[time-nuts] International time meridian

[Bill Hawkins]

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