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+.help gmsta Jun99 "Slalib Package"
+.nf
+
+      DOUBLE PRECISION FUNCTION slGMSA (DATE, UT)
+
+     - - - - - -
+      G M S A
+     - - - - - -
+
+  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.
+
+.fi
+.endhelp