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 _______________________________________________ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs