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# gst00b

erfa.gst00b(uta, utb)

  Greenwich apparent sidereal time (consistent with IAU 2000 resolutions but using the truncated nutation model IAU 2000B).

  Parameters
    uta double array
    utb double array
  Returns
    c_retval double array

  Notes

  Wraps ERFA function `eraGst00b`. The ERFA documentation is:

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

      Greenwich apparent sidereal time (consistent with IAU 2000
      resolutions but using the truncated nutation model IAU 2000B).

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

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

      Notes:

      1) The UT1 date uta+utb is a Julian Date, apportioned in any
         convenient way between the argument pair.  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.  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) The result is compatible with the IAU 2000 resolutions, except
         that accuracy has been compromised for the sake of speed and
         convenience in two respects:

         . UT is used instead of TDB (or TT) to compute the precession
           component of GMST and the equation of the equinoxes.  This
           results in errors of order 0.1 mas at present.

         . The IAU 2000B abridged nutation model (McCarthy & Luzum, 2003)
           is used, introducing errors of up to 1 mas.

      3) This GAST is compatible with the IAU 2000 resolutions and must be
         used only in conjunction with other IAU 2000 compatible
         components such as precession-nutation.

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

      5) The algorithm is from Capitaine et al. (2003) and IERS
         Conventions 2003.

      Called:
         eraGmst00    Greenwich mean sidereal time, IAU 2000
         eraEe00b     equation of the equinoxes, IAU 2000B
         eraAnp       normalize angle into range 0 to 2pi

      References:

         Capitaine, N., Wallace, P.T. and McCarthy, D.D., "Expressions to
         implement the IAU 2000 definition of UT1", Astronomy &
         Astrophysics, 406, 1135-1149 (2003)

         McCarthy, D.D. & Luzum, B.J., "An abridged model of the
         precession-nutation of the celestial pole", Celestial Mechanics &
         Dynamical Astronomy, 85, 37-49 (2003)

         McCarthy, D. D., Petit, G. (eds.), IERS Conventions (2003),
         IERS Technical Note No. 32, BKG (2004)

      This revision:  2021 May 11

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