Conversion from Universal Time to Sidereal Time. (double precision)
ut1 double Universal Time (strictly UT1) expressed as Modified Julian Date (JD-2400000.5) The result is the Greenwich Mean Sidereal Time (double precision, radians). The IAU 1982 expression (see page S15 of the 1984 Astronomical Almanac) is used, but rearranged to reduce rounding errors. This expression is always described as giving the GMST at 0 hours UT. In fact, it gives the difference between the GMST and the UT, which happens to equal the GMST (modulo 24 hours) at 0 hours UT each day. In this routine, the fractional UT is used directly as the argument for the standard formula, and the fractional part of the UT is added separately; note that the factor 1.0027379... does not appear.
slaDranrm Defined in slamac.h: D2PI, DS2R P.T.Wallace Starlink 10 December 1993