Ashley Yakeley wrote:
>I'd like to see an elastic "civil second" to which SI nanoseconds are
>added or removed. Perhaps this could be done annually: at the
>beginning of 2008, the length of the civil second for the year 2009
>would be set, with the goal of approaching DUT=0 at the end of 2009.

This was tried, between 1961 and 1972.  It sucked.  There is a big
problem with having two kinds of second in common use, especially with
such close values.  They get mixed up.  It is very convenient to have
civil days made up of SI seconds, even if the number of one in the other
has to vary in order to fit.

A technical issue: broadcast time signals are phase-locked with the
carrier, which is at some exact number of hertz.  If the time pulses are
every civil second, and that is now 1.000000015 s (as it was in 1961),
it can't be synchronised with the (say) 60 kHz carrier that must still
have exactly 60000 cycles per SI second.

The historical trend is towards using uniform time units.  It seems
curious now that when the atomic clock was invented astronomers opposed
calling it a "time" standard.  They wanted to keep the Earth's rotation
as the ultimate "time", and accepted atomic clocks only as *frequency*

It is much like the ancient Egyptians (IIRC) making the transition from
sundials to water clocks.  They had always marked out hours on sundials
subtending equal angles, so the actual temporal length of the hours
varied over the course of the day.  When the water clock gave them
an independent time reference, they were horrified that its uniform
hours didn't match sundial hours.  Much technical ingenuity went into
mechanical modifications to water clocks to make them accurately emulate
what went before.

>Actually I was going to suggest that everyone observe local apparent
>time, and include location instead of time-zone, but I think that
>would make communication annoying.

Yes.  It would be a big step backward.  Look up the history that led to
the adoption of standard timezones: it was prompted by the invention of
railways and telegraph.  The trend in this area is towards increasing
agreement of time over progressively larger geographical regions.
Projecting into the future, one can foresee the eventual abandonment of
timezones in favour of the universal use of Universal Time.


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