question: public static double greenwichSiderealTime ( double julianDate , GregorianCalendar time ) { double T = ( julianDate - 2451545.0 ) / 36525.0 ; double T0 = 6.697374558 + ( T * ( 2400.051336 + ( T + 2.5862E-5 ) ) ) ; T0 %= 24.0 ; if ( T0 < 0 ) { T0 += 24.0 ; } double UT = time . get ( Calendar . HOUR_OF_DAY ) + ( time . get ( Calendar . MINUTE ) + time . get ( Calendar . SECOND ) / 60.0 ) / 60.0 ; T0 += UT * 1.002737909 ; T0 %= 24.0 ; if ( T0 < 0 ) { T0 += 24.0 ; } return T0 ; }
target:  Calculate the greenwich sidereal time (GST). From Duffett-Smith, chapter 12.