Editing Leap second (section)

==Rationale==
{{See also|ΔT (timekeeping)}}
[[File:Deviation of day length from SI day.svg|thumb|left|Deviation of day length from SI based day with shorter days resulting from faster planetary rotation.]]
Leap seconds are irregularly spaced because the Earth's rotation speed changes irregularly. Indeed, the Earth's rotation is quite unpredictable in the long term, which explains why leap seconds are announced only six months in advance.

A [[mathematical model]] of the variations in the length of the solar day was developed by [[F. R. Stephenson]] and L. V. Morrison,<ref name="SM1995" /> based on records of [[eclipse]]s for the period 700 BC to 1623, telescopic observations of [[occultation]]s for the period 1623 until 1967 and atomic clocks thereafter. The model shows a steady increase of the mean solar day by 1.70&nbsp;ms (±0.05&nbsp;ms) per century, plus a periodic shift of about 4&nbsp;ms amplitude and [[period (physics)|period]] of about 1,500&nbsp;yr.<ref name="SM1995" /> Over the last few centuries, rate of lengthening of the mean solar day has been about 1.4&nbsp;ms per century, being the sum of the periodic component and the overall rate.<ref>{{cite web |author=Steve Allen |date=8 June 2011 |title=Extrapolations of the difference (TI – UT1) |url=http://www.ucolick.org/~sla/leapsecs/dutc.html |url-status=live |archive-url=https://web.archive.org/web/20160304193859/http://www.ucolick.org/~sla/leapsecs/dutc.html |archive-date=4 March 2016 |access-date=29 February 2016 |work=ucolick.org}}</ref>

The main reason for the slowing down of the Earth's rotation is [[tidal friction]], which alone would lengthen the day by 2.3&nbsp;ms/century.<ref name="SM1995" /> Other contributing factors are the movement of the Earth's [[crust (geology)|crust]] relative to its [[planetary core|core]], changes in [[mantle convection]], and any other events or processes that cause a significant redistribution of mass. These processes change the Earth's [[moment of inertia]], affecting the rate of rotation due to the conservation of [[angular momentum]]. Some of these redistributions increase Earth's rotational speed, shorten the solar day and oppose tidal friction. For example, [[glacial rebound]] shortens the solar day by 0.6&nbsp;ms/century and the [[2004 Indian Ocean earthquake and tsunami|2004 Indian Ocean earthquake]] is thought to have shortened it by 2.68 microseconds.<ref>{{cite web |last1=Cook-Anderson |first1=Gretchen |last2=Beasley |first2=Dolores |date=10 January 2005 |title=NASA Details Earthquake Effects on the Earth |url=http://www.nasa.gov/home/hqnews/2005/jan/HQ_05011_earthquake.html |url-status=live |archive-url=https://web.archive.org/web/20110127163105/http://www.nasa.gov/home/hqnews/2005/jan/HQ_05011_earthquake.html |archive-date=27 January 2011 |publisher=[[National Aeronautics and Space Administration]] (press release).}}</ref>

It is a mistake, however, to consider leap seconds as indicators of a slowing of Earth's rotation rate; they are indicators of the accumulated difference between atomic time and time measured by Earth rotation.<ref>{{cite web |last1=Chester |first1=Geoff |date=15 June 2015 |title=Wait a second… 2015 will be a little longer |url=https://www.doncio.navy.mil/chips/ArticleDetails.aspx?ID=6471 |access-date=4 March 2021 |website=CHIPS Articles: The Department of the Navy's Information Technology Magazine}}</ref>  The plot at the top of this section shows that in 1972 the average length of day was approximately {{val|86400.003}} seconds and in 2016 it was approximately {{val|86400.001}} seconds, indicating an overall increase in Earth's rotation rate over that time period.  Positive leap seconds were inserted during that time because the annual average length of day remained greater than {{val|86400}} SI seconds, not because of any slowing of Earth's rotation rate.<ref>{{cite web |last1=Plait |first1=Phil |date=31 December 2008 |title=Followup: Leap Seconds |url=https://www.discovermagazine.com/the-sciences/followup-leap-seconds |access-date=5 March 2021 |website=Discover Magazine: Bad Astronomy}}</ref>

In 2021, it was reported that Earth was spinning faster in 2020 and experienced the 28 shortest days since 1960, each of which lasted less than {{val|86399.999}} seconds.<ref>{{cite web |last1=Jones |first1=Graham |last2=Bikos |first2=Konstantin |date=6 January 2021 |orig-date=2020-12-23 |title=Earth is in a hurry in 2020 |url=https://www.timeanddate.com/time/earth-faster-rotation.html |access-date=6 March 2021 |website=timeanddate.com}}</ref>  This caused engineers worldwide to discuss a negative leap second and other possible timekeeping measures, some of which could eliminate leap seconds.<ref>{{cite news |last1=Knapton |first1=Sarah |date=4 January 2021 |title=The Earth is spinning faster now than at any time in the past half century |url=https://www.telegraph.co.uk/news/2021/01/04/earth-spinning-faster-now-time-past-half-century/ |url-access=subscription |url-status=live |archive-url=https://ghostarchive.org/archive/20220112/https://www.telegraph.co.uk/news/2021/01/04/earth-spinning-faster-now-time-past-half-century/ |archive-date=12 January 2022 |access-date=11 February 2021 |work=[[The Daily Telegraph]]}}{{cbignore}}</ref> The shortest day ever recorded was 29 June 2022, at 1.59 milliseconds less than 24 hours.<ref name="NHM 270324">{{cite news |last=Ashworth |first=James |date=27 March 2024 |title=Climate change is causing days to get longer by slowing down the Earth |url=https://www.nhm.ac.uk/discover/news/2024/march/climate-change-causing-days-get-longer-slowing-down-earth.html |publisher=Natural History Museum |location=London}}</ref> In a 2024 paper published in [[Nature (journal)|''Nature'']], Duncan Agnew of the [[Scripps Institution of Oceanography]] projects that the water from increasing [[ice cap]] melting will migrate to the equator and thus cause the rate of rotation to slow down again.<ref name="NHM 270324" />