<blockquote>
  <p>Units of time are actually quite arbitrary.</p>
</blockquote>

<p><strong>Their relationship to astronomical features is historical:</strong> </p>

<p>it's what people used when counting rotations and revolutions of the planet were the best tools to hand. Nowadays, they are legacy values: the "true" units are other things that happen to be feasible to measure using modern tools (wavelengths of light, resonances of atoms), with particular values chosen and scaled to match the old values for continuity.</p>

<p>And <em><code>other units</code></em> derived from them were purely matters of arithmetical convenience. Factors of 12 and 60 were common because they make it easy to do arithmetic in thirds and quarters, and when doing fractions, they reduce easily. So a value like a "second" is absolutely and utterly arbitrary; it applies to outer space exactly as well as it does here.</p>

<p><code>The second's arbitrary value is the fundamental unit of the metric system,</code> that that's what scientists use to communicate with each other. The numbers can be scaled for their convenience: as long as they're labeled, one system is as good as any other. (Especially with computers to do the work.) </p>

<p>Astronomers try to measure rotations and revolutions of distant bodies, and when speaking of them in public they might use "day" or "year" to help people visualize it. But when speaking to each other, they know what the technical terms mean, and avoid the ambiguity of layperson speak.</p>

<blockquote>
  <p>So in the end, it's all just <code>"seconds"</code>, scaled up or down. It may be
  converted to something else for public consumption or even with each
  other, but when it comes down to brass tacks they find it's easiest to
  all use the same unit, even if it's completely arbitrary.</p>
</blockquote>

<p>(It is worth noting that some physicists use a rather unfamiliar set of units call "natural units", under which important constants like the speed of light and the pull of gravity work out to precisely 1. These are convenient when you're pushing around equations, but require a fair bit of effort to translate into clocks and rulers when you want to build an actual experiment.)</p>

<blockquote>
  <p><strong>Another approach can be <code>Through an atomic clock.</code></strong></p>
</blockquote>

<p>Modern atomic clocks use atoms to determine the passing of time. So you would measure it using that. </p>

<blockquote>
  <p>For example 1 second is defined as --- the duration of 9192631770
  periods of the radiation corresponding to the transition between the
  two hyperfine levels of the ground state of the caesium 133 atom This
  mean that if you can make a laser shine on a ceasium 133 atom and by
  counting the periods of the light emitted you will now how many
  seconds have passed.</p>
</blockquote>

<p>As a interesting side note sins our orbit around the sun is not constant it is sometimes necessary by the scientific community to prolong a year by a second or so every now and then to make clocks match the moment of the earth. There called leap seconds</p>

CC BY-SA 3.0
2016-12-04T19:07:31.200Z
2016-12-04T19:07:31.200Z
Dastagir Husain Yasin
<p>We would use a clock.</p>

<p>Actually, we don't use the position of the sun for determining time. The apparent position of the sun doesn't change with enough regularity. Events on the Earth, such as Earthquakes, can change the length of a "day". The spinning of the Earth is a good clock, but not the best.</p>

<p>The best clocks use the frequency of light emitted by an atom (often a caesium atom). This frequency is very stable, far more stable than the Earth. If we were far from the sun, we could still use a clock to provide accurate timekeeping.</p>

<p>The clock would record local time, which would not keep in sync with Earth time if the clock were moving relative to Earth, due to relativistic effects.</p>

<p>Whether we would still use "days", "months", "years" etc is speculation. (and hence off topic) I'll note that our internal body clocks work best in a cycle of about 24 hours.</p>

