Editing Day (section)

== Variations in length ==
{{See|Leap second|Tidal acceleration}}
Mainly due to [[Tidal acceleration|tidal deceleration]] – the Moon's [[Gravity|gravitational pull]] slowing down the [[Earth's rotation]] – the Earth's [[Rotation period|rotational period]] is slowing.<ref name=":4">{{Cite journal |last1=McCarthy |first1=Dennis D. |last2=Hackman |first2=Christine |last3=Nelson |first3=Robert A. |date=2008-11-01 |title=The Physical Basis of the Leap Second |url=https://iopscience.iop.org/article/10.1088/0004-6256/136/5/1906 |journal=The Astronomical Journal |volume=136 |issue=5 |pages=1906–1908 |doi=10.1088/0004-6256/136/5/1906 |bibcode=2008AJ....136.1906M |s2cid=124701789 |issn=0004-6256 |access-date=2022-08-20 |archive-date=2022-05-31 |archive-url=https://web.archive.org/web/20220531162009/https://iopscience.iop.org/article/10.1088/0004-6256/136/5/1906 |url-status=live }}</ref> Because of the way the second is defined, the mean length of a solar day is now about 86,400.002 seconds, and is increasing by about 2 [[millisecond]]s per century.<ref name=":2">{{Cite journal |last=Arbab |first=Arbab I. |date=January 2009 |title=The Length of the Day: A Cosmological Perspective |url=http://www.ptep-online.com/2009/PP-16-02.PDF |journal=Department of Physics, Faculty of Science, University of Khartoum |volume=1 |access-date=2022-08-20 |archive-date=2022-08-20 |archive-url=https://web.archive.org/web/20220820051602/http://www.ptep-online.com/2009/PP-16-02.PDF |url-status=live }}</ref>

Since the rotation rate of the Earth is slowing, the length of a {{Abbr|SI|International System of Units}} second fell out of sync with a second derived from the rotational period.<ref name=":4" /> This created the need for [[leap second]]s, which insert extra seconds into [[Coordinated Universal Time|Coordinated Universal Time (UTC)]].<ref name=":4" /> Although typically 86,400 {{Abbr|SI|International System of Units}} seconds in duration, a civil day can be either 86,401 or 86,399 SI seconds long on such a day. Other than the two-millisecond variation from tidal deceleration, [[Day length fluctuations|other factors minutely affect the day's length]], which creates an irregularity in the placement of leap seconds.<ref>{{cite web |date=2013 |title=IERS science background |url=https://www.iers.org/IERS/EN/Science/EarthRotation/EarthRotation.html |url-status=live |archive-url=https://web.archive.org/web/20160829050135/https://www.iers.org/IERS/EN/Science/EarthRotation/EarthRotation.html |archive-date=August 29, 2016 |access-date=August 6, 2016 |publisher=[[IERS]] |location=Frankfurt am Main}}</ref> Leap seconds are announced in advance by the [[International Earth Rotation and Reference Systems Service|International Earth Rotation and Reference Systems Service (IERS)]], which measures the Earth's rotation and determines whether a leap second is necessary.

=== Geological day lengths ===
Discovered by paleontologist [[John W. Wells]], the day lengths of geological periods have been estimated by measuring sedimentation rings in coral [[fossil]]s,<ref name=":3" /><ref name=":2" /> due to some biological systems being affected by the tide.<ref name=":2" /> The length of a day at the Earth's formation is estimated at 6 hours.<ref name=":2" /> Arbab I. Arbab plotted day lengths over time and found a curved line.<ref name=":2" /> Arbab attributed this to the change of water volume present affecting Earth's rotation.<ref name=":2" />
{| class="wikitable"
|-
| '''Date'''
| '''Geological period'''
| '''Number of days per year'''<ref name=":3">{{Cite journal|url=http://articles.adsabs.harvard.edu//full/1969LAstr..83..411K/0000411.000.html|title=Paléo-Astronomie|author=J.Kovalesky ''Bureau des Longitudes''|journal=L'Astronomie|year=1969|volume=83|page=411|bibcode=1969LAstr..83..411K|access-date=5 June 2021|archive-date=21 December 2019|archive-url=http://archive.wikiwix.com/cache/20191221210557/http://articles.adsabs.harvard.edu//full/1969LAstr..83..411K/0000411.000.html|url-status=live}}</ref>
| '''Duration of the day'''
|-
| Present
| Current
| 365
| 24 hours
|-
| −100 million years
| [[Cretaceous]]
| 380
| 23 hours and 20 minutes
|-
| −200 million years
| [[Triassic]]
| 390
| 22 hours and 40 minutes
|-
| −300 million years
| [[Carboniferous]]
| 400
| 22 hours
|-
| −400 million years
| [[Devonian]]
| 410
| 21 hours and 20 minutes
|-
| −500 million years
| [[Cambrian]]
| 425
| 20 hours and 40 minutes
|}