[LEAPSECS] Consensus building 2

[Stephen Colebourne]

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.

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
* 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
* a UT1-day is the most commonly recognised form of a humanity-day
* 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

Stephen