.help gmsta Jun99 "Slalib Package"
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      DOUBLE PRECISION FUNCTION slGMSA (DATE, UT)

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      G M S A
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  Conversion from Universal Time to Greenwich mean sidereal time,
  with rounding errors minimized.

  double precision

  Given:
    DATE    d      UT1 date (MJD: integer part of JD-2400000.5))
    UT      d      UT1 time (fraction of a day)

  The result is the Greenwich mean sidereal time (double precision,
  radians, in the range 0 to 2pi).

  There is no restriction on how the UT is apportioned between the
  DATE and UT arguments.  Either of the two arguments could, for
  example, be zero and the entire date+time supplied in the other.
  However, the routine is designed to deliver maximum accuracy when
  the DATE argument is a whole number and the UT lies in the range
  0 to 1 (or vice versa).

  The algorithm is based on the IAU 1982 expression (see page S15 of
  the 1984 Astronomical Almanac).  This is always described as giving
  the GMST at 0 hours UT1.  In fact, it gives the difference between
  the GMST and the UT, the steady 4-minutes-per-day drawing-ahead of
  ST with respect to UT.  When whole days are ignored, the expression
  happens to equal the GMST at 0 hours UT1 each day.

  In this routine, the entire UT1 (the sum of the two arguments DATE
  and UT) is used directly as the argument for the standard formula.
  The UT1 is then added, but omitting whole days to conserve accuracy.

  See also the routine slGMST, which accepts the UT as a single
  argument.  Compared with slGMST, the extra numerical precision
  delivered by the present routine is unlikely to be important in
  an absolute sense, but may be useful when critically comparing
  algorithms and in applications where two sidereal times close
  together are differenced.

  Called:  slDA2P

  P.T.Wallace   Starlink   13 April 1998

  Copyright (C) 1998 Rutherford Appleton Laboratory
  Copyright (C) 1995 Association of Universities for Research in Astronomy Inc.

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