on the input measure of TT.
Many other forms of coordinate and proper times might be defined for
observers with other velocities and depths in gravitational
potentials.
Recent details of the meaning of all these can be found in
papers from.
http://www.iers.org/iers/publications/tn/tn29/
Software implementing the gory details can be found at
http://www.iau-sofa.rl.ac.uk/
========================
Okay, so much for the tour of time.
Now, to get to the subject of UTC.
UTC is, really, angle.
Its value has always been designed to indicate angle, although since
1972 it has been constrained to tick with TAI.
As seen above, this is consistent with all other forms of "Universal"
time, which have always been realized as measures of angle. To change
the meaning of UTC into a form of atomic, proper, or coordinate time
would create a substantially misleading misnomer capable of causing
significant confusion in the context of the other definitions of time.
========================
Who is in charge of the definition of UTC?
The CCIR Recommendation 460-4 (1986) says that UTC is maintained by
BIPM with assistance from IERS. The CCIR is a predecessor of ITU-R,
whence the document can currently be purchased.
The BIPM does maintain TAI, but indicates that UTC is within the purview
of IERS.
IERS says that UTC is defined by CCIR Rec. 460-4 (1986).
Is this arrangement an example of a happy cooperative, or tenuous
circularity?
To their credit, the documents about time from the IAU, IERS, and BIPM
are openly available to all. The authorship of the papers and
membership of the committees is visible online. The decisions are
made with respect to the practicalities of existing systems. In
contrast, everything from ITU-R is a closed process.
========================
It is clear that ITU-R is in charge of broadcast time and frequency,
and for practical purposes this means civil time. ITU-R could
redefine UTC within their document, or they could revise the
document to assert that some other timescale than UTC should be
broadcast.
Could it happen that ITU-R might chose to redefine UTC and IERS choose
not? The available documentation is not clear. If disagreement were
to happen would the unambiguous meaning of UTC be destroyed? Would
various national time broadcasts have to choose which of two versions
of UTC to transmit?
========================
If it is deemed that civil time should become a form of atomic time,
then I assert that it is more consistent and safer to define a new
timescale than to change UTC. The new timescale could be called
something like Civil Atomic Time (Temps Atomique Civil = TAC).
This is not to say that a new name and character for civil time is the
right thing to do, and it certainly does not solve all problems.
Whether done by changing the character or changing the name, dropping
leap seconds from civil time (and time broadcasts) will still cause
grief to many existing systems, especially telescopes. It must not
be done without a lead in period of many years.
--
Steve Allen UCO/Lick Observatory Santa Cruz, CA 95064
sla_at_ucolick.org Voice: +1 831 459 3046 http://www.ucolick.org/~sla
PGP: 1024/E46978C5 F6 78 D1 10 62 94 8F 2E 49 89 0E FE 26 B4 14 93
Received on Wed Feb 12 2003 - 01:50:27 PST