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Leap second coming up

Doug Kerr

Well-known member
A calendar year is defined in terms of an integral number of seconds (365•86,400, except in a "leap year", in which case it is 366• 86,400) , where the length of the second is defined in "atomic clock" terms.

But of course the intent of the overall time scheme is that we have consistency between this resulting "24-hour" time scale and the day-to-night reality of the earth's rotation.

A small discrepancy in the relationship between the two concepts might not seem to be troublesome, but if this discrepancy should grow without bound, it could be troublesome indeed. And current doctrine is that a discrepancy of over an entire second is undesirable.

To avert the emergence of a greater discrepancy, every so often, a "leap second" is inserted into the "UTC" time scheme. This is ordinarily done, when the need has been agreed by the cognizant scientific time organization, at the very end of the year or, if there is seen to be some "urgency" (hard to believe), at the end of June.

In fact, a leap second will be inserted at the very end of December 31, 2016 (just about 3 days from now). It will be the 27th leap second inserted since the scheme was instituted. (The immediately prior one was at the end of June, 2015; the next previous one was at the end of June, 2012.)

The way that works is as follows; we will work in terms of UTC (nominally the same as Greenwich Mean Time), with a "24-hour" clock.

Ordinarily, in successive seconds starting just before the end of December 31 (lets say, last year), the clock would read:

2015.12.31 23:59.57

2015.12.31 23:59.58

2015.12.31 23:59.59

2016.01.01 00:00:00 (Happy New Year!)

2016.01.01 00:00:01​

But this time, the clock will read (at successive seconds) as follows; I will put in bold the unusual member of this series:

2016.12.31 23:59.57

2016.12.31 23:59.58

2016.12.31 23:59.59

2016.12.31 23:59.60 (Yes, not just any old clock can show this!)

2017.01.01 00:00:00

2017.01.01 00:00:01​

In effect, the UTC clock has been "stretched out" for one second, so this last day of 2016 will have 86,401 seconds.

It is often said that the UTC clock "is stopped" for one seconds, but, as you can see from the sequence above, that is not the best way to look at it (and in fact that outlook just won't work if we are dealing with time in smaller increments than one second, as we of course do all the time in various systems).

It is also possible that it would be required to insert a "negative" leap second. This has never so far happened, but if it did, the sequence of clock readings (again at intervals of one second) would be, for a December event at the end of this year:

2016.12.31 23:59.57

2016.12.31 23:59.58

2017.01.01 00:00:00 (Note the "jump" here.)

2017.01.01 00:00:01​


Now of course the matter of leap seconds leads to zillions of complications in systems of all sorts, something that the field of computer science has increasingly improved its ability to deal with.

But, for example, in GPS (the Global Positioning System), and as well in the GSM cellular communication system (and other cellular systems as well), which get their time from the time of GPS, leap seconds are not applied to the "real clock" from which the systems operate.

However, since we may wish to get "actual wall time" (UTC) from the GPS or GSM signal, both those systems transmit a number that says "how many leap seconds have been applied to UTC". Then, a "wall clock" can apply that to the system time to get real UTC.

All this notwithstanding, there has been for a number of years respected engineering work addressing teh matter of do we need to do this at all, or is there another, less troublesome, was we can deal with this matter. I must admit I have not read the literature there, so I don't know what the "other way" is.

For the benefit of those in the U.S. who want to watch this happen on one of the on-line clocks that can show it directly, the event will occur as we approach 7:00 pm EST on December 31, 2016.

Best regards,

Doug
 

Asher Kelman

OPF Owner/Editor-in-Chief
Actual leap seconds are designed to tie the lines of longitude and prevent the earth springing back to flatness and to the time when HUGE plastic dynosaurs roamed the earth!

Asher
 

Doug Kerr

Well-known member
Hi, Asher,

Actual leap seconds are designed to tie the lines of longitude and prevent the earth springing back to flatness and to the time when HUGE plastic dynosaurs roamed the earth!

And of course, on the seventh day of Christmas, twenty-seven seconds a-leaping.

Best regards,

Doug
 
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