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      DOUBLE PRECISION FUNCTION slGMST (UT1)

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      G M S T
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  Conversion from universal time to sidereal time (double precision)

  Given:
    UT1    dp     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 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
  entire 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.

  See also the routine slGMSA, which delivers better numerical
  precision by accepting the UT date and time as separate arguments.

  Called:  slDA2P

  P.T.Wallace   Starlink   14 September 1995

  Copyright (C) 1995 Rutherford Appleton Laboratory
  Copyright (C) 1995 Association of Universities for Research in Astronomy Inc.

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