Editing Second (section)

==Timekeeping standards==
{{main|Time standard}}

A set of atomic clocks throughout the world keeps time by consensus: the clocks "vote" on the correct time, and all voting clocks are steered to agree with the consensus, which is called [[International Atomic Time]] (TAI). TAI "ticks" atomic seconds.<ref name="McCarthy 2009">{{cite book |last1=McCarthy |first1=Dennis D. |author-link1=Dennis McCarthy (scientist) |last2=Seidelmann |first2=P. Kenneth |title=Time: From Earth Rotation to Atomic Physics |date=2009 |location=Weinheim |publisher=Wiley }}</ref>{{Rp|207–218}}

Civil time is defined to agree with the rotation of the Earth. The international standard for timekeeping is [[Coordinated Universal Time]] (UTC). This time scale "ticks" the same atomic seconds as TAI, but inserts or omits [[leap second]]s as necessary to correct for variations in the rate of rotation of the Earth.<ref name="McCarthy 2009" />{{Rp|16–17, 207}}

A time scale in which the seconds are not exactly equal to atomic seconds is UT1, a form of [[universal time]]. UT1 is defined by the rotation of the Earth with respect to the Sun, and does not contain any leap seconds.<ref name="McCarthy 2009" />{{Rp|68, 232}} UT1 always differs from UTC by less than a second.

===Optical lattice clock===
{{Main|Optical lattice clock}}

While they are not yet part of any timekeeping standard, optical lattice clocks with frequencies in the visible light spectrum now exist and are the most accurate timekeepers of all.  A [[strontium]] clock with frequency 430&nbsp;[[Terahertz (unit)|THz]], in the red range of visible light, during the 2010s held the accuracy record: it gains or loses less than a second in 15 billion years, which is longer than the estimated age of the universe. Such a clock can measure a change in its elevation of as little as 2&nbsp;cm by the change in its rate due to [[gravitational time dilation]].<ref>{{cite web|last1=Vincent|first1=James|title=The most accurate clock ever built only loses one second every 15 billion years|url=https://www.theverge.com/2015/4/22/8466681/most-accurate-atomic-clock-optical-lattice-strontium|website=TheVerge|date=April 22, 2015|access-date=January 26, 2018|archive-url=https://web.archive.org/web/20180127084115/https://www.theverge.com/2015/4/22/8466681/most-accurate-atomic-clock-optical-lattice-strontium|archive-date=January 27, 2018|url-status=live}}</ref>