Starguide - A Practical Guide to Astro-Photography

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 make use of Greenwich sidereal time (GST), which is sidereal time on the IERS Reference Meridian, less precisely called the Greenwich, or prime meridian. 

There are two varieties, mean sidereal time if the mean equator and equinox of date are used, or apparent sidereal time if the apparent equator and equinox of date are used. The former ignores the effect of nutation while the latter includes nutation. When the choice of location is combined with the choice of including nutation or not, the acronyms GMST, LMST, GAST, and LAST result.

The following relationships are true:[15]

local mean sidereal time = GMST + east longitude

local apparent sidereal time = GAST + east longitude

The new definitions of Greenwich mean and apparent sidereal time (since 2003, see above) are:

such that Θ is the Earth Rotation Angle, EPREC is the accumulated precession, and E0 is the equation of the origins, which represents accumulated precession and nutation. 

GMST going to be replaced by ERA (Earth Rotation Angle)

Follow this link to calculate you local GMST.

Reference:

[1] Wikipedia Sidereal Time (en)

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