4 ms·

"On Unix systems we measure time as the number of seconds since "the epoch": 00:00:00 UTC on January 1st, 1970.... this definition is not based on something sen

"On Unix systems we measure time as the number of seconds since "the epoch": 00:00:00 UTC on January 1st, 1970.... this definition is not based on something sensical such as, say, the objective frequency of vibration of a Cesium-133 atom, but on a convenient fraction of the time it takes a particular large rock to complete a full rotation around its own axis." Well, seconds have not been defined as "a convenient fraction of the time it takes a particular large rock to complete a full rotation around its own axis" for quite some time, and the origin is set to an abstract event in the past, which is not (as far as I know) subject to retroactive revision as a consequence of the vagarities of planetary or celestial motion (if it is, I would be fascinated to know more.)

lazide 4y ago
That’s a very recent change - and it’s not like 9192631770 transitions/hz (which is hilariously self referential!) is some obvious, natural value that ISN’T based on the historic ‘typical’ length of the day based on our rotation around the sun. A second being 1/86400th of a day (24 hrs * 60 minutes * 60 seconds per minute) is still essentially true, and based, essentially still on the seasons and our movement around the sun (or relative movements between the various bodies in the solar system, depending). Being a chaotic natural system, we of course need to add fudge factors here and there to simplify the math day to day while keeping it aligned with observed reality, like leap seconds and all), at least where it intersects with reality in a material way.
- epicide 4y ago
  > it’s not like 9192631770 transitions/hz ... is some obvious, natural value No, but it is a phone number in the Raleigh area.
  - jsymolon 4y ago
    And would be awesome if it gave out the current time.
krisoft 4y ago
> seconds have not been defined as "a convenient fraction of the time it takes a particular large rock to complete a full rotation around its own axis" for quite some time That is true. > origin is set to an abstract event in the past That is also true. > which is not (as far as I know) subject to retroactive revision as a consequence of the vagarities of planetary or celestial motion I'm afraid you are wrong on that. The unix time is synced with UTC. UTC has so called "leap seconds" scheduled at irregular intervals by the International Earth Rotation and Reference Systems Service to keep it in sync with the Earth's actual movements. So in effect the unix timestamp is wrangled to sync with the Earth's motion. > if it is, I would be fascinated to know more https://en.wikipedia.org/wiki/Unix_time#UTC_basis https://en.wikipedia.org/wiki/Unix_time#UTC_basis https://en.wikipedia.org/wiki/Leap_second https://en.wikipedia.org/wiki/Leap_second
- whoville 4y ago
  > The unix time is synced with UTC No, it isn't. The very article you linked explains that Unix time is monotonically increasing and ignores leap seconds.
  - cellshade 4y ago
    "Unix time numbers are repeated in the second immediately following a positive leap second. The Unix time number 1483142400 is thus ambiguous: it can refer either to start of the leap second (2016-12-31 23:59:60) or the end of it, one second later (2017-01-01 00:00:00)." From the linked article.
    - whoville 4y ago
    - whoville 4y ago
- mannykannot 4y ago
  Thanks for the correction, this is quite important. I had taken statements that Unix and Posix time excludes or ignores leap seconds to mean that it increases monotonically in a continuous manner in sync with whatever atomic clock is the standard; on the contrary, it recognizes and implements leap-seconds with a discontinuity deviating from the steady tick of that standard. Examples: https://en.wikipedia.org/wiki/Unix_time#Leap_seconds https://en.wikipedia.org/wiki/Unix_time#Leap_seconds I also learned that 00:00:00 UTC on 1 January 1970 is a proleptic time, as UTC was not in effect then, though I am not sure that makes it subject to subsequent planetary motions.
thaumasiotes 4y ago
> Well, seconds have not been defined as "a convenient fraction of the time it takes a particular large rock to complete a full rotation around its own axis" for quite some time Seconds have not ever been defined that way, because the time it takes for the earth to complete a full rotation around its own axis (the "sidereal day") was never a question of much interest. It's mostly relevant to astronomers. Seconds were always defined in terms of the synodic day, the time it takes for a point on the earth that is aimed directly at the sun to rotate around the earth until it is once again aimed directly at the sun. They still are defined that way, in the sense that the only purpose of other definitions is to match the traditional definition as exactly as possible. If cesium started vibrating faster or slower, we'd change the "official definition" of a second until it matched the traditional definition. Given that fact, which definition is real and which isn't?
- mannykannot 4y ago
  That's an interesting point of view, but is it not the case that the length of the synodic day varies throughout the year, on account of Kepler's 2nd. law? - IIRC, this is the first-order effect in explaining the analemma. The length of the sidereal day is also variable, but more slowly (at least if we put aside atmospheric and earthquake effects, which I guess affect both equally.) I think we need both a way to reckon our everyday experience of passing time and a constant measure, and one cannot be just a special case of the other. Personally, I feel that introducing leap seconds into Unix time just muddied the waters. As to the sidereal day being of little interest outside of astronomy, James Watt's patron/partner Matthew Boulton built a sidereal clock and tried hard to find a purchaser (even having it shipped to Catherine the Great in hopes of sparking some interest) without success. It was on display in his Birmingham home when we visited a few years ago. https://www.thefreelibrary.com/Boulton+clocks+on+to+the+past%3b+AGENDA+%26+LETTERS.-a0200637119 https://www.thefreelibrary.com/Boulton+clocks+on+to+the+past...
  - thaumasiotes 4y ago
    > but is it not the case that the length of the synodic day varies throughout the year, on account of Kepler's 2nd. law? I don't see the relevance; the length of the sidereal day isn't constant either. Variation in the length of the synodic day is the reason a day may contain other than 86400 seconds. If days are not of constant length, you could vary the length of a second on a day-to-day basis, or you could define the length of a "reference day" and then define the second as a convenient fraction of that. We have taken the second approach (though the first was used historically). But we don't care about the vibration of cesium; if that were to change, we would adjust by changing the definition of a second, not by accepting that seconds were now of a different duration than before. Thus, the fact that cesium is referenced in an "official" definition of the duration of a second is meaningless. The officialness of that definition is illusory; in reality, seconds continue to be defined as a convenient fraction of an average day.

