[LEAPSECS] Consensus building 2
Warner Losh
imp at bsdimp.com
Wed Feb 2 21:29:45 EST 2011
On 02/02/2011 18:50, Stephen Colebourne wrote:
> OK, so we've got a little bogged down in redefining what appear to be
> well defined things, and whether a list like this should define things
> anyway. I'll give it one more go, but sadly I don't have the "patience
> of Job" if others don't also want consensus.
>
> Remember, I'm not an expert to the same degree as others on this list.
> Thus statements I make will be simplistic, but are the question is
> whether they are good enough for the problem at hand? Writing a
> clarification is only helpful if you propose a better statement to
> replace that you are objecting to.
Thanks for the update. It is a lot closer...
> A star is used for a new or amended line.
>
> General:
> * these points of consensus exist to aid the understanding of leap
> seconds not time in general
> * the terms seconds, minutes, hours and days are overloaded
> * relativistic effects do not significantly impact the understanding
> of leap seconds
> * definition: a time-line is the general passage of time
> * definition: instant - an instantaneous point on the time-line
> * definition: duration - the length of a portion of the time-line
> * definition: time-scale - a set of rules giving meaning to an instant
> * the length of time between two instants is a duration
>
> SI
> * definition: SI-second - a standardised unit of measurement for
> durations (defined in detail elsewhere)
> * the accurate measurement of an SI second is complex and typically
> achieved via cooperation
> - the SI-second forms the basis for many other fundamental units of measure
>
> TAI
> * definition: TAI-2008 - a time-scale commonly named TAI last revised
> in 2008 (defined in detail elsewhere)
> * the accurate measurement of TAI is complex and typically achieved
> via cooperation
Accurate creation of TAI is compled and achieved via cooperative
measurements. TAI isn't measured directly, but created after the fact
based on the measurements input. A subtle point, but one worth remembering.
> * the TAI-2008 time-scale is defined as a uniformly increasing count
> of TAI-seconds from a fixed epoch
> * definition: TAI-2008-second - the same as SI-second for the purposes
> of this discussion
> * TAI-2008 does not definitively imply a definition for minutes, hours and days
>
> Solar
> * definition: solar-time - time kept or measured by the Sun
> * apparent-solar-time and mean-solar-time are two forms of solar-time
> * the accurate measurement of solar-time is complex and typically
> achieved via cooperation
> * definition: mean-solar-day - the descriptive subdivision used when
> describing mean-solar-time
> * the length of a mean-solar-day in in SI-seconds varies over time
> * the length of a mean-solar-day in in SI-seconds is on average
> increasing with time
> * the length of a mean-solar-day is not a fixed number of SI-seconds
>
> UT
> * definition: UT - a time scale based on the rotation of the Earth
> (defined in detail elsewhere)
> * UT has a number of variants, the most common are UT1 and UTC
> * the accurate measurement of variants of UT is complex and typically
> achieved via cooperation
> * definition: UT1 - a smoothed variant of UT (defined in detail elsewhere)
> * definition: UT1-day - the subdivision used when expressing UT1
> * the length of a UT1-day in in SI-seconds varies over time
> * the length of a UT1-day in in SI-seconds is on average increasing with time
> * the length of a UT1-day is not a fixed number of SI-seconds
> * definition: UT1-second - a fraction 1/86400 of a UT1-day
>
> UTC-1972
> * definition: UTC-1972 - a time-scale commonly named UTC last revised
> in 1972 (defined in detail elsewhere)
> * the accurate measurement of UTC is complex and typically achieved
> via cooperation
> * the UTC-1972 time-scale is a continuous count of SI-seconds
> * the UTC-1972 time-scale defines UTC-1972-days
> * definition: UTC-1972-day - a duration either 86400 SI-seconds or
> 86401 SI-seconds long
> - the additional SI-second in a UTC-1972-day is a leap-second
> * the presence or absence of a leap-second is currently determined up
> to 6 months in advance
> * leap-seconds are added to UTC-1972 with the aim of keeping UT1 and
> UTC no more than 0.9 SI-seconds apart
> * leap-seconds are currently added about once every 18 to 24 months
> * UTC-1972 is an integral number of SI-seconds different from TAI-2008
>
> Humanity
> * definition: humanity-day - a non-scientific, commonly used term
> understood by 6bn humans
> - a humanity-day is interpreted in line with the rising and setting of the Sun
> * the legal definition of a day varies by country
> * the legal definition of a day may be based on UTC-day,
> mean-solar-day or humanity-day
>
> Maybe more controversial
> * UT1 is the most commonly recognised form of mean-solar-time
> * a UT1-day is the most commonly recognised form of mean-solar-day
Agreed up through here.
> * a UT1-day is the most commonly recognised form of a humanity-day
Not sure I'd agree on this point... We've had 40-odd years where the
humanity day has been a UTC day not a UT1 day. But the delta between
these is small. A humanity-day works well with anything that
approximates a UT1 day, since most people's humanity-day is local time
based not UT1 based. Local time is determined by timezones and is
generally close to an integral number of hours off of UT1 (or 1/2 hours
in the case of a few zones). Also some percentage of humanity lives
where the civil time differs from the local solar time by more than an
hour, sometimes for the entire year.
> * the duration of an SI-second is considered to be constant
> * the duration of a UT1-second is close to, but not equal to an SI-second
>
Agreed.
I think is is really good progress: one minor tweak and one point you
thought would be contentious that I'm contending...
> Stephen
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