On Mon 2007-01-01T17:42:11 +0000, Ed Davies hath writ:
> Sorry, maybe I'm being thick but, why?  Surely the IERS could announce
> all the leap seconds in 2007 through 2016 inclusive this week then
> those for 2017 just before the end of this year, and so on.  We'd have
> immediate 10 year scheduling.

For reasons never explained publicly this notion was shot down very
early in the process of the WP7A SRG.  It would almost certainly
exceed the current 0.9 s limit, and in so doing it would violate the
letter of ITU-R TF.460.

The IERS may not be a single entity so much as a confederation of
organizations competing for scientific glory and using the umbrella to
facilitate funding from each of their national governments.  Even if
the IERS were monolithic they would have to obtain approval for such a
change from the ITU-R, IAU, IUGG, and FAGS.  Given the tri/quadrennial
meeting schedules it seems unlikely that the IERS could obtain
approval much before year 2010.

> Maybe we can turn this question round.  Suppose the decision was made
> to simplistically schedule a positive leap second every 18 months for
> the next decade, what would be the effect of the likely worst case
> error?  First, what could the worst case error be?

McCarthy pretty much answered this question in 2001 as I reiterate here

> As Rob has pointed out recently on the list, 1 second in time equates
> to 15 seconds of arc in right ascension at the celestial equator for
> telescope pointing.
> For the professionals I'm not so sure but

Give us a few years of warning and I think we can cope.  No telescope
I know uses ICRS, we're all still using FK5 and/or FK4.  That means we
astronomers already know (or at least ought to know *) that we all
have to do a software update.

> For celestial navigation on the Earth, a nine second error in time
> would equate to a 4.1 km error along the equator.  Worth considering.

The format of the almanacs would be changed along with the change
in UTC such that by including one more addition there would be
no worse error than now.  This would be a change much smaller in
magnitude than what the Admiralty did in 1833.

> Is it really likely to be a problem, though?

I think not.  It's hard to prove not.
None of the agencies involved has the funding to mount a survey
which would motivate all affected parties to investigate.

(*) While standing near the UTC poster at ADASS I was accosted by a
software engineer whose PI had instructed that all observation times
be reduced to "heliocentric UTC".  Upon discussion it became clear
that the PI had not clearly distinguished between heliocentric and
barycentric.  Furthermore, there was no concept that UTC is only
defined at the surface of the earth and that the only suitable time
scales are TCB and TDB.  (TDB would be the natural result because
ticks along with UTC and because that's what the JPL ephemerides use.)
The need for pedagogy never ends.

Steve Allen                 <[EMAIL PROTECTED]>                WGS-84 (GPS)
UCO/Lick Observatory        Natural Sciences II, Room 165    Lat  +36.99858
University of California    Voice: +1 831 459 3046           Lng -122.06014
Santa Cruz, CA 95064        http://www.ucolick.org/~sla/     Hgt +250 m

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