lazide 4y ago

That’s a very recent change - and it’s not like 9192631770 transitions/hz (which is hilariously self referential!) is some obvious, natural value that ISN’T based on the historic ‘typical’ length of the day based on our rotation around the sun. A second being 1/86400th of a day (24 hrs * 60 minutes * 60 seconds per minute) is still essentially true, and based, essentially still on the seasons and our movement around the sun (or relative movements between the various bodies in the solar system, depending). Being a chaotic natural system, we of course need to add fudge factors here and there to simplify the math day to day while keeping it aligned with observed reality, like leap seconds and all), at least where it intersects with reality in a material way.

epicide 4y ago
> it’s not like 9192631770 transitions/hz ... is some obvious, natural value No, but it is a phone number in the Raleigh area.
- jsymolon 4y ago
  And would be awesome if it gave out the current time.

krisoft 4y ago

> seconds have not been defined as "a convenient fraction of the time it takes a particular large rock to complete a full rotation around its own axis" for quite some time That is true. > origin is set to an abstract event in the past That is also true. > which is not (as far as I know) subject to retroactive revision as a consequence of the vagarities of planetary or celestial motion I'm afraid you are wrong on that. The unix time is synced with UTC. UTC has so called "leap seconds" scheduled at irregular intervals by the International Earth Rotation and Reference Systems Service to keep it in sync with the Earth's actual movements. So in effect the unix timestamp is wrangled to sync with the Earth's motion. > if it is, I would be fascinated to know more https://en.wikipedia.org/wiki/Unix_time#UTC_basis https://en.wikipedia.org/wiki/Unix_time#UTC_basis https://en.wikipedia.org/wiki/Leap_second https://en.wikipedia.org/wiki/Leap_second

