[time-nuts] UTC ~ GMT

Rob Seaman seaman at noao.edu
Sun Jul 31 19:35:12 EDT 2005

Just checking in - glad to see the conversation continues.  I think a  
cafeteria reply might be warranted.  Press delete now if you disagree.

Poul-Henning Kamp says:

> My interpretation of this is that systems which assume that DUT <  
> 1s fail

This is one example of a larger class of risks that will arise from  
breaking the identification of civil time with GMT that has held  
since the nineteenth century.  The ad hoc interpretation of no  
individual or small group is sufficient to address the widespread  
implications of such a change to the world's clocks.  Any serious  
proposal should include a serious risk analysis.  Since we're talking  
about changing an international standard that has held for 30+ years,  
the onus falls on the group proposing a change.

> That's probably true, but since DUT is only relevant if you study  
> extraterrestial objects, we can safely assume that 99.9% or more of  
> those systems involve astronomers and optics.

The Sun is an extraterrestrial object.  We study the Sun not only due  
to basic scientific curiosity, but because everything we do on the  
Earth depends on the Sun.  Civil time is solar time because no other  
periodic phenomenon so dominates our lives.  Leap seconds are the  
mechanism that is currently used to synchronize our clocks to the  
solar day.  Your assumption of safety is in reality a source of  
extreme danger until proven otherwise.

Bill Hawkins says:

> The WSJ says that the UK still uses GMT. When the NASA person  
> called out "On my mark..." time during pre-launch activities, it  
> was specified as GMT.

Precisely.  The implicit and explicit identification of UTC with GMT  
is rampant in civil time usage.

> What relationship does GMT have to UTx time? The hill in Greenwich  
> is now just a museum, right?

An excellent question - not just a technical issue, but legal and  
historical and...

In a world without leap seconds, one interpretation is that Greenwich  
will start drifting monotonically out to sea.  One second of time is  
15 seconds of arc.  At the latitude of the UK, each missed leap  
second is thus equivalent to about 300 meters.  Feel free to correct  
me and to speculate on the direction of motion of the prime  
meridian.  A large part of the expense of discontinuing leap seconds  
is the confusion factor.  Think this is an absurd example?  It isn't  
if the proposal doesn't delve into its own (many) implications.   
Whatever else it is, the current proposal is embarrassing in what it  
does not address.

Bjorn Gabrielsson says:

> Legal time is often based on UTC. A slight adjustment of the  
> definition of UTC will not break legal timekeeping.

There are worlds of pain in the words "often" and "slight".  Lawsuits  
resulted from the nineteenth century transitions from apparent solar  
time to mean solar time, and then again from local time to standard  
time.  Anyone want to suggest that the twenty-first century is less  
litigious than the nineteenth?

> if system A breaks at DUT1 of 1.0seconds B & C might die at 5 and  
> 10 seconds, giving plenty of time to patch the systems. System A  
> does seem to live on the edge already, maybe a good candidate to  
> patch earlier...

Interesting attempt to convert the stochastic realization of failures  
into a strength.  To triage the remediation of systems A, B and C,  
somebody will have to perform a risk analysis first.

> Maybe the astronomers have not been following the lengthy  
> discussions among the real timekeepers of today. Whos fault is that?

Astronomers have been following - and leading - such discussions  
since this issue reared its ugly head.  It is not our fault that the  
policy makers don't choose to participate in the same forums.  This  
is not only true of time-nuts, but also, for instance, the leapsecs  
mailing list (http://rom.usno.navy.mil/archives/leapsecs.html).

I reject your premise that bureaucrats are now the "real timekeepers".

Bill Hawkins says:

> Please do something useful and tell me how I can defeat the effort  
> to remove leap seconds, or worse, put leaps off until they are more  
> acutely felt by the next generation.  [...] Let us have activism,  
> not argument.
Sometimes argument is the only activism available.  I believe  
"unofficial" discussion is the only reason the "official" process has  
not proceeded to its intended completion.

Rob Seaman
National Optical Astronomy Observatory

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