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{{short description|Extra second inserted to keep civil time in sync with the Earth's rotation}}
{{Use dmy dates|date=July 2023}}
[[File:Leapsecond2016.png|thumb|right|300px|[[Screenshot]] of the [[UTC]] clock from {{URL|https://time.gov/}} during the leap second on 31 December 2016.]]▼
A '''leap second''' is a one-[[second]] adjustment that is occasionally applied to [[Coordinated Universal Time]] (UTC), to accommodate the difference between precise time ([[International Atomic Time]] (TAI), as measured by [[atomic
▲[[File:Leapsecond2016.png|thumb|right|300px|Screenshot of the UTC clock from during the leap second on 31 December 2016.]]
|url=https://www.sciencenews.org/article/50-years-ago-leap-second |department=50 Years Ago |magazine=[[Science News]] |page=4}}</ref> All have so far been positive leap seconds, adding a second to a UTC day; while it is possible for a negative leap second to be needed, one has not happened yet.
Because the Earth's rotational speed varies in response to climatic and geological events,<ref>{{cite web |url=https://www.iers.org/IERS/EN/Science/EarthRotation/EarthRotation.html |title=IERS science background |publisher=[[IERS]] |___location=Frankfurt am Main |date=2013 |access-date=6 August 2016 |url-status=live |archive-url=https://web.archive.org/web/20160829050135/https://www.iers.org/IERS/EN/Science/EarthRotation/EarthRotation.html |archive-date=29 August 2016}}</ref> UTC leap seconds are irregularly spaced and unpredictable. Insertion of each UTC leap second is usually decided about six months in advance by the [[International Earth Rotation and Reference Systems Service]] (IERS), to ensure that the difference between the UTC and UT1 readings will never exceed 0.9 seconds.<ref name="Bulletin C 49">{{cite web|url=http://hpiers.obspm.fr/eoppc/bul/bulc/bulletinc.49|title=Bulletin C 49|last=Gambis|first=Danie|date=5 January 2015|publisher=[[IERS]]|___location=Paris|access-date=5 January 2015|url-status=live|archive-url=https://web.archive.org/web/20150530123243/https://hpiers.obspm.fr/eoppc/bul/bulc/bulletinc.49|archive-date=30 May 2015}}</ref><ref>{{cite web|url=https://www.theverge.com/2015/1/7/7508651/leap-second-2015-earths-rotation-slowing|title=2015 is getting an extra second and that's a bit of a problem for the internet|author1=James Vincent|website=[[The Verge]]|date=7 January 2015|url-status=live|archive-url=https://web.archive.org/web/20170317175523/http://www.theverge.com/2015/1/7/7508651/leap-second-2015-earths-rotation-slowing|archive-date=17 March 2017}}</ref>
▲A '''leap second''' is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC), to accommodate the difference between precise time (International Atomic Time (TAI), as measured by atomic clocks) and imprecise observed solar time (UT1), which varies due to irregularities and long-term slowdown in the Earth's rotation. The UTC time standard, widely used for international timekeeping and as the reference for civil time in most countries, uses TAI and consequently would run ahead of observed solar time unless it is reset to UT1 as needed. The leap second facility exists to provide this adjustment. The leap second was introduced in 1972. Since then, 27 leap seconds have been added to UTC, with the most recent occurring on December 31, 2016. All have so far been positive leap seconds, adding a second to a UTC day; while it is possible for a negative leap second to be needed, one has not happened yet.
This practice has proven disruptive, particularly in the twenty-first century and especially in services that depend on precise
▲This practice has proven disruptive, particularly in the twenty-first century and especially in services that depend on precise timestamping or time-critical process control. And since not all computers are adjusted by leap-second, they will display times differing from those that have been adjusted. After many years of discussions by different standards bodies, in November 2022, at the 27th General Conference on Weights and Measures, it was decided to abandon the leap second by or before 2035.
