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

From: Poul-Henning Kamp <>
Date: Sat, 07 Jan 2006 00:32:44 +0100

In message <>, Rob Seaman writes:

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

Good point.

> Civil Time = the common basis for diverse time usage worldwide


Civil Time is a legal term, and you have no power to redefine it.

        Civil Time =
        Legal Time =
            what the applicable law says the clock should show.

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

No, No, No.

        Solar Time ~ Mean Solar Time ~ UT1 =
                various astronomical/geophysical concepts.

        GMT = Legal time in the UK, defined by parliament.

        UTC = Standard defined by ITU-T, based on SI seconds.

        Universal Time = confusing term which comes handy when trying to
                manipulate discussions about leap second futures.

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

(Partially) Wrong again.

        Daylight Saving Time is a component of Legal time and very
        relevant to our discussion.

So lets try again, and let us focus on only the bits we need to look at:

                TAI(n) = TAI(n - 1) + 1 SI second

                Owned by BIPM / Metre Convention

                UTC(time) = TAI(time) + Leap(time)

                Owned by ITU.
                IERS evaluates Leap(time) according ITU definition

        Civil Time(country)

                Civil Time(time) = UTC(time) +
                             TimeZoneOffset(country, subdivision, time) +
                             SeasonalOffset(country, subdivision, time)

                Owned by government of country. Some politically
                backwards countries like Denmark have not after a century
                managed to get their laws aligned with reality, but that
                is merely a matter of political unexpediency.

At this level we don't need to look at UT1 or any of the other
timescales. 99.999% of the Earths population only see those
manifested as hidden variables in the Leap(time) function.

So what we are discussing here is only the Leap(time) function.

You will notice that this function has one argument only: time.

As surprising as this may seem, that is actually the way it is.

Leap(time) is a function that is defined as evaluating to an integer
number of SI seconds as a function of time and it is defined piecemal
with a horizon of 6 to 12 months from the present.

Once IERS have made up their mind, the function doesn't change
again, even if we find out that Earth Orientation was different
from what they thought it was.

Behind the scenes, IERS uses UT1 to decide how Leap(time) develops because
that is how ITU defined the function, but that is a hidden process because
nobody can evaluate it definitively apart from IERS.

If you look at the functions
        TimeZoneOffset(country, subdivision, time)
        SeasonalOffset(country, subdivision, time)

You will find that with a margin of a couple of hours to both sides
their values tend to make the statement "sun highest in the sky at
12:00" true. This trend is of course not accidental.

You will also appreciate that should the relevant governments feel the
desire, they can redefine both functions without consulting anybody but
the citizens of the country in question.

If in a hypothetical scenario, the citizens of country N notices
that because of continental drift, stupid politicians or the way
the UTC timescale is defined, the sun doesn't tend to be highest in
the sky around noon, they are perfectly free to elect some politicians
who with what goes for sufficient warning in that country can redefine
the two functions to make it more so.

Now tell me why you think Leap seconds are so important again.

Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk_at_FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
Received on Fri Jan 06 2006 - 15:43:35 PST

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