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# eo06a

erfa.eo06a(date1, date2)

  Equation of the origins, IAU 2006 precession and IAU 2000A nutation.

  Parameters
    date1 double array
    date2 double array
  Returns
    c_retval double array

  Notes

  Wraps ERFA function `eraEo06a`. The ERFA documentation is:

      - - - - - - - - -
       e r a E o 0 6 a
      - - - - - - - - -

      Equation of the origins, IAU 2006 precession and IAU 2000A nutation.

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

      Returned (function value):
                      double    the equation of the origins in radians

      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 equation of the origins is the distance between the true
         equinox and the celestial intermediate origin and, equivalently,
         the difference between Earth rotation angle and Greenwich
         apparent sidereal time (ERA-GST).  It comprises the precession
         (since J2000.0) in right ascension plus the equation of the
         equinoxes (including the small correction terms).

      Called:
         eraPnm06a    classical NPB matrix, IAU 2006/2000A
         eraBpn2xy    extract CIP X,Y coordinates from NPB matrix
         eraS06       the CIO locator s, given X,Y, IAU 2006
         eraEors      equation of the origins, given NPB matrix and s

      References:

         Capitaine, N. & Wallace, P.T., 2006, Astron.Astrophys. 450, 855

         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.