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# pb06

erfa.pb06(date1, date2)

  This function forms three Euler angles which implement general precession from epoch J2000.0, using the IAU 2006 model.

  Parameters:
    date1 double array
    date2 double array
  Returns:
    bzeta double array
    bz double array
    btheta double array

  Notes

  Wraps ERFA function `eraPb06`. The ERFA documentation is:

      - - - - - - - -
       e r a P b 0 6
      - - - - - - - -

      This function forms three Euler angles which implement general
      precession from epoch J2000.0, using the IAU 2006 model.  Frame
      bias (the offset between ICRS and mean J2000.0) is included.

      Given:
         date1,date2  double   TT as a 2-part Julian Date (Note 1)

      Returned:
         bzeta        double   1st rotation: radians cw around z
         bz           double   3rd rotation: radians cw around z
         btheta       double   2nd rotation: radians ccw around y

      Notes:

      1) The TT date date1+date2 is a Julian Date, apportioned in any
         convenient way between the two arguments.  For example,
         JD(TT)=2450123.7 could be expressed in any of these ways,
         among others:

                date1          date2

             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 method is best matched to the way
         the argument is handled internally and will deliver the
         optimum resolution.  The MJD method and the date & time methods
         are both good compromises between resolution and convenience.

      2) The traditional accumulated precession angles zeta_A, z_A,
         theta_A cannot be obtained in the usual way, namely through
         polynomial expressions, because of the frame bias.  The latter
         means that two of the angles undergo rapid changes near this
         date.  They are instead the results of decomposing the
         precession-bias matrix obtained by using the Fukushima-Williams
         method, which does not suffer from the problem.  The
         decomposition returns values which can be used in the
         conventional formulation and which include frame bias.

      3) The three angles are returned in the conventional order, which
         is not the same as the order of the corresponding Euler
         rotations.  The precession-bias matrix is
         R_3(-z) x R_2(+theta) x R_3(-zeta).

      4) Should zeta_A, z_A, theta_A angles be required that do not
         contain frame bias, they are available by calling the ERFA
         function eraP06e.

      Called:
         eraPmat06    PB matrix, IAU 2006
         eraRz        rotate around Z-axis

      This revision:  2021 May 11

      Copyright (C) 2013-2023, NumFOCUS Foundation.
      Derived, with permission, from the SOFA library.  See notes at end of file.