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(Please excuse my English)

A well known fact about Earth's rotation(around its axis) is that the rotation is slow down due to the tidal friction. And because of it, the length of Day is slowly increasing.
Originally, the definition of Second(time) was derived from the day length and Earth's rotation. The original definition of the second was 1 day / 24 / 60 / 60 = 1 day / 86400. However, the modern definition of the second is derived from the atom's oscillation period, for precise time keeping, independent of Earth's rotation, because of irregularity of Earth's rotation. Thus, compared to the original definition, the modern definition of the second has rigidly fixed value.

My question is : Someday in future, the length of a day will be longer than 86400 atomic seconds. Then what should future human do?
Changing the definition of the second will lead to incredibly complex situations. Also changing the definition of the day will cause tremendous complex situation.

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    $\begingroup$ This question isn't really about Earth Science. You might get better traction on Physics SE or HSM SE. $\endgroup$
    – Spencer
    Oct 17 '20 at 14:48
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    $\begingroup$ @Spencer : There are too many forums in StackExchange. How can I move this question to proper forum? I think Physics SE is not the proper forum. $\endgroup$
    – pdh0710
    Oct 17 '20 at 14:55
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    $\begingroup$ In this case, wait a little and see if anyone agrees with me. $\endgroup$
    – Spencer
    Oct 17 '20 at 15:03
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    $\begingroup$ I will say, this is about the measurement precision you can tolerate at different scales. The day is increasing slowly enough (1.7 ms/century) that it won't matter for a very long time. If humans are still around then, we can't predict what units they will be using. But even if there is academic continuity between then and now, they are more likely to recalibrate the length of the day, month, year, and other changing quantities than the second. $\endgroup$
    – Spencer
    Oct 17 '20 at 15:09
  • $\begingroup$ @Spencer : Yes, it will happen in a very far far future. After about 30000 years, the length of the day will be about 0.5 second longer than now. Anyway, why do you choose the day length than the second? $\endgroup$
    – pdh0710
    Oct 17 '20 at 15:40
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Someday in future, the length of a day will be longer than 86400 atomic seconds. Then what should future human do?

Having a time standard that works well with the ever improving capabilities of physics and having a time standard that stays in sync with the Earth's rotation are at odds with one another. One solution is to continue to do what we do now.

The length of a day is already longer than 86400 atomic seconds. Every once in a while a leap second is added to keep time as measured by the rotation of the Earth in sync with time as measured by an atomic clock. To date, 37 leap seconds have been added since 1972. This concept will continue to work, at least for the next few to several hundred years. Eventually the Earth's rotation rate will have slowed to such an extent that the leap second trick will no longer work.

Will the definition of the second be changed?

Almost certainly.

Scientists will eventually find something that is even better than is the unperturbed ground-state hyperfine transition frequency of the cesium-133 atom as a timekeeping device. That science advances is what science does. It has already been proposed to use pulsars rather than atomic clocks as a more precise timekeeping drive. One thing is certain: The second will not be redefined to be 1/86400 of a day.

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  • $\begingroup$ I'd always just naively figured leap seconds were mainly connected to the fine-tweaking of the length of the year [as leap years/the century rule are], but Wikipedia does show they're basically about day length. It seems feasible then that the adjustments would be led by this pattern... perhaps having one\a few seconds added at a regular interval for a while (say 1 second per yr, then every few months, month, week, and finally just extending it to adding a second to every day at the end?). Imagine Dec 31: 3... 2... 1... extra 1... Happy New Year!! $\endgroup$ Oct 18 '20 at 5:17
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    $\begingroup$ @JeopardyTempest Leap years are motivated by the desire to keep the calendar in sync with the Earth's revolution about the Sun. Leap seconds are motivated by the desire to keep the clock in sync with the Earth's rotation about its axis. The two concerns have little, if anything, to do with one another. $\endgroup$ Oct 18 '20 at 6:47
  • $\begingroup$ Good frame challenge. The second will be redefined, but only to make the existing value more precise. It won't be stretched to fit the linger day. $\endgroup$
    – Spencer
    Oct 20 '20 at 11:02

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