==History==
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Some of the objections to the proposed change have been addressed by its supporters. For example, Felicitas Arias, who, as Director of the [[International Bureau of Weights and Measures]] (BIPM)'s Time, Frequency, and Gravimetry Department, was responsible for generating UTC, noted in a press release that the drift of about one minute every 60–90 years could be compared to the 16-minute annual variation between true solar time and mean solar time, the one hour offset by use of daylight time, and the several-hours offset in certain geographically extra-large time zones.<ref>{{cite press release|url=http://www.bipm.org/utils/en/pdf/Press_Release_UTC_13October.pdf|title=The proposed redefinition of Coordinated Universal Time, UTC|publisher=[[BIPM]]|date=13 October 2011|url-status=live|archive-url=https://web.archive.org/web/20150118133939/http://www.bipm.org/utils/en/pdf/Press_Release_UTC_13October.pdf|archive-date=18 January 2015}}</ref>
Proposed alternatives to the leap second are the leap hour, which requires changes only once every few centuries;<ref>{{cite magazine |title=Scientists propose 'leap hour' to fix time system |url=https://www.newindianexpress.com/magazine/2008/dec/18/scientists-propose-leap-hour-to-fix-time-system-11669.html |magazine=New Scientist |via=The New Indian Express |access-date=3 September 2022 |orig-date=2008-12-18 |date=14 May 2012}}</ref> and the leap minute, with changes coming every half-century.<ref name="Martin-2024"
On 18 November 2022, the [[General Conference on Weights and Measures]] (CGPM) resolved to eliminate leap seconds by or before 2035. The difference between atomic and astronomical time will be allowed to grow to a larger value yet to be determined. A suggested possible future measure would be to let the discrepancy increase to a full minute, which would take 50 to 100 years, and then have the last minute of the day taking two minutes in a "kind of smear" with no discontinuity. The year 2035 for eliminating leap seconds was chosen considering Russia's request to extend the timeline to 2040, since, unlike the United States's [[global navigation satellite system]], [[GPS]], which does not adjust its time with leap seconds, Russia's system, [[GLONASS]], does adjust its time with leap seconds.<ref name="AFP">{{cite news |first= |date=18 November 2022 |title=Do not adjust your clock: scientists call time on the leap second |url=https://www.theguardian.com/world/2022/nov/18/do-not-adjust-your-clock-scientists-call-time-on-the-leap-second |website=[[The Guardian]] |department=World News |agency=[[Agence France-Presse]]}}</ref><ref name="gibney">{{cite journal |last=Gibney |first=Elizabeth |author-link=Elizabeth Gibney |date=18 November 2022 |title=The leap second's time is up: world votes to stop pausing clocks |journal=[[Nature (journal)|Nature]] |volume=612 |issue=7938 |page=18 |bibcode=2022Natur.612...18G |doi=10.1038/d41586-022-03783-5 |issn=0028-0836 |pmid=36400956 |doi-access=free}}</ref>
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==External links==
{{commons category|Leap second}}
*[https://www.iers.org/IERS/EN/Publications/Bulletins/bulletins.html IERS Bulletins, including Bulletin C (leap second announcements)]
*[http://www.leapsecond.com/ LeapSecond.com – A web site dedicated to precise time and frequency]
*[https://www.nist.gov/pml/time-and-frequency-division/leap-seconds-faqs NIST FAQ about leap year and leap second]
*[https://www.cl.cam.ac.uk/~mgk25/time/metrologia-leapsecond.pdf The leap second: its history and possible future]
*{{cite web|url=https://support.microsoft.com/help/2722715/support-for-the-leap-second|title=Support for the leap second|website=Microsoft Support|date=4 October 2018}}
*{{cite web|url=https://techcommunity.microsoft.com/t5/Networking-Blog/Leap-Seconds-for-the-IT-Pro-What-you-need-to-know/ba-p/339811|title=Leap Seconds for the IT Pro: What you need to know|author=Dan Cuomo|website=Windows Server – Networking Blog|date=17 October 2018}}
*{{cite web|url=https://techcommunity.microsoft.com/t5/Networking-Blog/Leap-Seconds-for-the-AppDev-What-you-should-know/ba-p/339813|title=Leap Seconds for the AppDev: What you should know|author=Travis Luke|website=Windows Server – Networking Blog|date=24 October 2018}}
*{{cite web|url=https://safran-navigation-timing.com/leap-second-readiness-tips-a-guide-for-managers-of-critical-it-infrastructure/|title=Leap Second Readiness Tips|publisher=Safran (formerly Orolia)|website=safran-navigation-timing.com|date=31 December 2018}}
* Judah Levine's Everyday Time and Atomic Time series
**{{cite web|url=https://nist.medium.com/everyday-time-and-atomic-time-part-one-9107b60fd9d0
|author=Judah Levine
|title=Everyday Time and Atomic Time: Part One
|publisher=National Institute of Standards and Technology
|date=31 March 2021}}
**{{cite web|url=https://nist.medium.com/everyday-time-and-atomic-time-part-2-65bb74d04f50
|author=Judah Levine
|title=Everyday Time and Atomic Time: Part Two
|publisher=National Institute of Standards and Technology
|date=7 April 2021}}
**{{cite web|url=https://nist.medium.com/everyday-time-and-atomic-time-part-3-24421c7a8c0f
|author=Judah Levine
|title=Everyday Time and Atomic Time: Part Three
|publisher=National Institute of Standards and Technology
|date=14 April 2021}}
**{{cite web|url=https://nist.medium.com/everyday-time-and-atomic-time-part-4-5d9b27820851
|author=Judah Levine
|title=Everyday Time and Atomic Time: Part Four
|publisher=National Institute of Standards and Technology
|date=21 April 2021}}
**{{cite web|url=https://nist.medium.com/everyday-time-and-atomic-time-part-5-1a9f063a8b1b
|author=Judah Levine
|title=Everyday Time and Atomic Time: Part Five
|publisher=National Institute of Standards and Technology
|date=28 April 2021}}
{{Time Topics}}
{{Time measurement and standards}}
{{New Year}}
{{DEFAULTSORT:Leap Second}}
[[Category:Timekeeping]]
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