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Considering the recent thread on the potential removal of leap seconds, a story in TheAge aussie paper seemed worth adding to the discussion:
Earth had its shortest day since records began last month, with 1.59 milliseconds shaved off the usual 24 hour spin on June 29 - raising the prospect that a negative leap second may soon be needed to keep clocks matched up with the heavens.
The Earth appears to be spinning slightly faster than normal.
Usually, Earth's average rotational speed decreases slightly over time and timekeepers have been forced to add 27 leap seconds to atomic time since the 1970s as the planet slows.
But since 2020, the phenomenon has reversed with records being frequently broken over the last two years. The previous fastest day was -1.47 milliseconds under 24 hours on July 19 2020 and it was almost broken again on July 26, when the day was -1.50 milliseconds shorter. While the effect is too small to be noticeable by humans, it can accumulate over time, potentially impacting modern satellite communication and navigation systems which rely on time being consistent with the conventional positions of the Sun, Moon and stars.
It means that it may soon be necessary to remove time, adding a negative leap second, and speeding up global clocks for the first time ever.
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(Score: 0) by Anonymous Coward on Tuesday August 02 2022, @03:42PM (3 children)
Okay, so eventually at some point in the future this problem will likely have to be dealt with in better way. Eventually, the earth's rotational speed will begin to slow appreciably due to tidal forces, so that even the semi-annual "leap seconds" won't be enough to "correct" things. But that's a long-term issue, and we have no idea how things may change with timekeeping and how we conceptualize it if we begin more intensive space exploration, etc. in the next few centuries.
For now, leap seconds are a "patch." Just like leap years are a "patch" that fixed a "bug" in the old Roman calendar. Then the Gregorian correction (skip leap years in years divisible by 100 but not divisible by 400) fixed the drifting "bug" a bit more. But it too will eventually not be enough to correct for drift of the equinoxes. That's far into the future, but the Gregorian system is a pretty good one, and not too terribly difficult to implement. (I say that, but how many systems are going to have issues with no leap year in the year 2100??)
Anyhow, the issue leap seconds seem to be dealing with is also a kind of temporary patch that will work for a reasonable timespan into the future -- we don't want the official time to drift "too far" off the alignment with other celestial objects (although "too far" is definitely a subjective phrase). But we know (if our general understanding of long-term trends with gravity holds up) that the long-term trend should be toward slowing rotation for the earth, which means adding seconds occasionally.
So why subtract ONE? Lots of code deals with positive leap seconds. Very little (from what I understand) accounts for the possibility of NEGATIVE leap seconds.
If the earth's "speed up" trend continues for several years and we end up starting to accumulate a significant difference in that direction, maybe we either need to take action (actually do some negative leap seconds) or just give up on the exact synchronization of time of day with planetary rotation completely.
I know there is a growing movement to abolish leap seconds entirely already, and I can see the argument for that both ways. It does make sense. But if we are going to have them, it makes sense to use them as corrections to stay on-course with the long-term trend, which (again, assuming we actually understand planetary rotation well) is going to require adding seconds.
Suddenly turning around and subtracting a second just because one or two years were a few milliseconds off from the long-term trend would be like saying, "Hey, let's skip leap year in 2024 because springtime temperatures arrived a little earlier in 2022 and 2023!" That's not why we have leap years... leap day keeps us in sync with the overall trend, not the annual whims of the earth.
Again, if our understanding of rotation starts to change in the coming decades, and rotational period really starts trending shorter for a significant period of time, then we need to change our thinking. But IF we're going to bother with leap seconds, at least let's use them to keep to the long-term trend.
(Score: 1) by aafcac on Tuesday August 02 2022, @04:52PM
It already has, it just hasn't happened appreciably during the period of time when we had accurate and precise clocks. Since the earth formed, there have been days and even weeks worth of extra days added to the revolution as things slowed down.
(Score: 3, Informative) by Anonymous Coward on Tuesday August 02 2022, @05:40PM
This is basically how the process already works.
UTC is (currently) specified to be within ±0.9 seconds of UT1. To achieve this, there are provisions for leap seconds in either direction. A negative leap second would only be inserted if the UT1-UTC difference is getting uncomfortably close to +0.9s
As you mention, the overall trend is a slowing rotation (primarily due to tidal drag from the moon) so a negative leap second has literally never been observed to date.
On the other hand, we have also never observed the difference going more positive year over year before. But the difference is still negative and rotational speedups would have to continue for many more years before we get close to +0.9s.
(Score: 3, Touché) by JoeMerchant on Tuesday August 02 2022, @06:41PM
Meanwhile, that giant comet plunging towards us from outside the orbital plane of the solar system is going to screw up all the other predictions when it hits...
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