Editing Universal Time (section)

==Alternate versions<span class="anchor" id="Versions"></span>==

'''UT1''' is the principal form of Universal Time.<ref name=Seago/> However, there are also several other infrequently used time standards that are referred to as ''Universal Time'', which agree within 0.03 seconds with UT1:<ref>{{cite web |last=Schlyter |first=Paul |title=Time Scales: UT1, UTC, TAI, ET, TT, GPS time |url=https://stjarnhimlen.se/comp/time.html |website=stjarnhimlen.se |access-date=24 May 2022}}</ref>

* '''UT0''' is Universal Time determined at an observatory by observing the diurnal motion of stars or extragalactic radio sources, and also from ranging observations of the Moon and artificial Earth satellites. The location of the observatory is considered to have fixed coordinates in a [[terrestrial reference frame]] (such as the [[International Terrestrial Reference Frame]]) but the position of the rotational axis of the Earth wanders over the surface of the Earth; this is known as [[polar motion]]. UT0 does not contain any correction for polar motion while UT1 does include them. The difference between UT0 and UT1 is on the order of a few tens of milliseconds. The designation ''UT0'' is no longer in common use.{{sfn|Urban|Seidelmann|2013|page=81}}
* '''UT1R''' is a smoothed version of UT1, filtering out periodic variations due to tides. It includes 62 smoothing terms, with periods ranging from 5.6 days to 18.6 years.{{sfn|IERS|n.d.}} UT1R is still in use in the technical literature but rarely used elsewhere.<ref>{{cite web |title=A Brief Note on Time Systems |url=https://www.astronomy.ohio-state.edu/pogge.1/Ast350/timesys.html |website=Department of Astronomy, [[Ohio State University]] |access-date=24 May 2022|quote=There are two other forms of Universal Time you are unlikely to encounter unless you dig into some very technical literature. UT1R [...]}}</ref>
* '''UT2''' is a smoothed version of UT1, filtering out periodic seasonal variations. It is mostly of historic interest and rarely used anymore. It is defined by
::<math>UT2 = UT1 + 0.022\cdot\sin(2\pi t) - 0.012\cdot\cos(2\pi t) - 0.006\cdot\sin(4\pi t) + 0.007\cdot\cos(4\pi t)\;\mbox{seconds}</math>
::where ''t'' is the time as fraction of the [[Year#Besselian year|Besselian year]].<ref>{{cite web|url=https://maia.usno.navy.mil/information/eo-values|title=Common Units and Conversions in Earth Orientation|website=IERS Rapid Service / Prediction Center, U.S. Naval Observatory}}</ref>