[tz] historic sub-second time offsets
eggert at cs.ucla.edu
Wed Nov 5 19:15:47 UTC 2014
On 11/05/2014 07:47 AM, Steve Allen wrote:
> 1960 is after the availability of cesium atomic chronometers and
> during the era when Heure Definitive was UT2.
Was heure définitive really UT2? My French is pretty weak, but page 253
of your source has a table entitled "Temps Universel 2 - Heure
Définitive", which makes it appear to be listing the difference between
UT2 and heure définitive. Also, I just now looked at a
seemingly-authoritative paper on this (see citation below), and its
pages 177-8 seem to say that heure définitive was closer to what is now
It's funny: just today I was wading through the HTML5 spec, which says:
"Times in dates before the formation of UTC in the mid twentieth
century must be expressed and interpreted in terms of UT1."
I'm not brave enough to put anything that specific into the tz
documentation. I suspect that nobody putting an accurate circa-1957
time stamp into an HTML5 document has ever consulted a UT2−UT1
difference table in order to conform to the HTML5 standard. And I'm a
bit puzzled as to why the HTML5 committee was so specific about which UT
variant to use before UTC was introduced.
Should we write to the HTML5 committee and say that they got their
pre-UTC timescales slightly wrong and they shouldn't have tried to be
that precise anyway? Or would that be too brave in the opposite
direction? (When was UT1 introduced anyway? :-)
Guinot B. History of the Bureau International de l'Heure. Polar Motion:
Historical and Scientific Problems, ASP Conference Series, Vol. 208,
also IAU Colloquium #178. 2000. pp 175-84.
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