Re: [LEAPSECS] leap second policy making (correction)

From: Demetrios <dnm_at_orion.usno.navy.mil>
Date: Wed, 30 Aug 2000 18:05:44 +0000 (UTC)

> This mailing list has been so quiet that I actually reread one of my
> own messages - and found this erroneous passage:

As the person who had this listserv set up, I have tried to be "laid back".
This was particularly easy to do because I was away from my desk for three
weeks. But I am somewhat surprised that the PTTI community haven't gotten
more involved. I will shortly be submitting a copy of the IAU's decision to
create a working group to study the issue, jointly with other international
bodies, and to report in 3 years.

>
> Note that currently UTC is tied to the location of the prime
> meridian. One interpretation of a floating UTC is a floating
> prime meridian. We may be generating a vast market for surveyors
> in Great Britain over the next century as the meridian moves a
> couple of miles east of the Airy Transit Circle.
> (Or is it west? Quick! Which way?)
>
> The estimate is that UTC will diverge from UT1 by about 140 seconds over
> the next century. That's two minutes of time, but about 35 minutes of arc.
> At the equator a minute of arc is a nautical mile. At the latitude of
> Great Britain, 140 seconds of time is about 20 miles.
>
> The final question remains, however. Which way does it move? If it's
> hard for us to have complete confidence in sorting out this not entirely
> pedagogical question in our own heads - should we be generating a
> situation in which laypeople (bilyuns of 'em) would have to invert
> the process in the future?

I don't think surveyers would be affected. The Greenwich meridian is now
just a (fat) line in the ground for tourists. The terrestrial reference frame
is nowadays defined by the assumed position of VLBI and GPS sites,
with due allowance for continental drift, glacial rebound, etc. (This
system was initially determined so as to coincide with best Greenwich
measurements, of course.)

But in answer to your question: if the Earth slows down and we do not
compensate, then every day the mean sun will arrive overhead at Greenwich
at a slightly later clock time. So when the clocks say noon, the mean
sun will be on the meridian at a more easterly position, and Paris Mean
Time will be resurrected :)

Demetrios Matsakis
Received on Wed Aug 30 2000 - 11:05:38 PDT