[ntp:questions] Re: Extracted fom comp.risks - What Time Is It?

H. Peter Anvin hpa at zytor.com
Mon Aug 18 05:35:05 UTC 2003

Followup to:  <bh96sf$quh$1 at nntp.webmaster.com>
By author:    "David Schwartz" <davids at webmaster.com>
In newsgroup: comp.protocols.time.ntp
> "Piotr Trojanek" <ptrojane at mion.elka.pw.edu.pl> wrote in message
> news:slrnbjelf1.b6d.ptrojane at mion.elka.pw.edu.pl...
> > In article <qDyYa.17$GF.676009 at news-text.cableinet.net>, David J Taylor
> wrote:
> > >  http://www.guardian.co.uk/uk_news/story/0,3604,985020,00.html
> >
> > "The Bureau International des Poids et Mesures in Paris sets TAI time
> > by monitoring the regular vibrations of caesium atoms in atomic clocks
> > around the world."
> >
> > (I hope it's OK to extract a single item - Piotr)
> >
> > does anybody know, how this monitoring is done?
> >
>     Google for 'cesium beam clock' and/or read articles like this one:
> http://www.bartleby.com/65/at/atomiccl.html

I think the big question is how do you *synchronize* these various

An atomic clock tells you what time it is in one particular location.
Any kind you move a clock you have to deal with time dilation issues,
and when you communicate its time using signals you have propagation
issues.  To make things even more complex, the speed of time actually
varies with the strength of gravity and therefore varies with
altitude, latitude and geological composition, although I *believe*
this is currently below the threshold of measurability (time dilation
due to moving an atomic clock is, however, very much measurable and
can amount to tens or even hundreds of nanoseconds on a
round-the-world journey.)


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