[LEAPSECS] Tripping over the International Date Line

[Steve Allen]

On Wed 2007-03-07T20:04:04 +0000, Zefram hath writ:

> In the absence of DST or

> other changes (the story predates DST), "day" in a civil context and

> without any qualifier must be taken to mean civil days at any fixed

> location.  Which location doesn't make any difference, of course, when

> using the day only as a unit of duration.  You'd have a hard time arguing

> in court to measure solar days at a moving location.


But this seems exactly the de-motivation for the IAU and IERS folks
paying any attention to the notion of Universal Time anymore.
The current paradigm for earth rotation eschews the validity of any
geographic entity as a reference point (other than in a least-squares
sense for defining the underlying and more "Platonically ideal" ITRS).

This notion has been growing ever since BIH was given authority to
monitor all broadcast time signals, and it can be traced back at least
as far as remarks between Janssen and Newcomb at the International
Meridian Conference.  In a world with tectonically moving plates there
can be no location which defines "day" at the microsecond level.

And, just to confuse the notion of day and night further, a pair of
papers by a guy who notes that even the IAU 2000 resolutions preserved
a lot of "classical cruft" with the implicit use of an ecliptic in the
precession model and the new explicit definition of an equator.

http://arxiv.org/abs/astro-ph/0611781
http://arxiv.org/abs/astro-ph/0611782

The upshot being that if you throw away all those classical notions
you can fit the earth rotation using 31 parameters instead of 46000.
Of course in this scheme there is only the earth and the quasars; the
sun need not apply.  It's also not clear whether the method leads to
anything like a useful long-term predictive model.

--
Steve Allen                 <sla at ucolick.org>                WGS-84 (GPS)
UCO/Lick Observatory        Natural Sciences II, Room 165    Lat  +36.99845
University of California    Voice: +1 831 459 3046           Lng -122.06025
Santa Cruz, CA 95064        http://www.ucolick.org/~sla/     Hgt +250 m