[LEAPSECS] Consensus building 2

[Stephen Colebourne]

Update including some comments sent earlier, and new entries on UTC
and local-time:

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
* definition; ISO-8601 - a standard format for expressing date-times
(defined in detail elsewhere)

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
- the duration of an SI-second is considered to be constant

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 is usually denoted using ISO-8601 with a day implied as
exactly 86400 TAI-seconds

Solar
- definition: solar-time - time measured by the rotation of the earth
relative to 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 SI-seconds varies over time and is
not a fixed number of SI-seconds
- the length of a mean-solar-day in SI-seconds is on average
increasing with time

UT
* definition: UT - a time scale based on the rotation of the Greenwich
meridian relative to the Sun (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 and is not
a fixed number of SI-seconds
- the length of a UT1-day in in SI-seconds is on average increasing with time
- definition: UT1-second - a fraction 1/86400 of a UT1-day
- the duration of a UT1-second is close to, but not equal to an SI-second
- UT1 is the most commonly recognised form of mean-solar-time
- a UT1-day is the most commonly recognised form of mean-solar-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 that is normally 86400
TAI-seconds but can be 86399 or 86401 SI-seconds
* the missing or additional TAI-second in a UTC-1972-day is a negative
or positive leap-second
* a leap-second is removed or inserted at the end of a UTC-1972-day
* an inserted leap-second has a representation of 23:59:60
- the presence or absence of a leap-second is currently determined up
to 6 months in advance
* leap-seconds are used 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
* UTC-1972 is usually denoted using ISO-8601 without inclusion of the
difference to TAI-2008

Local time
* definition: local-time - the time-scale local to a region of the Earth
* definition: offset - the duration that local-time differs from the
locally recognised legal standard time-scale
* definition: time-zone - a region of the Earth where local-time is coordinated
* definition: time-zone-rules - rules defining how the offset changes
along the time-line
* definition: UTC-1972-offset - the duration that local-time differs
from UTC-1972
* the offset from local-time to TAI-2008 can be calculated
* definition: TAI-2008-offset - the duration that local-time differs
from TAI-2008
* given either a TAI-2008-offset or a UTC-1972-offset and a
leap-second table the other may be calculated

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 at a single Earth location
- the legal definition of a day varies by country
- the legal definition of a day may be based on UTC-1972-day,
mean-solar-day or humanity-day
* local-time, offsets and time-zones are widely used by humanity
* the TAI-2008-offset is rarely used by humanity

Maybe more controversial
* one of the forms of UT is commonly considered to be the basis of a
humanity-day


Agree/disagree away..

Stephen