I love this decision, and the provided rationales:
in many countries there are differences of more than one hour between local apparent solar time and civil time due to the choice of time zone, and changes of an hour are regularly introduced when daylight-saving time is used,
a workshop organized by the Consultative Committee for Time and Frequency (CCTF) and IERS in March 2025, which included experts on Earth rotation, estimated that the probability of a negative leap second will increase rapidly in the near future, reaching a 30 % probability by 2035,
a negative leap second has not previously been applied, and it is considered to pose a high risk of causing anomalies and disruption to critical infrastructures that are largely unprepared, and the preparation would create, for the industries that rely on time synchronization, a need for financial investment, whose amount is estimated to be similar to their preparations for the millennium bug,
Leap seconds have been a thorn in my side for years and I am very happy to pass the buck down a few centuries. Having a fixed table for leap seconds that doesn't need to be updated for any mission's foreseeable future is a godsend.
Leap seconds are a classic YAGNI thing. Nobody, absolutely nobody needs them. If you need to track time in accordance with the movement of the celestial bodies so accurately that leap seconds matter, you need a bespoke time system anyway. Then you can also handle relativity and all the other joyous realities of our universe. We should have used TAI from the very beginning.
Leap seconds are a classic YAGNI thing. Nobody, absolutely nobody needs them.
That wasn’t true when they were introduced, though: leap seconds didn’t become obsolete until the 1990s (I roughly estimate).
Remember that in 1972, a lot of shipping still relied on celestial navigation. (Even very advanced craft such as the Apollo missions and the SR-71 Blackbird did as well!) Not everywhere was covered by radio navigation beacons and not all boats had the receiver equipment. Since the 1800s ships had relied on civil time being close enough to earth rotation angle that they could use it directly for navigation.
The way radio time signals in the 1960s handled the difference between UT1 and atomic time was a combination of rubber seconds (i.e., published frequency adjustments) with occasional discontinuous steps of up to about 200ms. (Some countries had “stepped atomic time” which had a fixed frequency and stepped more often.) This was a massive pain in the arse because frequency adjustments required literally retuning multi-megawatt radio transmitters, and it was very difficult for receivers to recover a precise reference frequency from the time signal transmissions.
As well as the technical difficulties, there were legal objections. AIUI the German PTB in particular believed that the law required them to implement SI without artificial deviations, which meant they were not allowed to use rubber seconds. (Why couldn’t they use stepped atomic time, I wonder.)
Unfortunately, leap seconds were cooked up as a back-room deal with very little review. (Nothing like as much multi-decade multi-agency multi-treaty coordination as was required to abolish them!) So they were implemented as a fait accompli without proper consideration of the alternatives; and it’s doubtful whether the disadvantages would have been apparent that long ago. (I wonder to what extent they appreciated that a standard for radio time broadcasts was also de facto a standard for civil time for all other purposes.)
So we ended up with a system that aimed to be close enough to UT1 for low-tech navigation, where “close enough” meant within one second for rough purposes, and 0.1s for greater precision, and “low-tech” meant just a radio receiver (possibly with a time signal decoder).
Part of the reason for the combination of Bulletin C leap seconds and Bulletin D n*0.1s offsets is so that they fit in a simple low-bit-rate radio time signal, so the offset between UT1 and UTC needs to fit in a very limited and bounded space. (But ten bits of unary is a very stupid encoding, especially when you only have about 60 bits per minute.)
What’s notable is that there was a period of great disruption in timekeeping between about 1940 and 1970, when (in quick succession) quartz clocks demonstrated that the earth does not rotate steadily; the second was redefined in terms of astronomical ephemeris to make it more stable; atomic clocks were invented and rapidly commercialized, and immediately proved to be much easier and more accurate than ephemeris time; the second was redefined again in terms of atomic clocks; and then civil time was redefined as a compromise between classic universal time and new atomic time. It’s curious that after this point everything became fixed again: the definition of the second was stable so there was little technical pressure to revamp time yet again, and the structures and overlapping responsibilities of the various international treaty organizations made it very difficult to un-bodge.
I haven’t been paying close attention to this story in the last year or two, so it’s a surprise that the change is happening sooner than expected (tho it wouldn’t have been a surprise if I had looked at the meeting agenda):
continuous UTC will become effective on 20 May 2027,
the maximum value for the difference |UT1-UTC| will be 3 600 seconds (1 hour), ensuring the long term continuity for UTC for several centuries,
The provisional plan agreed at previous treaty conferences (GCPM and WRC) was to make the change in 2035, but the prospect of a negative leap second encouraged them to make the change sooner.
the slope of UT1-UTC seems to be returning to normal, and (equivalently) the length-of-day is roughly trending slightly over 24h instead of slightly under
the mysterious change in behaviour of the dX and dY charts that matches the change in slope of UT1-UTC seems to have reverted as well
It’s hard to be sure if earth rotation is really back to its pre-2020 behaviour, because much of the change that I’m observing is in the blue prediction section of the charts.
dallen | a day ago
I love this decision, and the provided rationales:
Leap seconds have been a thorn in my side for years and I am very happy to pass the buck down a few centuries. Having a fixed table for leap seconds that doesn't need to be updated for any mission's foreseeable future is a godsend.
