During the discussions leading to the inception of the SI second the bigwig standards folks nixed the consideration of relativity, basically paraphrased as "If you physicists do not stop arguing about relativity we will never redefine time in a way that removes it from the astronomers."
Strange is what they said just after the CCIR incepted leap seconds at the SI subcommittee in charge of the subject:
"The CCIR may have overstepped its remit in defining the UTC"
and
"The process that led to UTC may have been illogical, but it was effective"
In that case UTC is the wrong answer to the question "What timescale will produce robust results for this application?" As soon as leap seconds were recommended by the CCIR in 1970 radionavigation system administrators announced they would switch to purely atomic TAI-10s and astronomical almanacs continued to use just plain UT. Proceedings of international standards bodies show them discussing how the new CCIR timescale with leap seconds (not yet officially named UTC) need not apply to anything other than radio broadcast time signals.
Calculating time deltas between events is fundamental for GPS navigation, tracking an incoming missile, coordinating the action of robotic systems, synchronization of telecommunications, ...
"ignoring leap seconds" is equivalent to "using a time scale that does not conform to UTC". Time scales other than UTC have 86400 seconds in a day. UTC is the exception.
"My system clock is wrong, what action shall I take to correct it?"
Common answers to that question are "step" or "smear". The notion that the clock could be wrong is not addressed in the design of most systems, and "smear" is widely accepted as the least harmful fix.
Yes, I'm wondering when/how "smearing" became widely accepted, since the Wikipedia page on leap seconds only says that there is a proprietary method that Google uses, and another that Amazon uses.
"UTC-SLS was proposed as a version of UTC with linear leap smearing, but it never became standard"
POSIX requires that we ignore leap seconds. POSIX had no alternatives to this choice because the information needed to do time right was not, and still is not, readily available via an authoritative and robust mechanism. No international recommendation has ever required the creation nor funding of a mechanism better than "This one agency in Paris will use the post office to send out letters to your national government time service agency at least 8 weeks in advance of a leap."
I was under the impression that it's fundamentally impossible to determine leap seconds in the future because the rate at which the Earth is slowing down its spin varies unpredictably.
At the inception of leap seconds it was unclear how well they could be predicted, but the agreement required 8 weeks of notice as a minimum. Since the inception of leap seconds the rotation of the earth has accelerated, not slowed.
>Since the inception of leap seconds the rotation of the earth has accelerated, not slowed
Ok, I'm not sure if you are correcting what I wrote or not. There is a sense in which what you write is correct, I think. But the long term trend is slowing, and since the inception of leap seconds, there have been ups and downs whether or not one averages over a year.
I think this is showing that the yearly cycle is larger than the change in speed in a century, and so are the cycles in the 365-day average over decade-timescales.
It is possible 8 weeks in advance to predict the difference between UTC and UT1 and keep it less than 0.9 seconds. Nothing more is required by international agreement.
A detailed look at the negotiations that led to leap seconds shows that they were not for maritime celestial navigation. During the process several different times the celestial navigation folks set a limit on how far radio broadcast time signals could deviate from astronomical time, and every one of those limits was violated as the negotiations proceeded. By the time the draft recommendation was given to 12th Plenary Assembly of the CCIR it allowed for leaps of multiple seconds, and that draft was amended on the floor to leaps of only one second before they voted to approve.
In 1950 astronomers pointed out that there would have to be two kinds of time, one to agree with calendar days and one to be as uniform as possible. Arguments over subsequent decades inexplicably decided that there could only be one kind of time specified by international agreements, and we ended up with a choice of two out of three characteristics in what we now call UTC. https://www.ucolick.org/~sla/leapsecs/picktwo.html
You website is a true hidden gold nugget, thanks for putting in the time to write all that up.
As an expert (as far as anyone around here is), what would your pick be for common civil time? Personally I feel like "precise time and simplicity" is almost obvious choice, but apparently it is not quite that clear cut.
"Simplicity" is rather subjective, even if you answer the question: simple for who?
Civil time has to stay in phase with the sun. As I see it, that's not negotiable. Inserting leap seconds, so that the nanosecond field of UTC remains the same as the nanosecond field of TAI, and the jumps that occur are negligible for ordinary people, seems to me overall the simplest solution, though I can see that UTC-SLS would be simpler for some people in some situations, and switching to leap minutes or leap hours would be simpler for people living now, who could then just ignore the problem. (Pollution and global warming and lots of other things can be treated in the same way, of course. Perhaps some of these things really will be easier to solve in the future, but I'd rather not rely on it.)
> "Simplicity" is rather subjective, even if you answer the question: simple for who?
I used simplicity in the meaning provided by link in parent comment refering to three desirable properties of time systems: "Every "day" has 86400 "seconds" (606024)."
> Civil time has to stay in phase with the sun. As I see it, that's not negotiable.
I don't see why that needs to be the case on a seconds level.
> switching to leap minutes or leap hours would be simpler for people living now, who could then just ignore the problem. (Pollution and global warming and lots of other things can be treated in the same way, of course. Perhaps some of these things really will be easier to solve in the future, but I'd rather not rely on it.)
Considering that need for leap hour would appear in over 500 years, I feel like trying predict the situation then is really borderline overarrogant.
Also leap hour would be basically a timezone shift, and I bet we will be doing timezone changes anyways in the next 500 years
Not at all. Just because it involves an "hour", rather than a "second", doesn't mean that it resembles a "timezone shift". It's totally different.
With a timezone shift, all that happens is that "09:00:00 +0900" is the same as "10:00:00 +1000". We can cope with that. But if you make UTC jump back an hour, then we have "09:59:59 +1000" followed by "09:00:00 +1000", and then the whole previous hour happens again. The internal timestamps in computer systems (typically expressed as a number of seconds since some epoch) repeat themselves for an hour. Causality is violated. Most computers stop working. You would probably have to switch them off beforehand to prevent data loss and even longer interruptions to service. You could shut down all the servers and desktop machines, stop all public transport and so on, but you can't just turn off the computers in embedded systems and satellites and so on...
So let's not try to arrogantly predict the situation in 500 years. Let's carry on with the established system of leap seconds, at least until someone comes up with a sensible alternative. Then there won't be a "situation" in (less than) 500 years.
