utils/SSEphem/astrtime.cc
double utc2gmst (long mjd, double utc)
{
    long ut1_mjd;
    double ut1;
    utc2ut1 (mjd, utc, ut1_mjd, ut1);
    return iauGmst82 (2400000.5, (double)ut1_mjd + ut1) / TWOPI;
}
// Greenwich Mean Sidereal Time is the hour angle of the average position of
// the vernal equinox from the Greenwich meridian.  The "average" position
// of the vernal equixox is defined by the equation on page 50 of the
// Explanatory Supplement to the Astronomical Almanac (1992) linking UT1 to
// GMST.  This equation is coded in the SOFA routine iauGmst82().
//    GMST (in seconds at UT1=0) = 24110.54841 + 8640184.812866 * T
//				   + 0.093104 * T^2 - 0.0000062 * T^3
//    where T is in Julian centuries from 2000 Jan. 1 12h UT1
//	    T = d / 36525
//	    d = JD - 2451545.0
//  The ratio of the rates of UT1 and GMST is the ratio on the earth
// rotation period as measured with respect to the mean solar position and
// with respect to distant stars plus the average drift rate of the vernal
// equinox (precession).
Greenwich Mean Sidereal Time is the hour angle of the average position of
the vernal equinox from the Greenwich meridian.  The "average" position
of the vernal equixox is defined by the equation on page 50 of the
Explanatory Supplement to the Astronomical Almanac (1992) linking UT1 to
GMST.  This equation is coded in the SOFA routine iauGmst82().
Greenwich Mean Sidereal Time is the hour angle of the average position of the vernal equinox from the Greenwich meridian . The " average " position of the vernal equixox is defined by the equation on page 50 of the Explanatory Supplement to the Astronomical Almanac ( 1992 ) linking UT1 to GMST . This equation is coded in the SOFA routine iauGmst82 () .
double utc2gmst (long mjd, double utc)
{
    long ut1_mjd;
    double ut1;
    utc2ut1 (mjd, utc, ut1_mjd, ut1);
    return iauGmst82 (2400000.5, (double)ut1_mjd + ut1) / TWOPI;
}
double utc2gmst ( long mjd , double utc ) { long ut1_mjd ; double ut1 ; utc2ut1 ( mjd , utc , ut1_mjd , ut1 ) ; return iauGmst82 ( 2400000.5 , ( double ) ut1_mjd + ut1 ) / TWOPI ; }
Greenwich Mean Sidereal Time is the hour angle of the average position of
the vernal equinox from the Greenwich meridian.
Greenwich Mean Sidereal Time is the hour angle of the average position of the vernal equinox from the Greenwich meridian .