[time-nuts] International time meridian

[Rob Kimberley]

Bill,

The  Bureau International des Poids et Mesures (BIPM) http://www.bipm.org
are the International coordinators of time, as well as other the standards
of length, mass, so you were right with your French Connection (good film by
the way!!). The Greenwich Meridian (0 degrees longitude) runs through the
middle of the Greenwich Observatory in London. Great place to visit if you
ever get over to the UK.

Check out the link given for more info, and www.npl.co.uk/time has useful
links also.

Best

Rob Kimberley

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Bill Hawkins
Sent: 14 January 2007 19:04
To: 'Discussion of precise time and frequency measurement'
Subject: [time-nuts] International time meridian

A non-hardware/software query:

Some time ago, I got it into my head that GMT was obsolete and that the time
meridian went through Paris, not Greenwich.
I answered a query in the Glowbugs list that way, and then used Google to
check up on that. Unlike others in lists, I feel bad about propagating
misinformation. But not bad enough to look it up first.

Wikipedia is not helpful about the status of GMT. As I understood it, the
prime meridian was defined in 1884, and GMT with it. Then atomic time
replaced the telescope that determined GMT in 1972.
The agreement was reached in Paris, no? Paris has the physical standards of
meter and kilogram, so why not atomic time.

Wikipedia says the prime meridian is still also the time meridian.
Paris is about 9 minutes ahead of London, and nobody made a nine minute step
change in social time. The wiki also says that GMT is still in use.

1/1/1977 was set as the base for average atomic time, according to the wiki,
after relativistic effects were compensated. So TAI marches monotonically
forward, while UTC subtracts leap seconds to maintain social time. Yes,
there are people who want to stop this irritating leap second business until
well after they are dead, but they have no "exit strategy" to deal with a
much bigger jump in social time at a later date.

In any event, social time is only occasionally (about annually) adjusted by
stellar observations, so technically the basis for GMT no longer exists.
Right?

Did see a 1999 article that said Paris was going to plant a boulevard of
trees along the Paris meridian that could be seen from outer space. Dunno if
they did it.

Also, satellites operate from a prime meridian that is offset by 102.5
meters to the east, because Earth based telescopes are aligned to the center
of gravity with a plumb bob. Satellites need to be aligned with the physical
center of the Earth, for some reason.

See http://en.wikipedia.org/wiki/Prime_Meridian

and http://en.wikipedia.org/wiki/International_Atomic_Time

Regards,
Bill Hawkins


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