Leap second

A leap second is an additionally inserted in the Coordinated Universal Time UTC second when required in order to synchronize it with the Universal Solar Time UT1 ( DUT1 <0.9 s ). It is defined and implemented by the International Earth Rotation and Reference Systems Service ( IERS ).

History

Until the 1950s, the second was defined as 1/86400 of a mean solar day. An ultimately derived from the Sun and from the Earth's rotation timescale to but not strictly uniform, since the rotational speed of the earth is subject to irregular fluctuations and a long-term slowdown. Such a non-uniform time scale is not useful for many technical and scientific purposes.

Since 1967, therefore, the second is set as the 9,192,631,770 times the period of the transition between the two hyperfine levels of the ground state of atoms of the nuclide 133Cs radiation corresponding. The uniform running time scale so produced is the International Atomic Time (TAI ). The Earth's rotation is nevertheless continue to be monitored; which derived from it uneven running time scale is the Universal Time UT1, which is needed eg for astronomical purposes.

In 1972, the difference between UTC and TAI was before the introduction of the leap second as 10 seconds. Thus, the difference is now at about 35 seconds.

Although the second was redefined several times in the course of time, they kept their length at each change in definition as well as possible. Therefore, the length of today's SI second is based ultimately on provisions of the length of the mean solar day, from the late 19th century. The variability of the mean solar day was not yet known, and their average length was determined from observational data, which extended mainly through the 18th and 19th centuries. As a result, today's second representative of the length of the astronomically determined second about the middle of what was then evaluated observation period, ie by 1820. Currently, the day a few milliseconds longer than it was then, so that the derived from the earth's rotation, UT1 seconds is slightly longer than the SI second, which reflects the situation 200 years ago. Therefore, it is necessary to balance insert more leap seconds in UTC ( positive leap seconds ) and omit (negative leap seconds ).

Need

The Earth's rotation is subject to constant fluctuations. In recent decades, it has accelerated, but in the long run it is slowed down by tidal friction. Therefore UT1 runs increasingly slowly while TAI runs strictly uniform.

The civil time used in everyday life expediently should be based on the day - night cycle, ie at the Earth's rotation and thus to UT1, on the other hand is for technical purposes a strictly uniform time scale, ie TAI desirable. As a compromise, the Coordinated Universal Time UTC was implemented. Unit of time for UTC is based on atomic clocks SI second as for TAI. By inserting leap seconds is achieved that UTC never removed more than 0.9 s of UT1. In this way, on the one hand ensures that the daily time scale, long-term drifts of the mean solar time, on the other hand is an exact atomic clock time unit.

Practical implementation

Leap seconds are set in Germany by the Physikalisch -Technische Bundesanstalt, which, however, this assumes only the internationally defined by the International Earth Rotation and Reference Systems Service ( IERS ) leap seconds. In the middle are leap seconds approximately every 18 months needed and favor on 31 December or 30 June, subordinated on March 31 or September 30, according 23:59:59 UTC (ie before 1:00 CET and 2:00 CEST ) inserted. Since the introduction of the system in 1972, only the time points were used in December and June.

The statement to insert a leap second is always given, when can be expected in the near future, that the difference between UTC and UT1 about 0.9 seconds increases. After 23:59:59 UTC of that day that extra second is added at 23:59:60, before the clock advances to 00:00:00 of the following day. This means that the day is with the leap second atom of 86401 seconds instead of the usual 86400th In the event that the Earth's rotation would be significantly faster, and negative leap seconds are provided. In this case the following day with 00:00:00 23:59:58 would immediately follow. This case is, however, never occurred. Although the Earth's rotation has since the late 1970s accelerated slightly (within the normal fluctuations ) so that leap seconds were less often necessary since. However, a day is still longer than the nominal atomic 86400 seconds, so presumably must continue to be made in more or less regular intervals compensation by positive leap seconds.

The correct representation of 31.12. 23:59:60 is rarely seen on commercial clocks. Due to the principle only Radio Watches would be suitable and even these are usually designed so that they ignore the transmission of 23:59:60 and simply instead of the correct display of a second longer to 23:59:60 23:59:59 or 00: 00:00 persevere. This is for most manufacturers anyway desirable, because the display 23:59:60 can be interpreted due to lack of knowledge of the user about the leap second as None or defect ( mis).

Also on some computer systems leads to differences in the calculation of time differences, many systems provide the difference between " 31 December 1998 23:59:59 UTC " and" 1 January 1999 00:00:00 UTC " " 1 second " flawed to, or fail when one times like" 31 December 1998 23:59:60 UTC " or" 1 January 1999 00:59:60 CET " enters. However, this is only a problem when it comes to safety-critical systems or systems for continuous data acquisition and display, such as databases, computer clusters and spatially distributed / mirrored systems.

List of leap seconds

Future

Due to the irregularity of the slowing down of the Earth's rotation is to predict if and when a leap second is necessary only for the near future. In July 2008, the statement was given by the IERS for the 24th time, insert a leap second. This was inserted into the night of January 1, 2009 at 00:59:59 Clock CET ( 23:59:59 UTC clock the previous day). The date last leap second was added to the transition from 30 June to 1 July 2012.

It is the responsibility of the International Earth Rotation and Reference Systems Service for ( IERS ) to observe the Earth's rotation and to determine whether a leap second is necessary. Your determination is published in the Bulletin C, which published every six months.

At present, it is discussed whether the insertion of a leap second is to occur less frequently, as this procedure a source of error for various computer systems was, or even whether the system should be fundamentally changed. If an entire shift hours would be inserted around the year 2600, could be found on the frequent small - no need adjustments - to be carried out at irregular intervals. The decision was postponed to 2015.

The Global Positioning System (GPS ) is used since its introduction in 1980 is an atomic time scale without leap seconds, so as not to get out of sync. The GPS time is therefore the UTC to 16 seconds ahead of (as of 2012). The data from the GPS broadcast signal is added to make it available in GPS receivers and users of the UTC, the actual difference.

Others

The longest year in the Gregorian calendar in the time zone UTC ± 0, and west was in 1972. It was a leap year one day and two leap seconds longer than usual. The shortest year was 1582, when the 10 days between the 4th and the 15th October were skipped by the introduction of the Gregorian calendar.

The leap second June 30, 2012 caused many servers worldwide significant problems. Especially computer with certain Linux kernel versions were affected. Due to a bug in the kernel, there was a so-called deadlock. Mainly affected were Java -based programs and MySQL databases. In addition to the airline Qantas, which had to move about 50 flights, many large sites were affected, which already moved journals to a comparison with the millennium bug. The company Google escaped the problem by a trick in which over the entire day small time increments were summed for the timing of the NTP server, up to the time difference was level.