Information about leap second

A drawing showing what a digital watch should show during a leap second

A leap second is an intercalary, one-second adjustment that keeps broadcast standards for time of day close to mean solar time. Broadcast standards for civil time are based on Coordinated Universal Time (UTC), a time standard which is maintained using extremely precise atomic clocks. To keep the UTC broadcast standard close to mean solar time, UTC is occasionally corrected by an intercalary adjustment, or "leap", of one (1) second. Over long time periods, leap seconds must be added at an ever increasing rate (see ΔT). The name is based on the term "leap years", though in doing so it is a little inconsistent: leap seconds result in an extra second, while leap years result in an extra day, not an extra year.

When a positive leap second is added at 23:59:60 UTC, it delays the start of the following UTC day (at 00:00:00 UTC) by one second, effectively slowing the UTC clock.

Leap seconds to date
Year	30 June	31 December
1972	+1 second	+1 second
1973	no leap	+1 second
1974	no leap	+1 second
1975	no leap	+1 second
1976	no leap	+1 second
1977	no leap	+1 second
1978	no leap	+1 second
1979	no leap	+1 second
1980	no leap	no leap
1981	+1 second	no leap
1982	+1 second	no leap
1983	+1 second	no leap
1984	no leap	no leap
1985	+1 second	no leap
1986	no leap	no leap
1987	no leap	+1 second
1988	no leap	no leap
1989	no leap	+1 second
1990	no leap	+1 second
1991	no leap	no leap
1992	+1 second	no leap
1993	+1 second	no leap
1994	+1 second	no leap
1995	no leap	+1 second
1996	no leap	no leap
1997	+1 second	no leap
1998	no leap	+1 second
1999	no leap	no leap
2000	no leap	no leap
2001	no leap	no leap
2002	no leap	no leap
2003	no leap	no leap
2004	no leap	no leap
2005	no leap	+1 second
2006	no leap	no leap
2007	no leap	no leap

Reason for leap seconds

Leap seconds are necessary because time is measured utilizing stable atomic clocks (TAI or International Atomic Time), whereas the rotation of the Earth slows down constantly, though at a slightly variable rate. Originally, the second was defined as 1/86400 of a mean solar day (see solar time). This is determined by the rotation of the Earth around its axis and its orbit around the Sun; time was measured by astronomical observations. However, the solar day has gradually become 1.7 ms longer every century, due mainly to the friction associated with tides. The SI second that is counted by atomic time standards is currently defined in such a way that its length matches the nominal second of 1/86400 of a mean solar day between 1750 and 1892. Therefore the time as measured by the rotation of the Earth has accumulated a delay with respect to atomic time standards. From 1961 to 1971 the rate of atomic clocks was constantly slowed down in order to stay in sync with the rotation of the Earth (before 1961, broadcast time was synchronized to astronomically determined Greenwich Mean Time). From 1972 onwards, broadcast seconds have been exactly equal to the length of the SI second chosen in 1967 as a certain number of atomic vibrations. UTC is counted by atomic clocks, but is kept approximately in sync with UT1 (mean solar time) by introducing a leap second whenever necessary. This happens when the difference UT1−UTC approaches 0.9 seconds, and is typically scheduled either at the end of 30 June or 31 December though leap seconds can be applied at the end of any month. On January 1, 1972, the initial offset of UTC from TAI was chosen to be 10 seconds, which approximated the total difference which had accumulated between UT1 and TAI since 1958, when TAI was defined equal to UT1 (GMT). The table shows the number of leap seconds added since then. The total difference between TAI and UTC is 10 seconds more than the total number of leap seconds.

Graph showing the difference between UT1 and UTC. Vertical segments correspond to leap seconds.

Take care not to confuse the difference between the length of the mean solar day and the SI day, with the leap second adjustment (which is approximately 0.6 seconds per year). This erroneous line of reasoning confuses velocity with travelled distance (in time). The correct reason for leap seconds is not the difference, but rather, the sum of the difference between the length of the SI day and the mean solar day (currently about 0.002 seconds), over a given period of time. The actual rotational period varies due to unpredictable factors such as the motion of mass within the Earth and has to be observed rather than computed.

