Re: [LEAPSECS] Mechanism to provide tai-utc.dat locally

From: Steve Allen <sla_at_UCOLICK.ORG>
Date: Thu, 28 Dec 2006 23:29:01 -0800

On Thu 2006-12-28T22:07:08 -0700, M. Warner Losh hath writ:
> I know the benefits, but nobody has yet produced a study on why 0.9s
> was chosen.

That's pretty easy. In 1969 the CCIR subcommittee was preparing its
unilateral decision to switch from rubber seconds and 200 ms steps
to leap seconds. They were disregarding the other international
scientific organizations who had called for a multi-lateral meeting
on how to deal with time and frequency broadcasts.

When the CCIR plenipotentiary voted in early 1970 their resolution
said the limit was 0.7 seconds. When the IAU met in the middle of
that year they had no correspondence from the CCIR, so they could take
no action and provide no response to give input before the 1972 deadline.

The limit of 0.7 was somebody's idea of how well the astronomers
could do on a six month schedule given the observing techniques and
telecomm and computational means available in 1970. Unfortunately at
that time the earth rotation was almost as slow as it has ever been,
so it seemed likely that there would have to be a lot of leap seconds,
and whoever was in charge tended to jump the gun on inserting them such
that the DUT1 value was often very close to the 0.7 s limit.
In 1973 the IAU did have standing to respond to the CCIR and recommended
0.9 s, which the CCIR then implemented despite the fact that the WWV
coding for DUT1 could not go past 0.7 s.

So the indications are that 0.7, and then 0.9 was as close as
anyone believed was possible in the early 1970s on a semi-annual
schedule. It was a compromise then, and a few people were very,
very upset about the situation, but so far as I know no ships
ran aground as a result. The subject of leap seconds causing
planes to crash was raised then, too, and that didn't happen

This story may not be exactly right, but the existing public documents
make it seem pretty close to the truth.
Who exactly the principals were in these cases is still obscure;
some of them have died, and others have not. Maybe the future will
have access to existing memoirs of some of the dead folks, and maybe
some of those living can be exhorted to leave their views behind.

So we don't really need a study of the history of why 0.9 because
the record of what happened and who did what when is pretty good
and that's all kindof moot for solving the dissatisfaction now.

The point of keeping DUT1 small was mainly for consistency
with existing navigation practices, and also for ease of
broadcasting that difference. I don't think that either of
these constraints is relevant anymore.

Steve Allen                 <>                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     Hgt +250 m
Received on Thu Dec 28 2006 - 23:29:16 PST

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