Conversion from universal time UT1 to Greenwich mean sidereal time.
D = sla_GMST (UT1)
universal time (strictly UT1)
expressed as modified Julian Date
Greenwich mean sidereal time (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 UT;
in fact, it gives the difference between the GMST and the UT, which happens to equal
the GMST (modulo 24 hours) at UT
each day. In sla_GMST, the entire UT is used directly as the argument for the canonical
formula, and the fractional part of the UT is added separately; note that the factor
does not appear.
See also the routine sla_GMSTA, which delivers better numerical precision by
accepting the UT date and time as separate arguments.