[LEAPSECS] Lets get REAL about time.

[Tony Finch]

Rob Seaman <seaman at noao.edu> wrote:

>

> Well, no.  The geographic center of California is about exactly 8 hours

> (120 degrees) west of Greenwich.  Los Angeles is a couple of degrees

> East and San Francisco a similar distance West of this meridian.  Even

> for California plate tectonics can be ignored, and PST will always

> remain naturally 8 hours from GMT.

>

> Both PST and GMT are stationary with respect to Universal Time.

> Historians or futurists, scientists or other long term planners can

> predict the relative activities in these two places on a given future

> calendar date, or proleptically backwards indefinitely even over periods

> that the length-of-day varies, precisely because it is days that are

> being counted.

>

> If Coordinated Universal Time is redefined, the foundation of this

> trembles.  For a given UTC we would no longer even be able to predict

> the day it corresponds to [...]


I agree with Clive's response to this but I feel the need to unpack it a
bit more.

You're mixing up at least five timescales with very different
characteristics here, and expecting their relations to remain predictable
thousands of years into the future. You are also using rather imprecise
terminology which makes the tangle particularly knotty: by "GMT" I'm not
sure if you mean UTC or UT1 or British time in winter or Microsoft-style
British time at any time of year.

Anyway, you seem to be talking about standard time zones (not local mean
solar time, say) but you are ignoring the fact that standard time isn't
used for more than half of the year, and a place might choose to use a
time that's very different from their standard time. (And change their
mind back, as Britain has done a couple of times.) Over the next ten years
it is very possible for the time difference between California and Britain
to be anything from 7h (California on PDT, Britain on WET) to 10h
(California on PST, Britain on CEST). So even in the near future it is the
case that planners cannot predict the relative activities in these two
places.

Having said that, in the far future it is likely that British time will
still be roughly centred on UT1 and California time will be roughly
centred on UT1-8h, plus or minus a couple of hours each way. That is, the
timezone system will be used to keep them stationary wrt universal time,
though with a fair amount of slop - but that slop is the same as the
present-day slop.

So the foundation does not tremble. You only think it trembles because you
are falsely assuming that the diverging difference between TAI and UT1
will lead to people getting up in the night or miscounting the days of
their calendars.

Yes you won't be able to predict the relation between atomic time and
solar time - that is of course physically impossible. Yes you won't be
able to predict what the politicians will decree about time zone offsets
and daylight saving - and that is also necessarily impossible. But neither
of these impossibilities imply that local civil time will diverge from
local solar time.


A somewhat irrelevant aside: even if you are talking about local mean
solar time, over the timescales you are worrying about it is not the case
that 120 degrees equals 8h (28800 SI seconds) because tidal slowdown is
making it longer. And earthquakes and plate tectonics will add further
disturbances to the time difference between California and Britain even if
you use UT1 seconds. But this is all milliseconds rather than hours...

Tony.
-- 
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