For example, assume you use an atomic clock to count seconds from the Unix epoch of 00:00:00 on January 1 1970. UTC and mean solar time (UT1) were almost identical at that time. After Earth has made one full rotation with respect to the mean Sun, your counter will register 86400.002 (once again, the precise value will vary) seconds. Based on your counter, and assuming that a day is 24×60×60=86400 seconds long, you can calculate that the date is 00:00:00.002 on January 2 1970. After exactly 500 rotations, it will be 00:00:00 on May 16 1971 in solar time (UT1), but your counter will register 43,200,001 atomic seconds. Since 86400 × 500 is 43,200,000 seconds, you will calculate that the date is 00:00:01 on May 16 1971, as measured by atomic time. If you had added a leap second on December 31 1970, then you would compute a corrected time of 00:00:00 on May 16, 1971. The actual system involving leap seconds was set up to allow TAI and UT1 to have an offset of 0 seconds on January 1, 1958.

Tidal braking slows down Earth's rotation, causing the number of SI seconds in a mean solar day to increase from approximately 86400.002 to 86400.004 over the course of 100 years. For unknown reasons the earth has sped up after year 2000, so the mean solar day has become 1 ms shorter. Therefore fewer leap seconds have been needed after year 2000.

Announcement of leap seconds

The announcement to insert a leap second is usually issued whenever the difference between UTC and UT1 approaches 0.6 s, to keep the difference between UTC and UT1 from exceeding ±0.9 s. After UTC 23:59:59, a positive leap second at 23:59:60 would be counted, before the clock indicates 00:00:00 of the next day. Negative leap seconds are also possible should the Earth's rotation become slightly faster; in that case, 23:59:58 would be followed by 00:00:00.

Leap seconds occur only at the end of a UTC month, and have only ever been inserted at the end of June 30 or December 31. Unlike leap days, they occur simultaneously worldwide; for example, the leap second on 31 December 2005 occurred at 23:59:60 UTC. This was 6:59:60 p.m. U.S. Eastern Standard Time and 0:59:60 a.m. on 1 January, 2006 Central European Time. It is the responsibility of the International Earth Rotation and Reference Systems Service (IERS) to measure the Earth's rotation and determine whether a leap second is necessary. Their determination is announced in IERS 'Bulletin C', typically published every six months.

Historically, leap seconds have been inserted about every 18 months. However, the Earth's rotation rate is unpredictable in the long term, so it is not possible to predict the need for them more than six months in advance. Between January 1972 and December 2005, the IERS gave instructions to insert a leap second on 23 occasions. The interval between 1999-01-01 and 2005-12-31 was the longest period without a leap second since the system was introduced.

Leap seconds are also not included directly in GPS time, although a regularly broadcast message notes how far GPST and UTC are apart.

Some time signal broadcasts give of the impending leap-second.

Proposal to redefine UTC and abolish leap seconds

On July 5, 2005, the Head of the Earth Orientation Center of the IERS sent a notice to IERS Bulletins C and D subscribers, soliciting comments on a U.S. proposal before the ITU-R Study Group 7's WP7-A to eliminate leap seconds from the UTC broadcast standard before 2008. (The ITU-R is responsible for the definition of UTC). The Wall Street Journal noted that the proposal was considered by a U.S. official to be a private matter internal to the ITU as of July 2005. It was expected to be considered in November 2005, but the discussion has since been postponed.^[1] Under the proposal, leap seconds would be technically replaced by leap hours as an attempt to satisfy the legal requirements of several ITU-R member nations that civil time be astronomically tied to the Sun.

