5 ms·

I'm wondering how atomic clocks in general get set in the first place. Did, at some point, scientists calculate when the sun was exactly overhead Greenwich and

I'm wondering how atomic clocks in general get set in the first place. Did, at some point, scientists calculate when the sun was exactly overhead Greenwich and call that noon? Because I would think that that calculation would have a multi-second error, so setting your clock to the second would be pointless. Or maybe there is no such thing as "actual physical time" and there is only what people have agreed to call the standard. But in that case why do time sources, such as time.gov and time.windows.com, still give different times? I would think Microsoft would have fixed any bugs in their NTP implementation by now, so it's not that. If it's just politics about nobody wanting to move to someone else's time, then there's no way to tell which source is the real standard, so your most practical choice is to synchronize your clock to the times you deal with. That is, set your clock to your clock at work, or an average of your friends clocks, or whatever source they get their time from. I don't mind the existence of central time sources, because they are better than having to go out and find someone else's clock, but they shouldn't call themselves official if they aren't actually official. So, does anybody know how the starting time for central time sources is chosen, and if any source is worthy of being called "the real time"?

dfc 15y ago
UTC is a "paper clock." The calculation of UTC has nanosecond error, not multisecond error. The most recent data for the calculation of UTC: ftp://ftp2.bipm.org/pub/tai/publication/cirt.290 Time is a really tricky and interesting topic. I would start with: "From Sundials to Atomic Clocks: Understanding Time and Frequency" http://www.nist.gov/timefreq/general/pdf/1796.pdf http://www.nist.gov/timefreq/general/pdf/1796.pdf "Originally published in 1977, this full-length book provides a comprehensive, easy-to-understand introduction to the field of time and frequency. Readers of nearly all ages and educational backgrounds should find it enjoyable. 306 pages."
phillmv 15y ago
Well… it's an arbitrary standard. We just call a day one full rotation of the earth, and we just happen to divide it into 24hr periods with 60 subdivisions. Theoretically, you could measure the rotation of the earth and call that a day, but I have no idea how you would measure that down to the millisecond. As to "Real Time", I don't know about the specific legalities, but the standard goes as follows: everyone uses UTC http://en.wikipedia.org/wiki/Coordinated_Universal_Time http://en.wikipedia.org/wiki/Coordinated_Universal_Time which is based off the International Atomic Time http://en.wikipedia.org/wiki/International_Atomic_Time http://en.wikipedia.org/wiki/International_Atomic_Time I too would be interested in how they picked the beginning epoch, i.e. "this moment right here is midnight from which all other seconds shall be referenced against" but in the grand scheme of things it doesn't matter - the only thing that matters is the standard we agreed to measure against.
- dfc 15y ago
  "Theoretically, you could measure the rotation of the earth and call that a day" This is not theoretical; this is known as sidereal time and is measured to the nanosecond... http://en.wikipedia.org/wiki/Sidereal_time http://en.wikipedia.org/wiki/Sidereal_time
  - mceachen 15y ago
    Except that "…due to nutation, an actual sidereal day is not quite so constant." Yeah. Nutation. Nutation "…is a rocking, swaying, or nodding motion in the axis of rotation of a largely axially symmetric object, such as a gyroscope, planet, or bullet in flight…" http://en.wikipedia.org/wiki/Nutation http://en.wikipedia.org/wiki/Nutation
troels 15y ago
The rotation of Earth is actually not as stable a thing as one might expect. See for example http://en.wikipedia.org/wiki/Leap_second http://en.wikipedia.org/wiki/Leap_second
- dfc 15y ago
  NIST recently published the new ntp.leap file and it looks like we will have one this July... 3550089600 35 # 1 Jul 2012 [1]
  - apaprocki 15y ago
    You also need to make sure your IANA tzdata is updated to account for this. I think this leap second was first added in tzdata2012a, but I'd have to check: ftp://ftp.iana.org/tz/data/leapseconds
    - acqq 15y ago
      If you use Unix time, you don't have to care: http://en.wikipedia.org/wiki/Unix_time http://en.wikipedia.org/wiki/Unix_time "Unix time, (...) is (...) defined as the number of seconds elapsed since midnight Coordinated Universal Time (UTC) of Thursday, January 1, 1970 (...) not counting leap seconds"
      - apaprocki 15y ago
        You might not be able to ignore leap seconds if you are interacting with an external system (software or hardware) / API that expects actual UTC. General rule applies to everything.. know your system, know what systems you are connected to, know what standards you need to adhere to. Microsoft apparently rejected the idea of using leap seconds because Windows just isn't accurate enough: The W32Time service uses the Network Time Protocol (NTP) in Microsoft Windows Server 2003, in Windows Server 2003 R2, in Windows Server 2008, and in Windows Server 2008 R2. ... The W32Time service cannot reliably maintain sync time to the range of 1 to 2 seconds. Such tolerances are outside the design specification of the W32Time service. http://support.microsoft.com/kb/939322 http://support.microsoft.com/kb/939322