CC BY-SA 3.0
2016-12-06T06:11:24.207Z
Sigh. That earthquakes can change the length of day is overhyped. This is purely theoretical; it has not been observed. The reason it hasn't been observed is that the background of changes in length of day due to snow falling on Siberia, snow melting in Siberia, air and water exchanging angular momentum with the Earth, etc., are so much larger (and so very noisy)  than is the tiny change from even the largest of earthquakes.
David Hammen
CC BY-SA 3.0
2016-12-06T13:44:41.347Z
There's a lot of half-truths and some false statements in this answer. It should really be cleaned up. For example, (cesium) atomic clocks to not measure time based on the frequency of light, but rather the frequency of hyperfine state transitions. For another, motion relative to earth only has relativistic effects when that motion involves acceleration. And we do use the position of the Sun to some degree. Otherwise, why would we add leap days and seconds to our calendars all the time? It is to sync our measurements back up with changes in the astronomical ephemerises.
zephyr
CC BY-SA 3.0
2016-12-04T20:10:01.427Z
2016-12-04T20:10:01.427Z
James K
<blockquote>
  <p>The sun is used to determine and measuring time in our solar system.</p>
</blockquote>

<p>Correction: The Sun (or more precisely, the Earth's rotation) <strong>was</strong> used to determine and measuring time in our solar system. Over two thousand years ago, keeping time via the appearance of the Sun was known to be problematic. The Babylonians, Egyptians, and other ancient cultures knew this. This became problematic with the invention of accurate clocks about four hundred years ago. Kepler argued that a clock rather than the position of the Sun was the proper way to keep time. Clocks and sundials disagree by as much as 16.5 minutes due to the equation of time.</p>

<p>Over a hundred years ago, the Earth's rotation rate was found to be less even less rock solid than thought. The second was redefined to be based on the Earth's orbit about the Sun as opposed to the Earth's average rotation rate. This, too, proved unreliable as we developed ever more accurate timekeeping devices.</p>

<p>Nowadays we keep time with atomic clocks. To keep time in sync with the rotating Earth, people worldwide observe the difference between the smoothed Earth rotation rate and what atomic clocks tell us. We occasionally have to introduce a leap second to maintain the fiction of a solar-based concept of time. For example, the last minute of 2016 will be 61 seconds long. (Leap seconds are a plague upon humanity. Leap minutes or leap hours would have been a better solution. Ask google. Or ask me. I'm currently dealing with a leap second problem in real life. It is a massive PITA.)</p>

<p>Even atomic clocks are a bit problematic. A minor problem is that some atomic clocks are more accurate than others. Even more problematic is that even the most accurate of atomic clocks won't keep time with one another due to the ever changing shape of the Earth. While our last ice age ended about twelve thousand years ago, the Earth has not quite recovered from the effects of the kilometer-plus thick sheets of ice that formerly covered parts of the Earth. The ongoing post glacial rebound affects the rate at which different atomic clocks tick.</p>

CC BY-SA 3.0
2016-12-06T17:18:38.087Z
Then how would the Babylonians and Egyptians know the time at night? Did they divide the day into hours ,using reference from day?How did it happen?
FlyingBird
CC BY-SA 3.0
2016-12-06T18:20:59.560Z
@FlyingBird: The Babylonians and Egyptians used something that many people can no longer see, which is a sky full of stars. That said, the penchant for accurate timekeeping is quite recent, at most 500 years old. Ancient peoples had little need to know the hour of the day, and even less need to know the hour of the night.
David Hammen
CC BY-SA 3.0
2016-12-06T07:09:32.127Z
2016-12-06T07:09:32.127Z
David Hammen
<p>The sun is used to determine and measuring time in our solar system. What if we were to travel outside the solar system i.e. to interstellar space, or other galaxies and other celestial bodies (out of our solar system). How would we keep a tab on time then?</p>

