Defining our terms (was Re: [LEAPSECS] Longer leap second notice)

From: Rob Seaman <seaman_at_NOAO.EDU>
Date: Fri, 6 Jan 2006 15:48:20 -0700

I said (and continue to assert):

>> civil time (as we know it) IS mean solar time

John Cowan demurs:

> Why do you persist in claiming that "all parties must certainly
> agree" on something that is precisely the point most in dispute?
>
> No communication with you is possible until you accept that there
> *is* a controversy over this.

There is a controversy. It is not over the identification of civil
time with solar time. The controversy is over how close a tolerance
is required between the two and how frequently they must be
resynchronized.

...or else, what have we been talking about all this time?

Poul-Henning Kamp also takes exception:

> Civil time is at best local solar time +/- up to several hours in
> either direction depending who you are and where you are.

The one thing Civil Time is NOT is Local Apparent Solar Time.

Perhaps what we need is simply to define our terms. A lot of the
friction on LEAPSECS undoubtedly comes from conflicting meanings.

        Civil Time = the common basis for diverse time usage worldwide

Civil Time is not what real clocks read - it is the idealized master
clock.

        Solar Time ~ Mean Solar Time ~ Universal Time ~ GMT ~ UTC ~ UT1 =
various approximations to the baseline of Earth orientation

For our purposes, we are interested in Mother Earth's bottom line -
the long term trend - not in her daily or seasonal or annual wiggles.

        Standard Time ~ Local Mean Time ~ Local Apparent Time ~ Sidereal
Time ~ Daylight Saving Time, etc = not pertinent to our discussion

For instance, Daylight Saving Time is a trivial seasonal offset to
Standard Time. Standard Time in one time zone is as good as in
another time zone. Offsets from Standard Time to Local Mean Time are
simple functions of longitude. The equation of time is a
straightforward reflection of the shape of the Earth's orbit and of
the Earth's inclination to that orbit. These are all irrelevant (if
interesting) details.

During all this, Civil Time continues to click steadily away in
synchronization with our home planet. The question is how steadily -
and how far it is permitted to drift before action is taken - and
what kind of action is appropriate - and who gets to make these
decisions.

Rob Seaman
National Optical Astronomy Observatory
Received on Fri Jan 06 2006 - 14:48:46 PST