Editing Sidereal time (section)

===Mean and apparent varieties===
[[File:Sidereal Clock made for Sir George Augustus William Shuckburgh.jpg|thumb|One of the two known surviving sidereal angle clocks in the world, made by [[John Arnold (watchmaker)|John Arnold]] & Son. It was previously owned by Sir [[George Shuckburgh-Evelyn]]. It is on display in the [[Royal Observatory, Greenwich]], London.]]

Although ERA is intended to replace sidereal time, there is a need to maintain definitions for sidereal time during the transition, and when working with older data and documents.

Similarly to mean solar time, every location on Earth has its own local sidereal time (LST), depending on the longitude of the point. Since it is not feasible to publish tables for every longitude, astronomical tables use Greenwich sidereal time (GST), which is sidereal time on the [[IERS Reference Meridian]], less precisely termed the Greenwich, or [[Prime meridian (Greenwich)|Prime meridian]]. There are two varieties, '''mean sidereal time''' if the mean equator and equinox of date are used, and '''apparent sidereal time''' if the apparent equator and equinox of date are used. The former ignores the effect of [[astronomical nutation]] while the latter includes it. When the choice of location is combined with the choice of including astronomical nutation or not, the acronyms GMST, LMST, GAST, and LAST result.

The following relationships are true:{{Sfn|Urban|Seidelmann|2013|page=80}}
{{block indent|1=local mean sidereal time = GMST + east longitude}}
{{block indent|1=local apparent sidereal time = GAST + east longitude}}

The new definitions of Greenwich mean and apparent sidereal time (since 2003, see above) are:
<math display="block">\mathrm{GMST}(t_U,t)=\theta(t_U)-E_\mathrm{PREC}(t)</math>
<math display="block">\mathrm{GAST}(t_U,t)=\theta(t_U)-E_0(t)</math>

such that ''θ'' is the Earth Rotation Angle, ''E''<sub>PREC</sub> is the accumulated precession, and ''E''<sub>0</sub> is equation of the origins, which represents accumulated precession and nutation.{{Sfn|Urban|Seidelmann|2013|pages=78–79}} The calculation of precession and nutation was described in Chapter 6 of Urban & Seidelmann.

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″ (6 h 42 m 28.8291 s). The GAST was 6 h 43 m 20.7109 s. For GMST the hour and minute were the same but the second was 21.1060.{{Sfn|Astronomical Almanac|2016|page=B21}}