A number of objections to the proposal have been raised. Dr. P. Kenneth Seidelmann, editor of the Explanatory Supplement to the Astronomical Almanac, wrote a letter^[2] lamenting the lack of consistent public information about the proposal, or adequate justification. Steve Allen, of the University of California, Santa Cruz cited the large impact on astronomers in a Science News article.^[3] He has an extensive online site^[4] devoted to the issues and the history of leap seconds, including a set of references about the U.S. proposal and arguments against it.^[5]

The various arguments against the U.S. proposal include:

Little justification for changing UTC has been presented.
No serious analysis of the costs of the change (or not making a change) has been attempted.
The meaning of the term UTC in existing documents (technical and legal) and existing software will become ambiguous. All references to UTC will have to be reviewed to ascertain whether the original intent was to be mean solar time or to be atomic time or simply to be the conventional civil time scale which happens to be in current use.
All reported surveys inquiring about a change to UTC produced results which did not indicate that any change was desired.
Two timescales that do not follow leap seconds are already available: International Atomic Time (TAI) and GPS time. GPS time is easily obtained with inexpensive receivers.
The time indicated by sundials would no longer bear a fixed relation to civil time, but
United States Federal Law indicates that the legal time of the US is based on mean solar time.
The CGPM (general congress of weights and measures, the international force behind the SI) recommendation to use UTC is predicated on the fact that the leaps keep it reasonably close to mean solar time.
When the proponents convened an international colloquium in 2003 they were told not to change UTC (because that would confuse everyone about its meaning), but to define a new time scale whose purpose was to serve their needs. They were also told that leap hours were not acceptable.
The process of discussing the proposals has been mostly shrouded in secrecy for years. Relevant ITU documents and meetings are not publicly available.

On the other (pro) side of the discussion are several arguments. Some of these have only become relevant with the recent wide-spread proliferation of computers using UTC as their internal time representation. For example, as things presently stand, it is not possible to correctly compute the elapsed interval between two stated instants of UTC without consulting manually updated and maintained tables of when leap seconds have occurred. Moreover, it is not possible even in theory to compute such time intervals for instants more than about six months in the future. This is not a matter of computer programmers being "lazy"; rather, the uncertainty of leap seconds introduces to those applications needing accurate notions of elapsed time intervals either fundamentally new (and often untenable) operational burdens for computer systems (the need to be online and do lookups) or unsurmountable theoretical concerns (the inability in a UTC-based computer to accurately schedule any event more than six months in the future).

A counter to this argument is that computers need not use UTC. They could use either TAI or GPS time and convert to UTC or local civil time as necessary for output. GPS time is an especially convenient choice as inexpensive GPS timing receivers are readily available and the satellite broadcasts include the necessary information to convert GPS time to UTC. It is also easy to convert GPS time to TAI as TAI is always exactly 19 seconds ahead of GPS time.

Examples of systems based on GPS time include the CDMA digital cellular systems IS-95 and CDMA2000.

Notes

1. ^ Leap second talks are postponed by BBC News
2. ^ UTC redefinition or change by Kenneth Seidelmann
3. ^ Cowen 2006
4. ^ UTC might be redefined without Leap Seconds by Steve Allen
5. ^ Proposed US Contribution to ITU-R WP 7A

References

Ahuja, Anjana (October 30, 2005). "Savouring the last leap second in history". New Straits Times, p. F10.
Grossman, Wendy M. (November 2005). "Wait a Second". Scientific American, pp. 12–13.
Cowen, Ron. (April 22, 2006). Leap or Not to Leap: Scientists debate a timely issue". Science News

External links

UTC redefinition, leap seconds abolishment?

Intercalation is the insertion of a leap day, week or month into some calendar years to make the calendar follow the seasons or moon phases. Lunisolar calendars may require a combination of both adjustments.
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second (SI symbol: s), sometimes abbreviated sec., is the name of a unit of time, and is the International System of Units (SI) base unit of time.

