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# gst06a

erfa.gst06a(uta, utb, tta, ttb)

  Greenwich apparent sidereal time (consistent with IAU 2000 and 2006 resolutions).

  Parameters
    uta double array
    utb double array
    tta double array
    ttb double array
  Returns
    c_retval double array

  Notes

  Wraps ERFA function `eraGst06a`. The ERFA documentation is:

      - - - - - - - - - -
       e r a G s t 0 6 a
      - - - - - - - - - -

      Greenwich apparent sidereal time (consistent with IAU 2000 and 2006
      resolutions).

      Given:
         uta,utb    double    UT1 as a 2-part Julian Date (Notes 1,2)
         tta,ttb    double    TT as a 2-part Julian Date (Notes 1,2)

      Returned (function value):
                    double    Greenwich apparent sidereal time (radians)

      Notes:

      1) The UT1 and TT dates uta+utb and tta+ttb respectively, are both
         Julian Dates, apportioned in any convenient way between the
         argument pairs.  For example, JD(UT1)=2450123.7 could be
         expressed in any of these ways, among others:

                 uta            utb

             2450123.7           0.0       (JD method)
             2451545.0       -1421.3       (J2000 method)
             2400000.5       50123.2       (MJD method)
             2450123.5           0.2       (date & time method)

         The JD method is the most natural and convenient to use in
         cases where the loss of several decimal digits of resolution
         is acceptable (in the case of UT;  the TT is not at all critical
         in this respect).  The J2000 and MJD methods are good compromises
         between resolution and convenience.  For UT, the date & time
         method is best matched to the algorithm that is used by the Earth
         rotation angle function, called internally:  maximum precision is
         delivered when the uta argument is for 0hrs UT1 on the day in
         question and the utb argument lies in the range 0 to 1, or vice
         versa.

      2) Both UT1 and TT are required, UT1 to predict the Earth rotation
         and TT to predict the effects of precession-nutation.  If UT1 is
         used for both purposes, errors of order 100 microarcseconds
         result.

      3) This GAST is compatible with the IAU 2000/2006 resolutions and
         must be used only in conjunction with IAU 2006 precession and
         IAU 2000A nutation.

      4) The result is returned in the range 0 to 2pi.

      Called:
         eraPnm06a    classical NPB matrix, IAU 2006/2000A
         eraGst06     Greenwich apparent ST, IAU 2006, given NPB matrix

      Reference:

         Wallace, P.T. & Capitaine, N., 2006, Astron.Astrophys. 459, 981

      This revision:  2021 May 11

      Copyright (C) 2013-2021, NumFOCUS Foundation.
      Derived, with permission, from the SOFA library.  See notes at end of file.