[LEAPSECS] Civil timekeeping before 1 January 1972
zefram at fysh.org
Sun Mar 8 13:09:59 EDT 2015
Brooks Harris wrote:
>It seems to me NTP and POSIX as well as other timescales concerned
>with "civil time", are essentially disconnected from "reality",
>expressing "idealized" measurement scales.
That's very much what they're not. TT is idealised, and TAI less so.
>think none of the "civil" timescales are counting in UT - they are
>measured in SI Seconds, even when prolpetic to 1972.
NTP doesn't deal with pre-1972 time at all. (No, the nominal epoch
POSIX time_t notionally can represent pre-1972 times, but in practice no
Unix system of that era was synchronised to UTC. Any use of time_t for
precise pre-1972 time is heavily retrospective, and the interpretation
is more governed by the application than by the POSIX standard. Wild
pre-1972 Unix time_t values heavily predate the POSIX standard, and their
interpretation has little to do with UTC. They are understood to be vague
UT with usually very poor synchronisation (via the operator's wristwatch).
Actual civil time, as used in the real world, was rarely precise pre-1972,
certainly not much to do with SI seconds. Historically civil time has,
over the long term and with large short-term excursions, been synchronised
to the solar day, so effectively to UT.
>I understand the proper SI second sprung into existence as of 1972.
You understand incorrectly. As with the existence of TAI (which I
discussed in my previous message), the development of the SI second
happened in stages. We can start in 1948, with the definition of
Ephemeris Time, which is the first time we got a definition for the
second that was not derived from the solar day. The ephemeris second is
instead defined in terms of the Earth's orbit around the Sun. ET was
adopted by the IAU in 1952. In 1958 the caesium second was defined,
the definition being chosen to match the value of the ephemeris second.
SI was established in 1960, and at that time adopted the ephemeris second
as the SI second. In 1967 SI was revised to adopt the caesium second
as the SI second, and in 1997 the definition was refined to account for
So the value of the SI second was established in 1948; the present
definition originates in 1958 but its full present form only goes back to
1997; SI (and so the concept of "SI second") originated in 1960; and the
SI second acquired its present basic definition in 1967. Any of these
dates could be taken as the inception of the SI second, but the only one
that could really be described as the SI second "springing into existence"
is the establishment of SI in 1960. Nothing relevant happened in 1972.
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