dfc 15y ago

UTC is a "paper clock." The calculation of UTC has nanosecond error, not multisecond error. The most recent data for the calculation of UTC: ftp://ftp2.bipm.org/pub/tai/publication/cirt.290 Time is a really tricky and interesting topic. I would start with: "From Sundials to Atomic Clocks: Understanding Time and Frequency" http://www.nist.gov/timefreq/general/pdf/1796.pdf http://www.nist.gov/timefreq/general/pdf/1796.pdf "Originally published in 1977, this full-length book provides a comprehensive, easy-to-understand introduction to the field of time and frequency. Readers of nearly all ages and educational backgrounds should find it enjoyable. 306 pages."

phillmv 15y ago

Well… it's an arbitrary standard. We just call a day one full rotation of the earth, and we just happen to divide it into 24hr periods with 60 subdivisions. Theoretically, you could measure the rotation of the earth and call that a day, but I have no idea how you would measure that down to the millisecond. As to "Real Time", I don't know about the specific legalities, but the standard goes as follows: everyone uses UTC http://en.wikipedia.org/wiki/Coordinated_Universal_Time http://en.wikipedia.org/wiki/Coordinated_Universal_Time which is based off the International Atomic Time http://en.wikipedia.org/wiki/International_Atomic_Time http://en.wikipedia.org/wiki/International_Atomic_Time I too would be interested in how they picked the beginning epoch, i.e. "this moment right here is midnight from which all other seconds shall be referenced against" but in the grand scheme of things it doesn't matter - the only thing that matters is the standard we agreed to measure against.

dfc 15y ago
"Theoretically, you could measure the rotation of the earth and call that a day" This is not theoretical; this is known as sidereal time and is measured to the nanosecond... http://en.wikipedia.org/wiki/Sidereal_time http://en.wikipedia.org/wiki/Sidereal_time
- mceachen 15y ago
  Except that "…due to nutation, an actual sidereal day is not quite so constant." Yeah. Nutation. Nutation "…is a rocking, swaying, or nodding motion in the axis of rotation of a largely axially symmetric object, such as a gyroscope, planet, or bullet in flight…" http://en.wikipedia.org/wiki/Nutation http://en.wikipedia.org/wiki/Nutation

troels 15y ago

The rotation of Earth is actually not as stable a thing as one might expect. See for example http://en.wikipedia.org/wiki/Leap_second http://en.wikipedia.org/wiki/Leap_second

dfc 15y ago
NIST recently published the new ntp.leap file and it looks like we will have one this July... 3550089600 35 # 1 Jul 2012 [1]
- apaprocki 15y ago
  You also need to make sure your IANA tzdata is updated to account for this. I think this leap second was first added in tzdata2012a, but I'd have to check: ftp://ftp.iana.org/tz/data/leapseconds
  - acqq 15y ago
    If you use Unix time, you don't have to care: http://en.wikipedia.org/wiki/Unix_time http://en.wikipedia.org/wiki/Unix_time "Unix time, (...) is (...) defined as the number of seconds elapsed since midnight Coordinated Universal Time (UTC) of Thursday, January 1, 1970 (...) not counting leap seconds"
    - apaprocki 15y ago
      You might not be able to ignore leap seconds if you are interacting with an external system (software or hardware) / API that expects actual UTC. General rule applies to everything.. know your system, know what systems you are connected to, know what standards you need to adhere to. Microsoft apparently rejected the idea of using leap seconds because Windows just isn't accurate enough: The W32Time service uses the Network Time Protocol (NTP) in Microsoft Windows Server 2003, in Windows Server 2003 R2, in Windows Server 2008, and in Windows Server 2008 R2. ... The W32Time service cannot reliably maintain sync time to the range of 1 to 2 seconds. Such tolerances are outside the design specification of the W32Time service. http://support.microsoft.com/kb/939322 http://support.microsoft.com/kb/939322