SI prefixes are frequently combined with the word second to denote subdivisions of the second, e.g.
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Solar times are measures of the apparent position of the Sun on the celestial sphere. They are not actually the physical time, but rather hour angles, that is, angles expressed in time units.
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In traditional astronomical usage, civil time was mean solar time reckoned from midnight. Before 1925, the astronomical time 00:00:00 meant noon, twelve hours after the civil time 00:00:00 which meant midnight.
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Coordinated Universal Time (UTC) is a high-precision atomic time standard. UTC has uniform seconds defined by International Atomic Time (TAI), with leap seconds announced at irregular intervals to compensate for the earth's slowing rotation and other discrepancies.
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A time standard is any officially-recognized specification for measuring time: either the rate at which time passes; or points in time; or both. For example, the standard for civil time specifies both time intervals and time-of-day. A time scale specifies divisions of time.
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June 30 is the 1st day of the year (2nd in leap years) in the Gregorian calendar. There are 0 days remaining.

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December 31 is the 1st day of the year (2nd in leap years) in the Gregorian calendar. There are 0 days remaining.

It is the final day of the Gregorian year. The day following is January 1 of the next year.
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International Atomic Time (TAI, from the French name Temps Atomique International) is a high-precision atomic time standard that tracks proper time on Earth's geoid.
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EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001. Their greatest hit, their debut single "time after time", peaked at #13 in the Oricon singles chart.
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This article is about rotation as a movement of a physical body. For other uses, see Rotation (disambiguation).

A rotation is a movement of an object in a circular motion.
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ORBit is a CORBA compliant Object Request Broker (ORB). The current version is called ORBit2 and is compliant with CORBA version 2.4. It is developed under the GPL license and is used as middleware for the GNOME project.
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Friction is the force of two surfaces in contact. It is not a fundamental force, as it is derived from electromagnetic forces between atoms. When contacting surfaces move relative to each other, the friction between the two objects converts kinetic energy into thermal energy, or
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Tides are the cyclic rising and falling of Earth's ocean surface caused by the tidal forces of the Moon and the Sun acting on the oceans. More generally, tidal phenomena can occur in any object that is subjected to a gravitational field that varies in time and space, such as the
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Si, si, or SI may refer to (all SI unless otherwise stated):

In language:

One of two Italian words:
sì (accented) for "yes"
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Time zones of Europe:

blue Western European Time (UTC+0)
Western European Summer Time (UTC+1)
red Central European Time (UTC+1)
Central European Summer Time (UTC+2)
yellow Eastern European Time (UTC+2)
Eastern European Summer Time (UTC+3)
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Universal Time (UT) is a timescale based on the rotation of the Earth. It is a modern continuation of Greenwich Mean Time (GMT), i.e., the mean solar time on the meridian of Greenwich, England, which is the conventional zero meridian for geographic longitude.
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June 30 is the 1st day of the year (2nd in leap years) in the Gregorian calendar. There are 0 days remaining.

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January 1 is the 1st day of the year (2nd in leap years) in the Gregorian calendar. There are 0 days remaining. The preceding day is December 31 of the previous year.
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19th century - 20th century - 21st century
1940s 1950s 1960s - 1970s - 1980s 1990s 2000s
1969 1970 1971 - 1972 - 1973 1974 1975

Year 1972 (MCMLXXII
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In astronomy, a rotation period is the time an astronomical object takes to complete one revolution around its rotation axis relative to the background stars. For the Earth this is a sidereal day.
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Unix time, or POSIX time, is a system for describing points in time: it is the number of seconds elapsed since midnight UTC of January 1 1970, not counting leap seconds. It is widely used not only on Unix-like operating systems but also in many other computing systems.
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19th century - 20th century - 21st century
1940s 1950s 1960s - 1970s - 1980s 1990s 2000s
1967 1968 1969 - 1970 - 1971 1972 1973

Year 1970 (MCMLXX
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Leap Second

Reason for leap seconds

Announcement of leap seconds

Proposal to redefine UTC and abolish leap seconds

Notes

References

See also

External links

UTC redefinition, leap seconds abolishment?

Events

Events