There are a lot of timescales. Astronomers are as heavy users of even
interval time as of Earth orientation time. I guess the answer to
your question lies in the explanatory supplement to the astronomical
almanac. Recent IAU standards changes have yet to appear in a
similarly normative form. Steve Allen may also comment.
But the issues focus on the requirements of civil timekeeping, not the
various precision timekeeping users like astronomers. I won't repeat
my argument about why civil time is mean solar time, but I assert that
it is. In addition, we all grasp the various issues regarding even
interval timekeeping for computing. That adds up to two types of
civil time.
UTC does a pretty good job of capturing both flavors of time into a
single mechanism. It's ok by me, however, if there is a initiative to
split them. It is not ok if we try to pretend that one does not exist
and I assert this will become obvious to all in the fullness of time.
Rob
--
On Feb 14, 2008, at 7:07 AM, Tony Finch wrote:
On Wed, 13 Feb 2008, Rob Seaman wrote:
The "day" is a key concept in our civilization. The "mean solar
day" is
the natural way to implement this. Sundials have nothing to do with
the
mean solar day, but rather the apparent solar day.
How does the mean solar day relate to ephemeris time? Between the
period
where time signals were based on observing solar transitions and the
period where time signals were based on atomic clocks, official time
was
ephemeris time. ET is more uniform than the Earth's rotation so a
second
of ET is not 1/86400 day any more than an SI second is - and of
course the
final definition of the SI second was chosen to match ET.
If we can imagine what things might have been like if essens had a
different period, can we imagine how astronomers would have handled
the
discrepancy between ET and UT1 in the absence of atomic clocks?
Tony.
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