[tz] What's "right"?
brooks at edlmax.com
Thu Nov 12 23:04:08 UTC 2020
On 2020-11-12 5:05 PM, Guy Harris wrote:
> A practical scale of time for world-wide use has two essential elements: a realization of the unit of time and a continuous temporal reference. The reference used is International Atomic Time (TAI), a time scale calculated at the BIPM using data from some four hundred atomic clocks in over eighty national laboratories.
> In that context, what is a "time scale"? Does it assign an hour/minute/second value to each second?
> It then says
> TAI is a uniform and stable scale which does not, therefore, keep in step with the slightly irregular rotation of the Earth. For public and practical purposes it is necessary to have a scale that, in the long term, does. Such a scale is Coordinated Universal Time (UTC), which is identical with TAI except that from time to time a leap second is added to ensure that, when averaged over a year, the Sun crosses the Greenwich meridian at noon UTC to within 0.9 s.
> so UTC is also a "time scale". ITU-R Recommendation TF.460-6:
> A positive leap-second begins at 23h 59m 60s ...
> which suggests that *something* is assigning hour/minute/second values to seconds.
> It also says
> A positive or negative leap-second should be the last second of a UTC month ...
> suggesting that the clock is tied to a calendar in some fashion.
Notice in Rec 460 it says:
B International atomic time (TAI)
.... It is in the form of a continuous scale, e.g. in days, hours,
minutes and seconds from the origin 1 January 1958 .....
From the Rec 460 perspective, TAI is a timescale delineated in YMDhms
form, not as an uninterrupted count of seconds since 1958, as it is
often treated as and referred to. UTC is also in YMDhms form, and the
UTC-TAI value (10 initial offset + leap-seconds) is the difference
between the two YMDhms forms. It is not a difference from between some
two uninterrupted count of seconds. UTC exists only as YMDhms. This
distinction between timescales represented as an uninterrupted numbered
count and YMDhms labels is not always made clear.
Rec 460 is stating that the alignment point between TAI and UTC in
YMDhms form includes an initial 10 second offset and can be expressed as:
1972-01-01 00:00:10 (TAI) = 1972-01-01T00:00:00 (UTC)
Note however, as Steve Allen points out, the term, or initialization,
"UTC" was not generally adopted until 1974.
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