[time-nuts] FW: Bulletin C number 30

[Magnus Danielson]

From: "Poul-Henning Kamp" <phk at phk.freebsd.dk>
Subject: Re: [time-nuts] FW: Bulletin C number 30 
Date: Tue, 05 Jul 2005 08:29:44 +0200
Message-ID: <33697.1120544984 at phk.freebsd.dk>

Hej Poul-Henning!

> In message <20050704.212118.87245102.cfmd at bredband.net>, Magnus Danielson write
> s:
> 
> >> And btw, it probably would not even be a leap-second for him, since
> >> general relativity would take its toll.  I'm not sure my grasp of the
> >> math is good enough to figure out how long his leap-second would be.
> >> 
> >> Instead, if we abandon leap-seconds, then we finally have a _truly_
> >> universal timescale.
> >> 
> >> It will not be locked to any more or less random piece of geophysics,
> >> anyone with a cesium clock and a set of gen-rel coordinates will be
> >> able to figure out what time it is, and time intervals can be measured
> >> and compared without weird gottchas.
> >
> >No. You are missing a detailed refinement in the definition of a second, it is
> >assumed that the Cesium clock is at sea-level.
> 
> Sea-level on this planet, yes.

Whatever sea-level on this planet is. It's not at the sea-level at all. You
have to measure the gravity and compensate for that to some constructed and
standardised sea-level. The gravity and sea-level maps of the earth is
interesting to study and when you think of it in this context you really start
to wonder. There are pretty accurate gravity instruments that are among other
things used to measure when vulcano bursts is due to happend and stuff like
that.

> If you are on a different planet in a different orbit and a differnet
> rotation period (and axis!), general relativity takes a toll.
> 
> If you bring a HP5071A to Mars, it will give you a wrong length of
> seconds.

Indeed. There is an 24 hour cycle which is due to the shift in gravity position
relative to the sun as the earth turns. The amplitude is however quite small
and I'd guess that the temperature shifts from the same cycle makes larger
influence on most clocks.

Relativity bites you many times when trying to do this.

The Universal in UTC is to indicate our world, the earth, and not to be
confused with the universe as in deep space. It was never meant to include
Mars for instance.

> >Considering how time-zones is set, I start to wonder. Look at the time zones
> >in South America and you see what I mean.
> 
> Just look at Europe :-)

I try not to! ;O)

> >> The other half is that leap-seconds are just not testable in a computer
> >> setting, and therefore I am sure that any cost of dropping them will
> >> be totally offset by the savings in the IT industry.
> >
> >I am not even sure that the IT industry is really spending a whole lot on this
> >issue. Most of them is to the best of my knowledge fairly ignorant to this
> >among other problems. Just having charging systems track UTC through NTP would
> >be a huge step forward IMHO.
> 
> Well, I think the disruption is obscured by the lack of implementation.
> As more systems implement leapseconds (through NTP or otherwise) we will
> see more applications which didn't do it quite right.

Which is a better wording of my point. ;O)

When different IT systems can be off by minutes (I have received the reports
about that some telcos have this problem for instance) I think there are a
bigger issues to handle first.

But, Werner is bringing out a very important point, if you *do* care about
doing it right, you need to do it properly and that involves caring about many
different things, including the relative brokenness of many systems and the
consequence when the systems missalign. Some of the systems could potentially
be fixed thought. NTP with the propper patches on kernel and software in
general will indeed solve parts of the problem, at least in the UNIX world.

Cheers,
Magnus