[LEAPSECS] Toasting Unix timestamp 1234567890
sla at ucolick.org
Sat Feb 14 19:17:54 EST 2009
The whole subject of an atomically precise Unix epoch is pedantic to
the point of irrelevance, for it is another example of the attempt to
declare a definitive propleptic time scale in the absence of any
such thing being generally available at the time.
On 1970-01-01 the authority for world time was BIH, and all of the
inputs to them were published in Bulletin Horaire. I'm not sure there
was an equivalent publication by the USNO, for prior to the IERS all
the international authority was concentrated at BIH, with the other
observatories being sovereign for their own countries, and
contributors to BIH, but not authoritative beyond that.
At that date it was only just dawning on astronomers that VLBI would
be a really good way to measure earth orientation. The majority
of the input EOP information was from the transit circles.
At that date there was no TAI, just BIH's TA.
At that date the in-force CCIR reccomendation specified that broadcast
time signals should follow UT2. The BIH had been using something
referred to as UTC which was supposed to predict UT2, but the Soviets
and Chinese had their own version of UTC.
Then, as now, the CCIR recommendation was acknowledging that the
authority for the maintenance of the time scale was another agency.
In 1970 the definition of UT2 was entirely in the hands of BIH,
whereas with UTC since 1972 it remains unclear whether the CCIR/ITU-R
control the definition of the name UTC per se, or just the definition
of the broadcast time signals.
At that date the human-audible WWV and WWVH broadcasts in the US were
announced as GMT, and I don't know what WWVB or any other national
transmissions were doing.
And as of 1970-01-01 the TA seconds were of a length systematically
shorter than the TAI seconds have been since 1977-01-01.
So as of 1970-01-01 the precise time scale means whatever you want it
to mean, and the only way to correlate with the time actually
available is to dig out the Bulletin Horaire.
But at least then the BIH was the place to go for both types of time
scales, astronomical and atomic. The way things were split in 1988
leaves us with IERS for the former and BIPM for the latter.
The current actions of the ITU-R seem on the whole to be commendable,
much more so than the CCIR was in 1970, for the ITU-R is insisting on
consensus. It's the delegates to WP7A, their machinations within
their own national bodies who approve their positions, and the
contributors of position papers who are hard to grok.
Steve Allen <sla at ucolick.org> WGS-84 (GPS)
UCO/Lick Observatory Natural Sciences II, Room 165 Lat +36.99855
University of California Voice: +1 831 459 3046 Lng -122.06015
Santa Cruz, CA 95064 http://www.ucolick.org/~sla/ Hgt +250 m
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