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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