[LEAPSECS] How good could civil timekeeping be?

Rob Seaman seaman at noao.edu
Thu Feb 14 12:19:54 EST 2008

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.


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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