Editing Sidereal time (section)

===Earth rotation angle{{anchor|ERA}}===

The '''Earth rotation angle''' ('''ERA''') measures the rotation of the Earth from an origin on the celestial equator, the ''Celestial Intermediate Origin'', also termed the ''Celestial Ephemeris Origin'',<ref>{{cite dictionary |url=https://www.iers.org/IERS/EN/Service/Glossary/celestialIntermediateOrigin.html?nn=14894 |title=Celestial Intermediate Origin (CIO) |dictionary=Glossary of the [[IERS]] Conventions (2010)}}</ref> that has no instantaneous motion along the equator; it was originally referred to as the ''non-rotating origin''. This point is very close to the equinox of J2000.<ref>{{cite dictionary |url=https://www.iers.org/IERS/EN/Service/Glossary/ceo.html?nn=14894 |title=Celestial Ephemeris Origin |dictionary=Glossary of the [[IERS]] Conventions (2010)}}</ref>

ERA, measured in [[radian]]s, is related to [[Universal time|UT1]] by a simple linear relation:{{sfn|Urban|Seidelmann|2013|page=78}}
<math display="block">\theta(t_U)=2\pi(0.779\,057\,273\,2640+1.002\,737\,811\,911\,354\,48\cdot t_U)</math>
where ''t<sub>U</sub>'' is the [[Julian day|Julian UT1 date]] (JD) minus 2451545.0.
The linear coefficient represents the [[Earth's rotation speed]] around its own axis.

ERA replaces ''Greenwich Apparent Sidereal Time'' (GAST). The origin on the celestial equator for GAST, termed the true [[Equinox (celestial coordinates)|equinox]], does move, due to the movement of the equator and the ecliptic. The lack of motion of the origin of ERA is considered a significant advantage.{{Sfn|Urban|Seidelmann|2013|page=6}}

The ERA may be converted to other units; for example, the ''Astronomical Almanac for the Year 2017'' tabulated it in degrees, minutes, and seconds.{{Sfn|Astronomical Almanac|2016|pages=B21–B24}}

As an example, the ''Astronomical Almanac for the Year 2017'' gave the ERA at 0 h 1 January 2017 UT1 as 100° 37′ 12.4365″.{{Sfn|Astronomical Almanac|2016|page=B21}} Since [[Coordinated Universal Time]] (UTC) is within a second or two of UT1, this can be used as an anchor to give the ERA approximately for a given civil time and date.