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Mainly due to the slowing down of earth rotation, it is about time to add another (the 35th) leap second to UTC in order to keep its time of day close to the mean solar time. NASA features an explanation of why the leap seconds have to be inserted, how earth rotation can be measured precisely and why it is impossible to give precise predictions on when the next one will happen.
The length of day is influenced by many factors, mainly the atmosphere over periods less than a year. Our seasonal and daily weather variations can affect the length of day by a few milliseconds over a year. Other contributors to this variation include dynamics of the Earth's inner core (over long time periods), variations in the atmosphere and oceans, groundwater, and ice storage (over time periods of months to decades), and oceanic and atmospheric tides. Atmospheric variations due to El Niño can cause Earth's rotation to slow down, increasing the length of day by as much as 1 millisecond, or a thousandth of a second.
VLBI [Very Long Baseline Interferometry] tracks these short- and long-term variations by using global networks of stations to observe astronomical objects called quasars. The quasars serve as reference points that are essentially motionless because they are located billions of light years from Earth. Because the observing stations are spread out across the globe, the signal from a quasar will take longer to reach some stations than others. Scientists can use the small differences in arrival time to determine detailed information about the exact positions of the observing stations, Earth's rotation rate, and our planet's orientation in space.
Original Submission
(Score: 3, Interesting) by bob_super on Tuesday June 30 2015, @12:38AM
https://en.wikipedia.org/wiki/Coordinated_Universal_Time [wikipedia.org]
https://en.wikipedia.org/wiki/Universal_Time [wikipedia.org]
UT1 is continuous and drifts from true noon as the earth slows down. UTC isn't continuous but tracks the earth to less than a second.
UTC is a royal pain when doing broadcast TV, especially in the US.
Don't get me started on pre-1972 seconds...
(Score: 2) by bob_super on Tuesday June 30 2015, @12:43AM
ARGH!
Wrong brain, Sorry!
https://en.wikipedia.org/wiki/International_Atomic_Time [wikipedia.org]
TAI is continuous, based on "seconds since"
UT1 is earth rotation
UTC is based on TAI, such that |UT1-UTC| 1s (leap seconds needed)
(Score: 2) by kaszz on Tuesday June 30 2015, @12:50AM
So people should use TAI for GPS and broadcast etc. And UTC for alarm clocks etc. And everybody will be happy?
(Score: 2) by bob_super on Tuesday June 30 2015, @12:57AM
In a nutshell, but this perfect world is screwed because humans who only occasionally care for 27 seconds, use machines who always do.
And instead of going TAI, humans have designed machines who seem to all have a different way to handle UTC leap seconds...
The most fun part of the dialog when broadcasters were trying to standardize time, is "how does a machine which just rebooted know what the time is". Trust me, it's a whole lot more complex than it sounds.
(Score: 2) by TheLink on Tuesday June 30 2015, @08:00AM
Most programs can handle that better than they can handle leap seconds - to them it's just like suddenly the CPU got faster - most modern programs should be able to deal with CPUs running faster and slower. Most programs and their use cases can also handle ten different seconds throughout the day each taking 10% longer than it should normally. Or just have 10000 seconds that each take 0.01% longer.
Leap seconds are mostly a stupid idea. Who wants their clock to ever show something like "23:59:60" ?
(Score: 3, Insightful) by iwoloschin on Tuesday June 30 2015, @11:07AM
Are you actually going to notice it showing the extra second? I mean, literally, look away for a second and you miss it!
(Score: 2) by bob_super on Tuesday June 30 2015, @03:23PM
> Who wants their clock to ever show something like "23:59:60" ?
Someone who likes the idea that the time is right?
The Incas had the whole planetary rotation figured out hundreds of years ago. By introducing amazingly better precision in the measurement of time, should we have to give up on the basics?
(Score: 2) by tangomargarine on Tuesday June 30 2015, @05:33PM
how does a machine which just rebooted know what the time is". Trust me, it's a whole lot more complex than it sounds.
CMOS.
"Is that really true?" "I just spent the last hour telling you to think for yourself! Didn't you hear anything I said?"