BenjaminRi | 8 hours ago
Leap seconds are a classic YAGNI thing. Nobody, absolutely nobody needs them. If you need to track time in accordance with the movement of the celestial bodies so accurately that leap seconds matter, you need a bespoke time system anyway. Then you can also handle relativity and all the other joyous realities of our universe. We should have used TAI from the very beginning.
fanf | 7 hours ago
That wasn’t true when they were introduced, though: leap seconds didn’t become obsolete until the 1990s (I roughly estimate).
Remember that in 1972, a lot of shipping still relied on celestial navigation. (Even very advanced craft such as the Apollo missions and the SR-71 Blackbird did as well!) Not everywhere was covered by radio navigation beacons and not all boats had the receiver equipment. Since the 1800s ships had relied on civil time being close enough to earth rotation angle that they could use it directly for navigation.
The way radio time signals in the 1960s handled the difference between UT1 and atomic time was a combination of rubber seconds (i.e., published frequency adjustments) with occasional discontinuous steps of up to about 200ms. (Some countries had “stepped atomic time” which had a fixed frequency and stepped more often.) This was a massive pain in the arse because frequency adjustments required literally retuning multi-megawatt radio transmitters, and it was very difficult for receivers to recover a precise reference frequency from the time signal transmissions.
As well as the technical difficulties, there were legal objections. AIUI the German PTB in particular believed that the law required them to implement SI without artificial deviations, which meant they were not allowed to use rubber seconds. (Why couldn’t they use stepped atomic time, I wonder.)
Unfortunately, leap seconds were cooked up as a back-room deal with very little review. (Nothing like as much multi-decade multi-agency multi-treaty coordination as was required to abolish them!) So they were implemented as a fait accompli without proper consideration of the alternatives; and it’s doubtful whether the disadvantages would have been apparent that long ago. (I wonder to what extent they appreciated that a standard for radio time broadcasts was also de facto a standard for civil time for all other purposes.)
So we ended up with a system that aimed to be close enough to UT1 for low-tech navigation, where “close enough” meant within one second for rough purposes, and 0.1s for greater precision, and “low-tech” meant just a radio receiver (possibly with a time signal decoder).
Part of the reason for the combination of Bulletin C leap seconds and Bulletin D n*0.1s offsets is so that they fit in a simple low-bit-rate radio time signal, so the offset between UT1 and UTC needs to fit in a very limited and bounded space. (But ten bits of unary is a very stupid encoding, especially when you only have about 60 bits per minute.)
What’s notable is that there was a period of great disruption in timekeeping between about 1940 and 1970, when (in quick succession) quartz clocks demonstrated that the earth does not rotate steadily; the second was redefined in terms of astronomical ephemeris to make it more stable; atomic clocks were invented and rapidly commercialized, and immediately proved to be much easier and more accurate than ephemeris time; the second was redefined again in terms of atomic clocks; and then civil time was redefined as a compromise between classic universal time and new atomic time. It’s curious that after this point everything became fixed again: the definition of the second was stable so there was little technical pressure to revamp time yet again, and the structures and overlapping responsibilities of the various international treaty organizations made it very difficult to un-bodge.
fanf | 21 hours ago
I haven’t been paying close attention to this story in the last year or two, so it’s a surprise that the change is happening sooner than expected (tho it wouldn’t have been a surprise if I had looked at the meeting agenda):
The provisional plan agreed at previous treaty conferences (GCPM and WRC) was to make the change in 2035, but the prospect of a negative leap second encouraged them to make the change sooner.
However it seems that the near-term likelihood of negative leap second has become much lower this year. If you look at the earth orientation parameters finals.data (IAU2000) plots observe:
It’s hard to be sure if earth rotation is really back to its pre-2020 behaviour, because much of the change that I’m observing is in the blue prediction section of the charts.
chrismorgan | 20 hours ago
Please, you should have. Titles like this are a pain.
I have suggested the subtitle: “On the technical actions needed to ensure the continuity of UTC”.
[OP] JulianWgs | 16 hours ago
Sorry & thank you for the suggestion :)
gettalong | 16 hours ago
Wow, that is one long sentence...