UT and the IAU

From: Steve Allen <sla_at_ucolick.org>
Date: Sat, 10 May 2003 02:09:31 -0700

In 1925 the publications of nautical almanacs and ephemerides made a
major change. They started counting the hours of the day as beginning
at midnight (as with most civil time) instead of at noon (as had been
the astronomical practice). Before 1925 the American Ephemeris and
Nautical Almanac (AENA) had called time starting at noon GMT, and
starting with 1925 (thru 1952) it called time starting at midnight
Greenwich Civil Time. The British almanac could not be persuaded not
to apply the term GMT to the new reckoning of hours starting at
midnight despite the high likelihood of confusion of the meaning of

In 1925 the IAU Temporary Committee on Time approved the following text:
    Astronomers have not yet arrived at sufficient agreement upon a
    single terminology which it is desirable to use for time, and are
    not in a position to lay down a rule on the subject.
    It is desirable that astronomers should state exactly which time
    they are using.

In 1928 IAU Commission 4 (Ephemerides) adopted the following text:
    The terms Greenwich Civil Time (G.C.T.), Weltzeit (W.Z.) and
    Universal Time (U.T.) denote time measured from Greenwich Mean
    Midnight, and are not ambiguous.
I believe that this is the original definition of UT. I am unaware of
any significant usage of WZ, and in the AENA GCT was replaced by UT in
1953. (In the mean time most of the world except for old astronomers
has forgotten that there ever was an ambiguous usage of GMT, and the
British interpretation of GMT=UT is broadly believed.)

The discovery of seasonal variations in earth rotation did not occur
until 1936/1937, so the expectation in 1928 was that UT would be
suitable both as a measure of angle ("time-of-day") and as a measure
of dynamical time. Within a decade it was clear that earth rotation
was not a good clock. The IAU began to take steps to distinguish
between rotation and time, and Ephemeris Time (ET) came into being.

Beginning in 1960 the almanacs started publishing ephemerides using
ET. The various types of UT were treated as measures of angle, and
this conforms with the original 1928 definition that indicates UT is
measured from midnight. At that time most civil time broadcasts were
providing UT2, and throughout the 1960s the rate of UT2 was adjusted
in a way that precluded its interpretation as a uniform timescale. UT
in all its forms was a measure of "time-of-day", not time.

The definition of UTC as atomic time with leap seconds to keep in step
with UT was codified in 1970, implemented in 1972, and the rules were
refined over the next few years.

So, inasmuch as the IAU controls the definition of types of "Universal
Time", the IAU has always held that UT means angle. As such, all
forms of UT have always required input from astronomers.

Early determinations of UT were optical measurements from transit
instruments, but by 1970 IAU Commission 31 (Time) remarked that the
"exotic systems such as VLBI" were proving important. In 1973 a joint
session of IAU commissions 19, 31, and 24 recommended that
"multi-national and global networks of stations" be established and
"operated for a long enough time" to evaluate their results and that
"additional technical developments be undertaken" to improve their

Naturally this required money. During the 1980s the deployment of
Mark III VLBI correlators was greatly accelerated by the influx of
NASA dollars as part of the "Mission to Earth". VLBI provides
information on all sorts of esoteric astronomical objects and
geophysical effects. Nevertheless, it was always safe to explain to a
congressman that these dollars to VLBI were directly contributing to
the ability to tell what time it is. "Time-of-day" is a concretely
relevant datum that anyone on the street can grok.

If UTC, or even merely civil time, ceases to rely on astronomical
input, will astronomers be less able to justify why legislators should
spend tax monies on "the next generation of Mark III correlators"?

Will the IAU quietly allow the re-definition of a form of Universal
Time in such a fashion that it diverges from all past usage and also
kills a cash cow?

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 Sat May 10 2003 - 02:09:48 PDT

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