<blockquote>
<p>Someday in future, the length of a day will be longer than 86400 atomic seconds. Then what should future human do?</p>
</blockquote>
<p>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.</p>
<p>The length of a day is already longer than 86400 atomic seconds. Every once in a while a <a href="https://en.wikipedia.org/wiki/Leap_second" rel="nofollow noreferrer">leap second</a> 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.</p>
<blockquote>
<p>Will the definition of the second be changed?</p>
</blockquote>
<p>Almost certainly.</p>
<p>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.</p>

CC BY-SA 4.0
2020-10-18T05:17:25.023Z
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](https://en.wikipedia.org/wiki/Leap_second) 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!!
JeopardyTempest
CC BY-SA 4.0
2020-10-18T06:47:33.920Z
@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.
David Hammen
CC BY-SA 4.0
2020-10-20T11:02:54.540Z
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.
Spencer
CC BY-SA 4.0
2020-10-18T02:38:15.840Z
2020-10-18T09:20:27.427Z
2020-10-18T09:20:27.427Z
David Hammen
<p>(Please excuse my English)</p>
<p>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.<br />
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.</p>
<p>My question is : Someday in future, the length of a day will be longer than 86400 atomic seconds. Then what should future human do?<br />
Changing the definition of the second will lead to incredibly complex situations. Also changing the definition of the day will cause tremendous complex situation.</p>

CC BY-SA 4.0
2020-10-17T14:48:36.263Z
This question isn't really about Earth Science. You might get better traction on Physics SE or HSM SE.
Spencer
CC BY-SA 4.0
2020-10-17T14:55:26.927Z
@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.
pdh0710
CC BY-SA 4.0
2020-10-17T15:03:06.367Z
In this case, wait a little and see if anyone agrees with me.
Spencer
CC BY-SA 4.0
2020-10-17T15:09:10.053Z
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.
Spencer
CC BY-SA 4.0
2020-10-17T15:40:15.463Z
@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?
pdh0710
CC BY-SA 4.0
2020-10-17T18:27:01.963Z
Because the second is chosen to be the fundamental unit (such as the SI base unit of time), in part because it's nearer the timescale of most events that directly impact people. Physics constants would have to be adjusted... we talk about gravity in m/s^2, the speed of light in m/s, and there are many derived units currently developed on the definition of seconds (energy, power, pressure). The fact they already recast seconds into something unchanging shows it is the unit already picked to be the reliable one.
JeopardyTempest
CC BY-SA 4.0
2020-10-17T18:30:25.990Z
The problem with the foot (and even the original meter?) is its definition kept changing.  So science worked to pick basic units that were unmoving.  They've put a lot of attention into removing them from things that vary... they wouldn't want to have to change the definition of meters if the Earth's radius changed either.  And we already change the definition of the year regularly, so it also shows we're more willing to alter bigfee scaled units than small.
JeopardyTempest
CC BY-SA 4.0
2020-10-17T20:59:57.907Z
@JeopardyTempest : I agree that the second is more fundamental unit than the day.
pdh0710
CC BY-SA 4.0
2020-10-18T06:59:13.133Z
@pdh0710 When the French invented the metric system, they tried but failed to metricize two key concepts, angle and time. The initial metric proposal had the unit of angle measurements be one rotation and the unit of time measurements be a one day, both with standard metrication by factors of ten. That didn't work because unlike length and mass, widely used *de facto* standards that were not based on factors of ten already existed for angle and time.
David Hammen
CC BY-SA 4.0
2020-10-18T07:20:33.043Z
Highly relevant, but not a duplicate, and not voted as off-topic by anyone: [Changes in Earth's orbital and rotation speeds](https://earthscience.stackexchange.com/questions/13575). If the votes to close continue, I will vote to reopen. This is on topic.
David Hammen
CC BY-SA 4.0
2020-10-18T12:33:04.990Z
@DavidHammen : Oh, I didn't notice those facts, though I know about the French Revolutionary calendar also based on factors of 10.
pdh0710
<time>
Will the definition of the second be changed?
pdh0710
# Will the definition of the second be changed?

(Asked by: username_0 on 2020-10-17)

(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.

**Tags:** time

> **Comments:**
> **username_1**: This question isn't really about Earth Science. You might get better traction on Physics SE or HSM SE.  
> **username_0**: @username_1 : 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.  
> **username_1**: In this case, wait a little and see if anyone agrees with me.  
> **username_1**: 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.  
> **username_0**: @username_1 : 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?  
> **username_2**: Because the second is chosen to be the fundamental unit (such as the SI base unit of time), in part because it's nearer the timescale of most events that directly impact people. Physics constants would have to be adjusted... we talk about gravity in m/s^2, the speed of light in m/s, and there are many derived units currently developed on the definition of seconds (energy, power, pressure). The fact they already recast seconds into something unchanging shows it is the unit already picked to be the reliable one.  
> **username_2**: The problem with the foot (and even the original meter?) is its definition kept changing.  So science worked to pick basic units that were unmoving.  They've put a lot of attention into removing them from things that vary... they wouldn't want to have to change the definition of meters if the Earth's radius changed either.  And we already change the definition of the year regularly, so it also shows we're more willing to alter bigfee scaled units than small.  
> **username_0**: @username_2 : I agree that the second is more fundamental unit than the day.  
> **username_3**: @username_0 When the French invented the metric system, they tried but failed to metricize two key concepts, angle and time. The initial metric proposal had the unit of angle measurements be one rotation and the unit of time measurements be a one day, both with standard metrication by factors of ten. That didn't work because unlike length and mass, widely used *de facto* standards that were not based on factors of ten already existed for angle and time.  
> **username_3**: Highly relevant, but not a duplicate, and not voted as off-topic by anyone: [Changes in Earth's orbital and rotation speeds](https://earthscience.stackexchange.com/questions/13575). If the votes to close continue, I will vote to reopen. This is on topic.  
> **username_0**: @username_3 : Oh, I didn't notice those facts, though I know about the French Revolutionary calendar also based on factors of 10.  

---

## 1 answers

### Answer by username_3 on 2020-10-18. Score: 5

> 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](https://en.wikipedia.org/wiki/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.

> **Comments:**
> **username_2**: 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](https://en.wikipedia.org/wiki/Leap_second) 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!!  
> **username_3**: @username_2  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.  
> **username_1**: 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.  

---

