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# ecm06

erfa.ecm06(date1, date2)

  ICRS equatorial to ecliptic rotation matrix, IAU 2006.

  Parameters:
    date1 double array
    date2 double array
  Returns:
    rm double array

  Notes

  Wraps ERFA function `eraEcm06`. The ERFA documentation is:

      - - - - - - - - -
       e r a E c m 0 6
      - - - - - - - - -

      ICRS equatorial to ecliptic rotation matrix, IAU 2006.

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

      Returned:
         rm           double[3][3]   ICRS to ecliptic rotation matrix

      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 matrix is in the sense

            E_ep = rm x P_ICRS,

         where P_ICRS is a vector with respect to ICRS right ascension
         and declination axes and E_ep is the same vector with respect to
         the (inertial) ecliptic and equinox of date.

         P_ICRS is a free vector, merely a direction, typically of unit
         magnitude, and not bound to any particular spatial origin, such
         as the Earth, Sun or SSB.  No assumptions are made about whether
         it represents starlight and embodies astrometric effects such as
         parallax or aberration.  The transformation is approximately that
         between mean J2000.0 right ascension and declination and ecliptic
         longitude and latitude, with only frame bias (always less than
         25 mas) to disturb this classical picture.

      Called:
         eraObl06     mean obliquity, IAU 2006
         eraPmat06    PB matrix, IAU 2006
         eraIr        initialize r-matrix to identity
         eraRx        rotate around X-axis
         eraRxr       product of two r-matrices

      This revision:  2023 February 26

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