whoville 4y ago
> The unix time is synced with UTC No, it isn't. The very article you linked explains that Unix time is monotonically increasing and ignores leap seconds.
- cellshade 4y ago
  "Unix time numbers are repeated in the second immediately following a positive leap second. The Unix time number 1483142400 is thus ambiguous: it can refer either to start of the leap second (2016-12-31 23:59:60) or the end of it, one second later (2017-01-01 00:00:00)." From the linked article.
  - whoville 4y ago
  - whoville 4y ago
mannykannot 4y ago
Thanks for the correction, this is quite important. I had taken statements that Unix and Posix time excludes or ignores leap seconds to mean that it increases monotonically in a continuous manner in sync with whatever atomic clock is the standard; on the contrary, it recognizes and implements leap-seconds with a discontinuity deviating from the steady tick of that standard. Examples: https://en.wikipedia.org/wiki/Unix_time#Leap_seconds https://en.wikipedia.org/wiki/Unix_time#Leap_seconds I also learned that 00:00:00 UTC on 1 January 1970 is a proleptic time, as UTC was not in effect then, though I am not sure that makes it subject to subsequent planetary motions.

thaumasiotes 4y ago

> Well, seconds have not been defined as "a convenient fraction of the time it takes a particular large rock to complete a full rotation around its own axis" for quite some time Seconds have not ever been defined that way, because the time it takes for the earth to complete a full rotation around its own axis (the "sidereal day") was never a question of much interest. It's mostly relevant to astronomers. Seconds were always defined in terms of the synodic day, the time it takes for a point on the earth that is aimed directly at the sun to rotate around the earth until it is once again aimed directly at the sun. They still are defined that way, in the sense that the only purpose of other definitions is to match the traditional definition as exactly as possible. If cesium started vibrating faster or slower, we'd change the "official definition" of a second until it matched the traditional definition. Given that fact, which definition is real and which isn't?

mannykannot 4y ago
That's an interesting point of view, but is it not the case that the length of the synodic day varies throughout the year, on account of Kepler's 2nd. law? - IIRC, this is the first-order effect in explaining the analemma. The length of the sidereal day is also variable, but more slowly (at least if we put aside atmospheric and earthquake effects, which I guess affect both equally.) I think we need both a way to reckon our everyday experience of passing time and a constant measure, and one cannot be just a special case of the other. Personally, I feel that introducing leap seconds into Unix time just muddied the waters. As to the sidereal day being of little interest outside of astronomy, James Watt's patron/partner Matthew Boulton built a sidereal clock and tried hard to find a purchaser (even having it shipped to Catherine the Great in hopes of sparking some interest) without success. It was on display in his Birmingham home when we visited a few years ago. https://www.thefreelibrary.com/Boulton+clocks+on+to+the+past%3b+AGENDA+%26+LETTERS.-a0200637119 https://www.thefreelibrary.com/Boulton+clocks+on+to+the+past...
- thaumasiotes 4y ago
  > but is it not the case that the length of the synodic day varies throughout the year, on account of Kepler's 2nd. law? I don't see the relevance; the length of the sidereal day isn't constant either. Variation in the length of the synodic day is the reason a day may contain other than 86400 seconds. If days are not of constant length, you could vary the length of a second on a day-to-day basis, or you could define the length of a "reference day" and then define the second as a convenient fraction of that. We have taken the second approach (though the first was used historically). But we don't care about the vibration of cesium; if that were to change, we would adjust by changing the definition of a second, not by accepting that seconds were now of a different duration than before. Thus, the fact that cesium is referenced in an "official" definition of the duration of a second is meaningless. The officialness of that definition is illusory; in reality, seconds continue to be defined as a convenient fraction of an average day.