Re: [LEAPSECS] Comparing Time Scales

From: Rob Seaman <seaman_at_noao.edu>
Date: Sat, 4 Feb 2006 12:38:46 -0700

> So internally, once we hit the leap second time, we step backwards.

Lovely waxing crescent moon last night. I could see it out of the
north facing windows of our family room. It might be simpler in some
sense to pretend that the moon (and the sun for that matter) always
rises due east and set due west, but there it was, shining in the
window because its declination last night happened to lie above the
equator.

Whatever time does, it does not step backwards. It may be simpler -
in some sense - to build a clock or software interface that
accommodates the lunar correction to the solar day by stepping
backwards more or less contemporaneously with a leap second, but when
you get down to it, this is - well - a naive kludge.

> If you freeze time, then doing a time exchange during a leap second
> is going to give bogus results.

Time neither freezes...

> If you step it back, then it will give good results, but there are
> other bad effects.

...nor steps back during a leap second. Just like the 29th of
February, a leap second is an extra unit of time that is interpolated
into the grand eternal sweep of history and causality. Why not
design our computers, clocks and communications technology to
implement that simple fact - just like our calendars and palm pilots
recognize the fact of an extra day in February every four years?

Hey gang! Let's put on a Gedankenexperimenten! Imagine we are faced
with the prospect of a quadratically accelerating sequence of
negative, rather than positive, leap seconds. (We'll ignore the
trifling anthropic dilemma of the moon looming ever closer in our
sky.) Two questions to ponder: How would we implement these? And
would the ITU feel more driven - or rather less - to modify UTC as a
result?

To address the first question, we can start with the statutory
requirement from ITU 460-4:

        "2.2 A positive leap-second begins at 23h 59m 60s and ends at 0h 0m
0s of the first day of the following month. In the case of a negative
leap-second, 23h 59m 58s will be followed one second later by 0h 0m
0s of the first day of the following month"

As we can see from the current discussion, system designers don't
necessarily even try to implement statutory requirements. Notions
about freezing time or of stepping it back arise - it seems to me -
out of balking at the clear and explicit requirements of the task at
hand. But in the case of a negative leap second, there would be no
additional wrinkle in time to iron out or to double over - just the
simple need to omit a second from the count.

If we were merely faced with omitting a second every year or two,
would the requirement really seem particularly onerous from the point
of Posix or NTP or our other technology? Would GLONASS have noticed
the event even at the minimal level seen this New Year? What is it
about a 61s minute that is deemed more herculean than a 59s minute?

We manage to deal with months containing 28, 29, 30, and 31 days.
The great majority of months (4497/4800) aren't even denumerable with
the length of our week. Why then is a requirement that one minute
out of 800,000 accommodate one extra (or one fewer) second seen to be
such an imposition? Especially when anybody who does find it so can
simply choose to use TAI instead?

Eppur si muove!

Rob Seaman
NOAO
Received on Sat Feb 04 2006 - 11:40:29 PST