Why?

If you ask "most people", they will tell you that a second is one 86,400-th part of the
amount of time that the earth takes to rotate on its axis. Even though they don't know it,
such a definition leads to a variable duration of a second.

It seems to me that lots of people have timescales to suit their own needs - scientists
have TAI, and astronomers have all the rest :). But not everyone has a suitable
timescale - UTC is the timescale that's been introduced, it seems, for the benefit of
"most people", and it has serious shortcomings. If "most people" are already quite
happy with a variable second, then why not codify it? If "most people" want solar time,
and solar time is variable, then give 'em what they want!

Of course, the serious problem with a variable second is that of determining time
intervals - you cannot just subtract the earlier time from the later - you would also have
to calculate the appropriate scaling factor. But UTC isn't appropriate for determining
time intervals anyway! If you want time intervals, you use TAI - end of story. And, using
simple arithmetic with a timescale with a variable second would give an order of
magnitude better estimate of the amount of time between 2005 Dec 31 23:59:59.9 and
2006 Jan 01 00:00:00.1 than UTC does!

I suspect that mathematicians and computer programmers alike would prefer to use
quadratic equations than look-up tables, even if the latter are well-defined. Quadratic
equations are easier to work with than discontinuities. Also, continuing to use leap
seconds eventually leads to inserting leap seconds at the end of every month, week,
then day, though it may take 1,200 years before a leap second is required at the end of
each month. And, which would the general public prefer - a slightly, imperceptibly,
varying second that (attempts to) track the earth, or 23:59:60? Finally, over 400 years
ago, there was a change made to our calendar that is still in use today, and that looks
like it will still be in use 1,000 years from now (an impressive achievement by our
forebearers) - I think we should aim for such a long-term solution for the measurement
of the time of day.

Nah - just trying to be helpful!

Just to introduce myself - I'm a professional programmer and amateur astronomer, and
my interest in precise time started with my involvement in a (professional) experiment
years ago to detect a pulsar. Two great memories I have from that time, both of them
arising from my confusion as to what a GPS receiver I was using was actually saying to
me about the time. One was when I was comparing the output of the GPS to the radio
time station RWM in Moscow - I saw that there was an 11ms discrepancy. This, I
decided, was because I was about 3,300 km from Moscow! The second great memory
was when I saw a one-second discrepancy between my understanding of the GPS
output and the national telephone time service (I'm in Ireland). I eventually tracked down
the people responsible for that time service, and asked them if they knew if or how
there could be a one second error in their clock. I had to explain to them that a one-
second error was actually very important to me as I was trying to measure time to
millionths of a second. I got as far as them asking me "oh - do you want us to reset the
clock?" before I gave up!

(Hence my belief that people would rather have the compromise of a varying second
than the 23:59:60 compromise).

Chris.

At 08:25 AM 7/17/2005, obyrne@iol.ie wrote...

UTC will tell you that there is EXACTLY 1.2 seconds between those two points. No estimation needed, so it would be infinitely better than some loosely defined variable second.

One must understand that UTC does not follow the convention that all minutes have 60 seconds. UTC requires that 59 or 61 second minutes be allowed during leap second events. It also requires one to track when leap seconds occur. There is no ambiguity and no imprecision.

Neither can be guaranteed to keep UTC in sync to UT1 by the defined amount. There is a very high degree of confidence that leap seconds will accumulate over the long term, but over a shorter term, we may actually need to remove a leap second, as the rotation of the earth seems to be on a current trend of speeding up. http://www.ucolick.org/~sla/leapsecs/dutc.html#atomic.png

There are no discontinuities with UTC, at least not if one uses the mathematical definition of "discontinuity." All time is fully accounted for, and there are no gaps. If one claims that leap seconds make UTC "discontinuous" by some other definition, one must then admit that leap days make most other time scales discontinuous also, for they are logically equivalent, differing only in the amount of time before irregular corrections need to be applied (years for leap seconds/UTC, millenia for leap days/Gregorian).

Nor is UTC entirely unpredictable or random, for we can with some degree of precision (but not exactly), know how much it will differ from TAI at any point in the near future (several millenia). "Irregular" might be a better term to refer to the effect leap seconds have on UTC.

23:59:60, since it only occurs infrequently and properly implemented has zero effect on interval measurement. The problem is not how to handle leap seconds, but in getting the infrastructure to actually do so. How many RTC chips handle 23:59:60? Operating systems? Applications? What channels are available to automatically distribute notice of upcoming leap second events? The people/organizations which are complaining about leap seconds have had 33 years to work this out, but haven't for the most part. Now they want to fundamentally change UTC (breaking systems which DO use UTC as intended) to make up for their inaction.

The Gregorian system slips about 1 day every 4000 years. Considering that is shorter than recorded history, and very much less than human existance, I consider that less than "long term."