On Tue 2003-01-28T23:10:07 -0500, John Cowan hath writ:
> Dr. Mark Calabretta scripsit:
>
> > Much of the problem boils down to the question of why we would want
> > to continue to pretend that a mean solar day has exactly 86400 SI
> > seconds when in fact, it has 86400+epsilon SI seconds.
>
> I at least care that a *civil* day be 86400 SI seconds in length.
> Mean solar days don't affect me to speak of.

Mark's suggestion verges on yet another solution that might be adopted
by some for the sake of civil time.  Any telescope with a polar mount
already has a clock that keeps another kind of time -- sidereal time.
This is counted in sidereal seconds, of which there are always 86400
during one rotation of the planet (an interval which is currently
about 86160 SI seconds, and getting longer).  In this sense sidereal
seconds are not really a measure of elapsed time, for they vary in
duration; they are actually a measure of earth rotation.  But in the
old days of telescope control the difference between earth rotation
and time duration was less than the drift of any clock.

If in the far future when the earth has slowed significantly it is
still deemed important for civil time to have 86400 civil seconds in a
day, then that future society will have decided, like the astronomers,
to keep two kinds of seconds in common use.  It may not even be
necessary to look to the far future if the proposal to drop leap
seconds were to cause some (religious?)  societies to reject the use
of UTC for civil time.

Which is more important...
        for civil time to be counted in SI seconds?
        for civil time to track the rotation of earth smoothly?

Mark's alternative resembles the civil time solution adopted by the
martian colonists in Kim Stanley Robinson's "Red/Green/Blue Mars"
trilogy, where the clocks tick SI seconds, but every day at midnight
they stop for 39.5 minutes of "slip time" to let the planet catch up.

If we have decided that SI seconds are to be used,
which is more important...
        for civil time to keep noon from drifting?
        for civil time to increase constantly?

The above scenarios may seem infeasible in a real society, but if the
clocks of the future have access to TAI (for the sake of timestamping,
precision time, and legal time), and also have access to UT1 (for
civil time), then they could be preferred by some groups of our
descendants.

> What for?  Why should we (the people of the Earth) care about mean
> solar days?  For some purposes, apparent solar time is important, but
> most of the time it's civil time that counts.  Why should that be tied
> to mean solar days?

Partly because eventually the scheme of turning UTC into a constant
offset form of TAI requires a leap hour lest our descendants find
themselves having their midday meal at 18:00 local civil time.  This
sort of pushing the problem off onto our descendants implies that we
really don't have the right solution, just one that salves some needs
now.

> >       I suspect this would account for 99.9% of the world's clocks,
> >       including the clocks inside most computers, VCRs and microwave
> >       ovens; on your wrist; or next to your bed.
>
> Hmm.  How reasonable is it to expect this to change in future?

If the future is a world of ubiquitous networking where Bill Gates can
make every wristwatch and refrigerator magnet into a .NET client, then
we should very much expect this to change.

I doubt that we can definitively answer the questions posed above in a
way that will satisfy the needs and technological [lack of?]
constraints for all of our descendants.  For that reason I remain
a proponent of not modifying the meaning of time scales that work now.

--
Steve Allen          UCO/Lick Observatory       Santa Cruz, CA 95064
[EMAIL PROTECTED]      Voice: +1 831 459 3046     http://www.ucolick.org/~sla
PGP: 1024/E46978C5   F6 78 D1 10 62 94 8F 2E    49 89 0E FE 26 B4 14 93

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