On Sat 2014-11-01T16:50:57 -0400, Athena Madeleina hath writ: > So days may come and go, but UTC with or without leap seconds meets > its definition just fine - for those who just think of it as a > universally agreed-upon time reference that's coordinated by timing > labs. It is not amibuguous if this universal reference coincides > with UT1 to .9 seconds until 2020 and then less closely thereafter - > that's just the way it would work out.
I disagree, and so does the existing documentary record. In the appropriate contexts there are days of Terrestrial Time, International Atomic Time, Barycentric Coordinate Time, Geocentric Coordinate time, GPS system time, BeiDou system time, etc. Each of those days is 86400 SI seconds in its own reference frame. In other contexts there are days of Universal Time, Sidereal Time, Ephemeris Time. Each of those days is 86400 of its own kind of seconds. UTC is unique by being the only time scale where 86400 of its seconds is not the same as one of its days, and the complex web of reasons why UTC is different is most obvious in the explanatory note from the 1964 IAU General Assembly http://www.ucolick.org/~sla/leapsecs/note1964en.html In that note they clarify that Atomic Time and Universal Time are recognized as distinct concepts with technical definitions, both of which are needed in some context. Every other agency involved in the definition of radio broadcast time signals has statements in the documentary record in agreement with that. The two demands placed on radio broadcast time signals were 1) they must provide a measure of elapsed uniform atomic seconds 2) they must provide a measure of earth rotation as Universal Time The CCIR decided to eschew the entrenched notion of 86400 seconds = 1 day as a practical way to satisfy the two incompatible demands for the purposes of radio broadcast time signals. This is effectively the same as the doomed never to succeed World Calendar and its intercalary Worldsday. The subsequent proliferation of other time scales shows that UTC was not favored by users with precise technical needs of either flavor. In the past 50 years the needs of users of radio broadcast time signals have changed. It is no longer imperative that radio broadcast time signals directly provide Universal Time, and it has become imperative that the radio broadcasts provide uniform measure of elapsed time. This is saying that requirement 2) has lost importance, not that the definition of Universal Time has become deficient. UT is just not what is needed by the principal users of the broadcasts. The ITU-R cannot erase a century of textbooks, laws, statements, and algorithms which all recognize Universal Time as a subdivision of calendar days of earth rotation. Whereas it is true that Universal Time is not time, it is also true that Atomic Time is not date in any way that calendars have meant since antiquity. At WRC-15 next year the ITU-R has no choice about the nature of the radio broadcast time signals; they must become a purely atomic time scale. The ITU-R does have a choice about the name of the time scale in the radio broadcasts. If WRC-15 decides to change the nature and retain the name then they will be repeating the mistake their predecessors made in 1970: trying to pretend that one technical time scale can serve two incompatible technical purposes. I expect that will result in yet another generation of confusion and flame wars, until all of the people and systems who know the current definition of Universal Time are gone. -- Steve Allen <[email protected]> WGS-84 (GPS) UCO/Lick Observatory--ISB Natural Sciences II, Room 165 Lat +36.99855 1156 High Street Voice: +1 831 459 3046 Lng -122.06015 Santa Cruz, CA 95064 http://www.ucolick.org/~sla/ Hgt +250 m _______________________________________________ LEAPSECS mailing list [email protected] https://pairlist6.pair.net/mailman/listinfo/leapsecs