CC BY-SA 3.0
2016-12-04T21:35:12.997Z
there are palm sized atomic clocks these days, and casio watches are fine too, if you wish to measure timescales useful for human actions. humans have two circadian rythms, one of day and night and another in the day recently measured which is about 4 hours long, which is why we have a meal around every 4 hours.
bandybabboon
CC BY-SA 3.0
2016-12-05T13:49:24.600Z
Recommend closing because it's unclear what kind of "time measurement" he's asking about.  day/night cycle?  general relativity frame dragging?  etc
Carl Witthoft
CC BY-SA 3.0
2016-12-06T17:21:36.370Z
__and other __**celestial bodies**__(out of our solar system). How would we keep a tab on time then?__
FlyingBird
CC BY-SA 3.0
2016-12-06T17:29:01.483Z
__and other __**celestial bodies**__(out of our solar system). How would we keep a tab on time then?__ < br/> here I mention about <br/>1.black holes <br/>2.wormholes<br/>3.universe  traversal,etc.etc
FlyingBird
<general-relativity>
Time measurement outside solar system?
FlyingBird
# Time measurement outside solar system?

(Asked by: username_0 on 2016-12-04)

The sun is used to determine and measuring time in our solar system. What if we were to travel outside the solar system i.e. to interstellar space, or other galaxies and other celestial bodies (out of our solar system). How would we keep a tab on time then?

**Tags:** general-relativity

> **Comments:**
> **username_1**: there are palm sized atomic clocks these days, and casio watches are fine too, if you wish to measure timescales useful for human actions. humans have two circadian rythms, one of day and night and another in the day recently measured which is about 4 hours long, which is why we have a meal around every 4 hours.  
> **username_2**: Recommend closing because it's unclear what kind of "time measurement" he's asking about.  day/night cycle?  general relativity frame dragging?  etc  
> **username_0**: __and other __**celestial bodies**__(out of our solar system). How would we keep a tab on time then?__  
> **username_0**: __and other __**celestial bodies**__(out of our solar system). How would we keep a tab on time then?__ < br/> here I mention about <br/>1.black holes <br/>2.wormholes<br/>3.universe  traversal,etc.etc  

---

## 3 answers

### Answer by username_3 on 2016-12-04. Score: 4

> Units of time are actually quite arbitrary.

**Their relationship to astronomical features is historical:**

it's what people used when counting rotations and revolutions of the planet were the best tools to hand. Nowadays, they are legacy values: the "true" units are other things that happen to be feasible to measure using modern tools (wavelengths of light, resonances of atoms), with particular values chosen and scaled to match the old values for continuity.

And *`other units`* derived from them were purely matters of arithmetical convenience. Factors of 12 and 60 were common because they make it easy to do arithmetic in thirds and quarters, and when doing fractions, they reduce easily. So a value like a "second" is absolutely and utterly arbitrary; it applies to outer space exactly as well as it does here.

`The second's arbitrary value is the fundamental unit of the metric system,` that that's what scientists use to communicate with each other. The numbers can be scaled for their convenience: as long as they're labeled, one system is as good as any other. (Especially with computers to do the work.)

Astronomers try to measure rotations and revolutions of distant bodies, and when speaking of them in public they might use "day" or "year" to help people visualize it. But when speaking to each other, they know what the technical terms mean, and avoid the ambiguity of layperson speak.

> So in the end, it's all just `"seconds"`, scaled up or down. It may be
> converted to something else for public consumption or even with each
> other, but when it comes down to brass tacks they find it's easiest to
> all use the same unit, even if it's completely arbitrary.

(It is worth noting that some physicists use a rather unfamiliar set of units call "natural units", under which important constants like the speed of light and the pull of gravity work out to precisely 1. These are convenient when you're pushing around equations, but require a fair bit of effort to translate into clocks and rulers when you want to build an actual experiment.)

> **Another approach can be `Through an atomic clock.`**

Modern atomic clocks use atoms to determine the passing of time. So you would measure it using that.

> For example 1 second is defined as --- the duration of 9192631770
> periods of the radiation corresponding to the transition between the
> two hyperfine levels of the ground state of the caesium 133 atom This
> mean that if you can make a laser shine on a ceasium 133 atom and by
> counting the periods of the light emitted you will now how many
> seconds have passed.

As a interesting side note sins our orbit around the sun is not constant it is sometimes necessary by the scientific community to prolong a year by a second or so every now and then to make clocks match the moment of the earth. There called leap seconds


---

### Answer by username_4 on 2016-12-04. Score: 3

We would use a clock.

Actually, we don't use the position of the sun for determining time. The apparent position of the sun doesn't change with enough regularity. Events on the Earth, such as Earthquakes, can change the length of a "day". The spinning of the Earth is a good clock, but not the best.

The best clocks use the frequency of light emitted by an atom (often a caesium atom). This frequency is very stable, far more stable than the Earth. If we were far from the sun, we could still use a clock to provide accurate timekeeping.

The clock would record local time, which would not keep in sync with Earth time if the clock were moving relative to Earth, due to relativistic effects.

Whether we would still use "days", "months", "years" etc is speculation. (and hence off topic) I'll note that our internal body clocks work best in a cycle of about 24 hours.

> **Comments:**
> **username_5**: Sigh. That earthquakes can change the length of day is overhyped. This is purely theoretical; it has not been observed. The reason it hasn't been observed is that the background of changes in length of day due to snow falling on Siberia, snow melting in Siberia, air and water exchanging angular momentum with the Earth, etc., are so much larger (and so very noisy)  than is the tiny change from even the largest of earthquakes.  
> **username_6**: There's a lot of half-truths and some false statements in this answer. It should really be cleaned up. For example, (cesium) atomic clocks to not measure time based on the frequency of light, but rather the frequency of hyperfine state transitions. For another, motion relative to earth only has relativistic effects when that motion involves acceleration. And we do use the position of the Sun to some degree. Otherwise, why would we add leap days and seconds to our calendars all the time? It is to sync our measurements back up with changes in the astronomical ephemerises.  

---

### Answer by username_5 on 2016-12-06. Score: 3

> The sun is used to determine and measuring time in our solar system.

Correction: The Sun (or more precisely, the Earth's rotation) **was** used to determine and measuring time in our solar system. Over two thousand years ago, keeping time via the appearance of the Sun was known to be problematic. The Babylonians, Egyptians, and other ancient cultures knew this. This became problematic with the invention of accurate clocks about four hundred years ago. Kepler argued that a clock rather than the position of the Sun was the proper way to keep time. Clocks and sundials disagree by as much as 16.5 minutes due to the equation of time.

Over a hundred years ago, the Earth's rotation rate was found to be less even less rock solid than thought. The second was redefined to be based on the Earth's orbit about the Sun as opposed to the Earth's average rotation rate. This, too, proved unreliable as we developed ever more accurate timekeeping devices.

Nowadays we keep time with atomic clocks. To keep time in sync with the rotating Earth, people worldwide observe the difference between the smoothed Earth rotation rate and what atomic clocks tell us. We occasionally have to introduce a leap second to maintain the fiction of a solar-based concept of time. For example, the last minute of 2016 will be 61 seconds long. (Leap seconds are a plague upon humanity. Leap minutes or leap hours would have been a better solution. Ask google. Or ask me. I'm currently dealing with a leap second problem in real life. It is a massive PITA.)

Even atomic clocks are a bit problematic. A minor problem is that some atomic clocks are more accurate than others. Even more problematic is that even the most accurate of atomic clocks won't keep time with one another due to the ever changing shape of the Earth. While our last ice age ended about twelve thousand years ago, the Earth has not quite recovered from the effects of the kilometer-plus thick sheets of ice that formerly covered parts of the Earth. The ongoing post glacial rebound affects the rate at which different atomic clocks tick.

> **Comments:**
> **username_0**: Then how would the Babylonians and Egyptians know the time at night? Did they divide the day into hours ,using reference from day?How did it happen?  
> **username_5**: @username_0: The Babylonians and Egyptians used something that many people can no longer see, which is a sky full of stars. That said, the penchant for accurate timekeeping is quite recent, at most 500 years old. Ancient peoples had little need to know the hour of the day, and even less need to know the hour of the night.  

---

